Sudoers Manual
NAME
sudoers
—
list of which users may execute
what
DESCRIPTION
The sudoers file is composed of two types of entries: aliases (basically variables) and user specifications (which specify who may run what).
When multiple entries match for a user, they are applied in order. Where there are multiple matches, the last match is used (which is not necessarily the most specific match).
The sudoers grammar will be described below in Extended Backus-Naur Form (EBNF). Don't despair if you are unfamiliar with EBNF; it is fairly simple, and the definitions below are annotated.
Quick guide to EBNF
EBNF is a concise and exact way of describing the grammar of a language. Each EBNF definition is made up of production rules. E.g.,
symbol ::= definition
|
alternate1
| alternate2
...
Each production rule references others and thus makes up a grammar for the language. EBNF also contains the following operators, which many readers will recognize from regular expressions. Do not, however, confuse them with “wildcard” characters, which have different meanings.
?
- Means that the preceding symbol (or group of symbols) is optional. That is, it may appear once or not at all.
*
- Means that the preceding symbol (or group of symbols) may appear zero or more times.
+
- Means that the preceding symbol (or group of symbols) may appear one or more times.
Parentheses may be used to group symbols together. For clarity, we will use single quotes ('') to designate what is a verbatim character string (as opposed to a symbol name).
Aliases
There are four kinds of aliases:
User_Alias
, Runas_Alias
,
Host_Alias
and
Cmnd_Alias
.
Alias ::= 'User_Alias' User_Alias (':' User_Alias)* | 'Runas_Alias' Runas_Alias (':' Runas_Alias)* | 'Host_Alias' Host_Alias (':' Host_Alias)* | 'Cmnd_Alias' Cmnd_Alias (':' Cmnd_Alias)*User_Alias ::= NAME ‘=’ User_List
Runas_Alias ::= NAME ‘=’ Runas_List
Host_Alias ::= NAME ‘=’ Host_List
Cmnd_Alias ::= NAME ‘=’ Cmnd_List
NAME ::= A-Z*
Each alias definition is of the form
Alias_Type NAME = item1, item2, ...
where
Alias_Type is
one of User_Alias
,
Runas_Alias
, Host_Alias
, or
Cmnd_Alias
. A NAME
is a
string of uppercase letters, numbers, and underscore characters
(‘_
’). A NAME
must start
with an uppercase letter. It is possible to put several alias definitions of
the same type on a single line, joined by a colon
(‘:
’). E.g.,
Alias_Type NAME = item1, item2, item3 : NAME = item4, item5
The definitions of what constitutes a valid alias member follow.
User_List ::= User | User ',' User_ListUser ::= ‘!’* user name | ‘!’* #uid | ‘!’* %group | ‘!’* %#gid | ‘!’* +netgroup | ‘!’* %:nonunix_group | ‘!’* %:#nonunix_gid | ‘!’* User_Alias
A User_List
is made up of one or more user
names, user ids (prefixed with ‘#
’),
system group names and ids (prefixed with
‘%
’ and
‘%#
’ respectively), netgroups
(prefixed with ‘+
’), non-Unix group
names and IDs (prefixed with ‘%:
’ and
‘%:#
’ respectively) and
User_Alias
es. Each list item
may be prefixed with zero or more ‘!
’
operators. An odd number of ‘!
’
operators negate the value of the item; an even number just cancel each
other out.
A user name
, uid
,
group
, gid
,
netgroup
, nonunix_group
or
nonunix_gid
may be enclosed in double quotes to
avoid the need for escaping special characters. Alternately, special
characters may be specified in escaped hex mode, e.g. \x20 for space. When
using double quotes, any prefix characters must be included inside the
quotes.
The actual nonunix_group
and
nonunix_gid
syntax depends on the underlying
implementation. For instance, the QAS AD backend supports the following
formats:
- Group in the same domain: "%:Group Name"
- Group in any domain: "%:Group Name@FULLY.QUALIFIED.DOMAIN"
- Group SID: "%:S-1-2-34-5678901234-5678901234-5678901234-567"
Note that quotes around group names are optional. Unquoted strings
must use a backslash (‘\
’) to escape
spaces and special characters. See
Other
special characters and reserved words for a list of characters that need
to be escaped.
Runas_List ::= Runas_Member | Runas_Member ',' Runas_ListRunas_Member ::= ‘!’* user name | ‘!’* #uid | ‘!’* %group | ‘!’* %#gid | ‘!’* %:nonunix_group | ‘!’* %:#nonunix_gid | ‘!’* +netgroup | ‘!’* Runas_Alias
A Runas_List
is similar to a
User_List
except that instead of
User_Alias
es it can contain
Runas_Alias
es. Note that
user names and groups are matched as strings. In other words, two users
(groups) with the same uid (gid) are considered to be distinct. If you wish
to match all user names with the same uid (e.g. root and toor), you can use
a uid instead (#0 in the example given).
Host_List ::= Host | Host ',' Host_ListHost ::= ‘!’* host name | ‘!’* ip_addr | ‘!’* network(/netmask)? | ‘!’* +netgroup | ‘!’* Host_Alias
A Host_List
is made up of one or more host
names, IP addresses, network numbers, netgroups (prefixed with
‘+
’) and other aliases. Again, the
value of an item may be negated with the
‘!
’ operator. If you do not specify a
netmask along with the network number, sudo
will
query each of the local host's network interfaces and, if the network number
corresponds to one of the hosts's network interfaces, the corresponding
netmask will be used. The netmask may be specified either in standard IP
address notation (e.g. 255.255.255.0 or ffff:ffff:ffff:ffff::), or CIDR
notation (number of bits, e.g. 24 or 64). A host name may include
shell-style wildcards (see the Wildcards
section below), but unless the host name
command on
your machine returns the fully qualified host name, you'll need to use the
fqdn option for wildcards to be useful. Note that
sudo
only inspects actual network interfaces; this
means that IP address 127.0.0.1 (localhost) will never match. Also, the host
name “localhost” will only match if that is the actual host
name, which is usually only the case for non-networked systems.
Cmnd_List ::= Cmnd | Cmnd ',' Cmnd_Listcommand name ::= file name | file name args | file name ‘""’
Cmnd ::= ‘!’* command name | ‘!’* directory | ‘!’* "sudoedit" | ‘!’* Cmnd_Alias
A Cmnd_List
is a list of one
or more command names, directories, and other aliases. A command name is a
fully qualified file name which may include shell-style wildcards (see the
Wildcards section below). A simple file
name allows the user to run the command with any arguments he/she wishes.
However, you may also specify command line arguments (including wildcards).
Alternately, you can specify ""
to
indicate that the command may only be run
without command
line arguments. A directory is a fully qualified path name ending in a
‘/
’. When you specify a directory in a
Cmnd_List
, the user will be able to run any file
within that directory (but not in any sub-directories therein).
If a Cmnd
has associated command line
arguments, then the arguments in the Cmnd
must match
exactly those given by the user on the command line (or match the wildcards
if there are any). Note that the following characters must be escaped with a
‘\
’ if they are used in command
arguments: ‘,
’,
‘:
’,
‘=
’,
‘\
’. The special command
“sudoedit
” is used to permit a user to
run sudo
with the -e
option
(or as sudoedit
). It may take command line arguments
just as a normal command does.
Defaults
Certain configuration options may be changed from their default
values at run-time via one or more Default_Entry
lines. These may affect all users on any host, all users on a specific host,
a specific user, a specific command, or commands being run as a specific
user. Note that per-command entries may not include command line arguments.
If you need to specify arguments, define a
Cmnd_Alias
and reference that instead.
Default_Type ::= 'Defaults' | 'Defaults' '@' Host_List | 'Defaults' ':' User_List | 'Defaults' '!' Cmnd_List | 'Defaults' '>' Runas_ListDefault_Entry ::= Default_Type Parameter_List
Parameter_List ::= Parameter | Parameter ‘,’ Parameter_List
Parameter ::= Parameter ‘=’ Value | Parameter ‘+=’ Value | Parameter ‘-=’ Value | ‘!’* Parameter
Parameters may be
flags,
integer
values,
strings,
or
lists.
Flags are implicitly boolean and can be turned off via the
‘!
’ operator. Some integer, string and
list parameters may also be used in a boolean context to disable them.
Values may be enclosed in double quotes ("") when they contain
multiple words. Special characters may be escaped with a backslash
(‘\
’).
Lists have two additional assignment operators,
+=
and -=
. These operators
are used to add to and delete from a list respectively. It is not an error
to use the -=
operator to remove an element that
does not exist in a list.
Defaults entries are parsed in the following order: generic, host and user Defaults first, then runas Defaults and finally command defaults.
See SUDOERS OPTIONS for a list of supported Defaults parameters.
User specification
User_Spec ::= User_List Host_List '=' Cmnd_Spec_List \ (':' Host_List '=' Cmnd_Spec_List)*Cmnd_Spec_List ::= Cmnd_Spec | Cmnd_Spec ‘,’ Cmnd_Spec_List
Cmnd_Spec ::= Runas_Spec? SELinux_Spec? Tag_Spec* Cmnd
Runas_Spec ::= ‘(’ Runas_List? (’:’ Runas_List)? ‘)’
SELinux_Spec ::= (‘ROLE=role’ | ‘TYPE=type’)
Tag_Spec ::= (‘NOPASSWD:’ | ‘PASSWD:’ | ‘NOEXEC:’ | ‘EXEC:’ | ‘SETENV:’ | ‘NOSETENV:’ | ‘LOG_INPUT:’ | ‘NOLOG_INPUT:’ | ‘LOG_OUTPUT:’ | ‘NOLOG_OUTPUT:’)
A user specification determines which commands a user may run (and as what user) on specified hosts. By default, commands are run as root, but this can be changed on a per-command basis.
The basic structure of a user specification is “who where = (as_whom) what”. Let's break that down into its constituent parts:
Runas_Spec
A Runas_Spec
determines the user and/or
the group that a command may be run as. A fully-specified
Runas_Spec
consists of two
Runas_List
s (as defined
above) separated by a colon (‘:
’) and
enclosed in a set of parentheses. The first
Runas_List
indicates which users the command may be
run as via sudo
's
-u
option. The second defines a list of groups that
can be specified via sudo
's
-g
option. If both
Runas_List
s are specified,
the command may be run with any combination of users and groups listed in
their respective
Runas_List
s. If only the
first is specified, the command may be run as any user in the list but no
-g
option may be specified. If the first
Runas_List
is empty but the second is specified, the
command may be run as the invoking user with the group set to any listed in
the Runas_List
. If no
Runas_Spec
is specified the command may be run as
root and no group may be specified.
A Runas_Spec
sets the default for the
commands that follow it. What this means is that for the entry:
dgb boulder = (operator) /bin/ls, /bin/kill, /usr/bin/lprm
The user dgb may run /bin/ls, /bin/kill, and /usr/bin/lprm—but only as operator. E.g.,
$ sudo -u operator /bin/ls
It is also possible to override a
Runas_Spec
later on in an entry. If we modify the
entry like so:
dgb boulder = (operator) /bin/ls, (root) /bin/kill, /usr/bin/lprm
Then user dgb is now allowed to run /bin/ls as operator, but /bin/kill and /usr/bin/lprm as root.
We can extend this to allow dgb to run
/bin/ls
with either the user or group set to
operator:
dgb boulder = (operator : operator) /bin/ls, (root) /bin/kill,\ /usr/bin/lprm
Note that while the group portion of the
Runas_Spec
permits the user to run as command with
that group, it does not force the user to do so. If no group is specified on
the command line, the command will run with the group listed in the target
user's password database entry. The following would all be permitted by the
sudoers entry above:
$ sudo -u operator /bin/ls $ sudo -u operator -g operator /bin/ls $ sudo -g operator /bin/ls
In the following example, user tcm may run commands that access a modem device file with the dialer group.
tcm boulder = (:dialer) /usr/bin/tip, /usr/bin/cu,\ /usr/local/bin/minicom
Note that in this example only the group will be set, the command still runs as user tcm. E.g.
$ sudo -g dialer /usr/bin/cu
Multiple users and groups may be present in a
Runas_Spec
, in which case the user may select any
combination of users and groups via the -u
and
-g
options. In this example:
alan ALL = (root, bin : operator, system) ALL
user alan may run any command as either user root or bin, optionally setting the group to operator or system.
SELinux_Spec
On systems with SELinux support, sudoers entries may optionally have an SELinux role and/or type associated with a command. If a role or type is specified with the command it will override any default values specified in sudoers. A role or type specified on the command line, however, will supersede the values in sudoers.
Tag_Spec
A command may have zero or more tags associated with it. There are
ten possible tag values: NOPASSWD
,
PASSWD
, NOEXEC
,
EXEC
, SETENV
,
NOSETENV
, LOG_INPUT
,
NOLOG_INPUT
, LOG_OUTPUT
and
NOLOG_OUTPUT
. Once a tag is set on a
Cmnd
, subsequent
Cmnd
s in the
Cmnd_Spec_List
, inherit the tag unless it is
overridden by the opposite tag (in other words,
PASSWD
overrides NOPASSWD
and NOEXEC
overrides
EXEC
).
By default, sudo
requires that a user
authenticate him or herself before running a command. This behavior can be
modified via the NOPASSWD
tag. Like a
Runas_Spec
, the NOPASSWD
tag
sets a default for the commands that follow it in the
Cmnd_Spec_List
. Conversely, the
PASSWD
tag can be used to reverse things. For
example:
ray rushmore = NOPASSWD: /bin/kill, /bin/ls, /usr/bin/lprm
would allow the user ray to run /bin/kill, /bin/ls, and /usr/bin/lprm as root on the machine rushmore without authenticating himself. If we only want ray to be able to run /bin/kill without a password the entry would be:
ray rushmore = NOPASSWD: /bin/kill, PASSWD: /bin/ls, /usr/bin/lprm
Note, however, that the PASSWD
tag has no
effect on users who are in the group specified by the
exempt_group option.
By default, if the NOPASSWD
tag is applied to any of the entries for a user on the current host, he or
she will be able to run “sudo -l
”
without a password. Additionally, a user may only run
“sudo -v
” without a password if the
NOPASSWD
tag is present for all a user's entries
that pertain to the current host. This behavior may be overridden via the
verifypw and
listpw
options.
NOEXEC and EXEC
If sudo
has been compiled with
noexec support and the underlying operating system
supports it, the NOEXEC
tag can be used to prevent a
dynamically-linked executable from running further commands itself.
In the following example, user aaron may run /usr/bin/more and /usr/bin/vi but shell escapes will be disabled.
aaron shanty = NOEXEC: /usr/bin/more, /usr/bin/vi
See the Preventing
shell escapes section below for more details on how
NOEXEC
works and whether or not it will work on your
system.
These tags override the value of the
setenv option on a
per-command basis. Note that if SETENV
has been set
for a command, the user may disable the env_reset option
from the command line via the -E
option.
Additionally, environment variables set on the command line are not subject
to the restrictions imposed by env_check,
env_delete, or env_keep. As such, only
trusted users should be allowed to set variables in this manner. If the
command matched is ALL, the SETENV
tag is implied for that command; this default may be overridden by use of
the NOSETENV
tag.
These tags override the value of the log_input option on a per-command basis. For more information, see the description of log_input in the SUDOERS OPTIONS section below.
These tags override the value of the log_output option on a per-command basis. For more information, see the description of log_output in the SUDOERS OPTIONS section below.
Wildcards
sudo
allows shell-style
wildcards
(aka meta or glob characters) to be used in host names, path names and
command line arguments in the sudoers file. Wildcard
matching is done via the
POSIX
glob(3) and
fnmatch(3) routines. Note
that these are not regular expressions.
*
- Matches any set of zero or more characters.
?
- Matches any single character.
[...]
- Matches any character in the specified range.
[!...]
- Matches any character not in the specified range.
\x
- For any character ‘x’, evaluates to ‘x’. This
is used to escape special characters such as:
‘
*
’, ‘?
’, ‘[
’, and ‘]
’.
POSIX character classes may also be used if your system's
glob(3) and
fnmatch(3) functions
support them. However, because the ‘:
’
character has special meaning in sudoers, it must be
escaped. For example:
/bin/ls [[alpha]]*
Would match any file name beginning with a letter.
Note that a forward slash
(‘/
’) will not be
matched by wildcards used in the path name. This is to make a path like:
/usr/bin/*
match /usr/bin/who but not /usr/bin/X11/xterm.
When matching the command line arguments, however, a slash does get matched by wildcards since command line arguments may contain arbitrary strings and not just path names.
Wildcards in command line arguments should be used with care.
Because command line arguments are matched as a single, concatenated string,
a wildcard such as ‘?
’ or
‘*
’ can match multiple words. For
example, while a sudoers entry like:
%operator ALL = /bin/cat /var/log/messages*
will allow command like:
$ sudo cat /var/log/messages.1
It will also allow:
$ sudo cat /var/log/messages /etc/shadow
which is probably not what was intended.
Exceptions to wildcard rules
The following exceptions apply to the above rules:
""
- If the empty string
""
is the only command line argument in the sudoers entry it means that command is not allowed to be run with any arguments. - sudoedit
- Command line arguments to the
sudoedit
built-in command should always be path names, so a forward slash
(‘
/
’) will not be matched by a wildcard.
Including other files from within sudoers
It is possible to include other sudoers files
from within the sudoers file currently being parsed using
the #include
and #includedir
directives.
This can be used, for example, to keep a site-wide sudoers file in addition to a local, per-machine file. For the sake of this example the site-wide sudoers will be /etc/sudoers and the per-machine one will be /etc/sudoers.local. To include /etc/sudoers.local from within /etc/sudoers we would use the following line in /etc/sudoers:
#include /etc/sudoers.local
When sudo
reaches this line it will
suspend processing of the current file
(/etc/sudoers) and switch to
/etc/sudoers.local. Upon reaching the end of
/etc/sudoers.local, the rest of
/etc/sudoers will be processed. Files that are
included may themselves include other files. A hard limit of 128 nested
include files is enforced to prevent include file loops.
If the path to the include file is not fully-qualified (does not
begin with a ‘/
’, it must be located
in the same directory as the sudoers file it was included from. For example,
if /etc/sudoers contains the line:
#include sudoers.local
the file that will be included is /etc/sudoers.local.
The file name may also include the %h
escape, signifying the short form of the host name. In other words, if the
machine's host name is “xerxes”, then
#include /etc/sudoers.%h
will cause sudo
to include the file
/etc/sudoers.xerxes.
The #includedir
directive can be used to
create a sudo.d directory that the system package
manager can drop sudoers rules into as part of package
installation. For example, given:
#includedir /etc/sudoers.d
sudo
will read each file in
/etc/sudoers.d, skipping file names that end in
‘~
’ or contain a
‘.
’ character to avoid causing
problems with package manager or editor temporary/backup files. Files are
parsed in sorted lexical order. That is,
/etc/sudoers.d/01_first will be parsed before
/etc/sudoers.d/10_second. Be aware that because the
sorting is lexical, not numeric,
/etc/sudoers.d/1_whoops would be loaded
after
/etc/sudoers.d/10_second. Using a consistent number
of leading zeroes in the file names can be used to avoid such problems.
Note that unlike files included via
#include
, visudo
will not
edit the files in a #includedir
directory unless one
of them contains a syntax error. It is still possible to run
visudo
with the -f
flag to
edit the files directly.
Other special characters and reserved words
The pound sign (‘#
’) is used
to indicate a comment (unless it is part of a #include directive or unless
it occurs in the context of a user name and is followed by one or more
digits, in which case it is treated as a uid). Both the comment character
and any text after it, up to the end of the line, are ignored.
The reserved word ALL is a built-in
alias that always causes a match to succeed. It can be
used wherever one might otherwise use a Cmnd_Alias
,
User_Alias
, Runas_Alias
, or
Host_Alias
. You should not try to define your own
alias called ALL as the built-in alias
will be used in preference to your own. Please note that using
ALL can be dangerous since in a command context, it allows
the user to run any command on the system.
An exclamation point (‘!
’)
can be used as a logical not operator both in an
alias and in front of a Cmnd
. This
allows one to exclude certain values. Note, however, that using a
‘!
’ in conjunction with the built-in
ALL alias to allow a user to run “all but a
few” commands rarely works as intended (see
SECURITY NOTES below).
Long lines can be continued with a backslash
(‘\
’) as the last character on the
line.
White space between elements in a list as well as special
syntactic characters in a User Specification
(‘=
’,
‘:
’,
‘(
’,
‘)
’) is optional.
The following characters must be escaped with a backslash
(‘\
’) when used as part of a word
(e.g. a user name or host name): ‘!
’,
‘=
’,
‘:
’,
‘,
’,
‘(
’,
‘)
’,
‘\
’.
SUDOERS OPTIONS
sudo
's behavior
can be modified by Default_Entry
lines, as explained
earlier. A list of all supported Defaults parameters, grouped by type, are
listed below.
- always_set_home
- If enabled,
sudo
will set theHOME
environment variable to the home directory of the target user (which is root unless the-u
option is used). This effectively means that the-H
option is always implied. Note thatHOME
is already set when the the env_reset option is enabled, so always_set_home is only effective for configurations where either env_reset is disabled orHOME
is present in the env_keep list. This flag is off by default. - authenticate
- If set, users must authenticate themselves via a password (or other means
of authentication) before they may run commands. This default may be
overridden via the
PASSWD
andNOPASSWD
tags. This flag is on by default. - closefrom_override
- If set, the user may use
sudo
's-C
option which overrides the default starting point at whichsudo
begins closing open file descriptors. This flag is off by default. - compress_io
- If set, and
sudo
is configured to log a command's input or output, the I/O logs will be compressed using zlib. This flag is on by default whensudo
is compiled with zlib support. - env_editor
- If set,
visudo
will use the value of theEDITOR
orVISUAL
environment variables before falling back on the default editor list. Note that this may create a security hole as it allows the user to run any arbitrary command as root without logging. A safer alternative is to place a colon-separated list of editors in theeditor
variable.visudo
will then only use theEDITOR
orVISUAL
if they match a value specified ineditor
. This flag is off by default. - env_reset
- If set,
sudo
will run the command in a minimal environment containing theTERM
,PATH
,HOME
,MAIL
,SHELL
,LOGNAME
,USER
,USERNAME
andSUDO_*
variables. Any variables in the caller's environment that match theenv_keep
andenv_check
lists are then added, followed by any variables present in the file specified by the env_file option (if any). The default contents of theenv_keep
andenv_check
lists are displayed whensudo
is run by root with the-V
option. If the secure_path option is set, its value will be used for thePATH
environment variable. This flag is on by default. - fast_glob
- Normally,
sudo
uses the glob(3) function to do shell-style globbing when matching path names. However, since it accesses the file system, glob(3) can take a long time to complete for some patterns, especially when the pattern references a network file system that is mounted on demand (auto mounted). The fast_glob option causessudo
to use the fnmatch(3) function, which does not access the file system to do its matching. The disadvantage of fast_glob is that it is unable to match relative path names such as ./ls or ../bin/ls. This has security implications when path names that include globbing characters are used with the negation operator, ‘!
’, as such rules can be trivially bypassed. As such, this option should not be used when sudoers contains rules that contain negated path names which include globbing characters. This flag is off by default. - fqdn
- Set this flag if you want to put fully qualified host names in the
sudoers file when the local host name (as returned by
the
hostname
command) does not contain the domain name. In other words, instead of myhost you would use myhost.mydomain.edu. You may still use the short form if you wish (and even mix the two). This option is only effective when the “canonical” host name, as returned by thegetaddrinfo
() orgethostbyname
() function, is a fully-qualified domain name. This is usually the case when the system is configured to use DNS for host name resolution.If the system is configured to use the /etc/hosts file in preference to DNS, the “canonical” host name may not be fully-qualified. The order that sources are queried for hosts name resolution is usually specified in the /etc/nsswitch.conf, /etc/netsvc.conf, /etc/host.conf, or, in some cases, /etc/resolv.conf file. In the /etc/hosts file, the first host name of the entry is considered to be the “canonical” name; subsequent names are aliases that are not used by
sudoers
. For example, the following hosts file line for the machine “xyzzy” has the fully-qualified domain name as the “canonical” host name, and the short version as an alias.192.168.1.1 xyzzy.sudo.ws xyzzy
If the machine's hosts file entry is not formatted properly, the fqdn option will not be effective if it is queried before DNS.
Beware that when using DNS for host name resolution, turning on fqdn requires
sudoers
to make DNS lookups which renderssudo
unusable if DNS stops working (for example if the machine is disconnected from the network). Also note that just like with the hosts file, you must use the “canonical” name as DNS knows it. That is, you may not use a host alias (CNAME
entry) due to performance issues and the fact that there is no way to get all aliases from DNS.This flag is off by default.
- ignore_dot
- If set,
sudo
will ignore "." or "" (both denoting current directory) in thePATH
environment variable; thePATH
itself is not modified. This flag is off by default. - ignore_local_sudoers
- If set via LDAP, parsing of /etc/sudoers will be
skipped. This is intended for Enterprises that wish to prevent the usage
of local sudoers files so that only LDAP is used. This thwarts the efforts
of rogue operators who would attempt to add roles to
/etc/sudoers. When this option is present,
/etc/sudoers does not even need to exist. Since
this option tells
sudo
how to behave when no specific LDAP entries have been matched, this sudoOption is only meaningful for thecn=defaults
section. This flag is off by default. - insults
- If set,
sudo
will insult users when they enter an incorrect password. This flag is off by default. - log_host
- If set, the host name will be logged in the (non-syslog)
sudo
log file. This flag is off by default. - log_input
- If set,
sudo
will run the command in a pseudo tty and log all user input. If the standard input is not connected to the user's tty, due to I/O redirection or because the command is part of a pipeline, that input is also captured and stored in a separate log file.Input is logged to the directory specified by the iolog_dir option (/var/log/sudo-io by default) using a unique session ID that is included in the normal
sudo
log line, prefixed with “TSID=
”.Note that user input may contain sensitive information such as passwords (even if they are not echoed to the screen), which will be stored in the log file unencrypted. In most cases, logging the command output via log_output is all that is required.
- log_output
- If set,
sudo
will run the command in a pseudo tty and log all output that is sent to the screen, similar to the script(1) command. If the standard output or standard error is not connected to the user's tty, due to I/O redirection or because the command is part of a pipeline, that output is also captured and stored in separate log files.Output is logged to the directory specified by the iolog_dir option (/var/log/sudo-io by default) using a unique session ID that is included in the normal
sudo
log line, prefixed with “TSID=
”.Output logs may be viewed with the sudoreplay(8) utility, which can also be used to list or search the available logs.
- log_year
- If set, the four-digit year will be logged in the (non-syslog)
sudo
log file. This flag is off by default. - long_otp_prompt
- When validating with a One Time Password (OTP) scheme such as S/Key or OPIE, a two-line prompt is used to make it easier to cut and paste the challenge to a local window. It's not as pretty as the default but some people find it more convenient. This flag is off by default.
- mail_always
- Send mail to the mailto user every time a users runs
sudo
. This flag is off by default. - mail_badpass
- Send mail to the mailto user if the user running
sudo
does not enter the correct password. If the command the user is attempting to run is not permitted by sudoers and one of the mail_always, mail_no_host, mail_no_perms or mail_no_user flags are set, this flag will have no effect. This flag is off by default. - mail_no_host
- If set, mail will be sent to the mailto user if the invoking user exists in the sudoers file, but is not allowed to run commands on the current host. This flag is off by default.
- mail_no_perms
- If set, mail will be sent to the mailto user if the
invoking user is allowed to use
sudo
but the command they are trying is not listed in their sudoers file entry or is explicitly denied. This flag is off by default. - mail_no_user
- If set, mail will be sent to the mailto user if the invoking user is not in the sudoers file. This flag is on by default.
- noexec
- If set, all commands run via
sudo
will behave as if theNOEXEC
tag has been set, unless overridden by aEXEC
tag. See the description of NOEXEC and EXEC below as well as the Preventing shell escapes section at the end of this manual. This flag is off by default. - path_info
- Normally,
sudo
will tell the user when a command could not be found in theirPATH
environment variable. Some sites may wish to disable this as it could be used to gather information on the location of executables that the normal user does not have access to. The disadvantage is that if the executable is simply not in the user'sPATH
,sudo
will tell the user that they are not allowed to run it, which can be confusing. This flag is on by default. - passprompt_override
- The password prompt specified by passprompt will normally only be used if the password prompt provided by systems such as PAM matches the string “Password:”. If passprompt_override is set, passprompt will always be used. This flag is off by default.
- preserve_groups
- By default,
sudo
will initialize the group vector to the list of groups the target user is in. When preserve_groups is set, the user's existing group vector is left unaltered. The real and effective group IDs, however, are still set to match the target user. This flag is off by default. - pwfeedback
- By default,
sudo
reads the password like most other Unix programs, by turning off echo until the user hits the return (or enter) key. Some users become confused by this as it appears to them thatsudo
has hung at this point. When pwfeedback is set,sudo
will provide visual feedback when the user presses a key. Note that this does have a security impact as an onlooker may be able to determine the length of the password being entered. This flag is off by default. - requiretty
- If set,
sudo
will only run when the user is logged in to a real tty. When this flag is set,sudo
can only be run from a login session and not via other means such as cron(8) or cgi-bin scripts. This flag is off by default. - root_sudo
- If set, root is allowed to run
sudo
too. Disabling this prevents users from “chaining”sudo
commands to get a root shell by doing something like “sudo sudo /bin/sh
”. Note, however, that turning off root_sudo will also prevent root from runningsudoedit
. Disabling root_sudo provides no real additional security; it exists purely for historical reasons. This flag is on by default. - rootpw
- If set,
sudo
will prompt for the root password instead of the password of the invoking user. This flag is off by default. - runaspw
- If set,
sudo
will prompt for the password of the user defined by the runas_default option (defaults toroot
) instead of the password of the invoking user. This flag is off by default. - set_home
- If enabled and
sudo
is invoked with the-s
option theHOME
environment variable will be set to the home directory of the target user (which is root unless the-u
option is used). This effectively makes the-s
option imply-H
. Note thatHOME
is already set when the the env_reset option is enabled, so set_home is only effective for configurations where either env_reset is disabled orHOME
is present in the env_keep list. This flag is off by default. - set_logname
- Normally,
sudo
will set theLOGNAME
,USER
andUSERNAME
environment variables to the name of the target user (usually root unless the-u
option is given). However, since some programs (including the RCS revision control system) useLOGNAME
to determine the real identity of the user, it may be desirable to change this behavior. This can be done by negating the set_logname option. Note that if the env_reset option has not been disabled, entries in the env_keep list will override the value of set_logname. This flag is on by default. - setenv
- Allow the user to disable the env_reset option from the
command line via the
-E
option. Additionally, environment variables set via the command line are not subject to the restrictions imposed by env_check, env_delete, or env_keep. As such, only trusted users should be allowed to set variables in this manner. This flag is off by default. - shell_noargs
- If set and
sudo
is invoked with no arguments it acts as if the-s
option had been given. That is, it runs a shell as root (the shell is determined by theSHELL
environment variable if it is set, falling back on the shell listed in the invoking user's /etc/passwd entry if not). This flag is off by default. - stay_setuid
- Normally, when
sudo
executes a command the real and effective UIDs are set to the target user (root by default). This option changes that behavior such that the real UID is left as the invoking user's UID. In other words, this makessudo
act as a setuid wrapper. This can be useful on systems that disable some potentially dangerous functionality when a program is run setuid. This option is only effective on systems that support either the setreuid(2) or setresuid(2) system call. This flag is off by default. - targetpw
- If set,
sudo
will prompt for the password of the user specified by the-u
option (defaults toroot
) instead of the password of the invoking user. In addition, the time stamp file name will include the target user's name. Note that this flag precludes the use of a uid not listed in the passwd database as an argument to the-u
option. This flag is off by default. - tty_tickets
- If set, users must authenticate on a per-tty basis. With this flag
enabled,
sudo
will use a file named for the tty the user is logged in on in the user's time stamp directory. If disabled, the time stamp of the directory is used instead. This flag is on by default. - umask_override
- If set,
sudo
will set the umask as specified by sudoers without modification. This makes it possible to specify a more permissive umask in sudoers than the user's own umask and matches historical behavior. If umask_override is not set,sudo
will set the umask to be the union of the user's umask and what is specified in sudoers. This flag is off by default. - use_loginclass
- If set,
sudo
will apply the defaults specified for the target user's login class if one exists. Only available ifsudo
is configured with the--with-logincap
option. This flag is off by default. - use_pty
- If set,
sudo
will run the command in a pseudo-pty even if no I/O logging is being gone. A malicious program run undersudo
could conceivably fork a background process that retains to the user's terminal device after the main program has finished executing. Use of this option will make that impossible. This flag is off by default. - visiblepw
- By default,
sudo
will refuse to run if the user must enter a password but it is not possible to disable echo on the terminal. If the visiblepw flag is set,sudo
will prompt for a password even when it would be visible on the screen. This makes it possible to run things like “ssh somehost sudo ls
” since by default, ssh(1) does not allocate a tty when running a command. This flag is off by default.
Integers:
- closefrom
- Before it executes a command,
sudo
will close all open file descriptors other than standard input, standard output and standard error (ie: file descriptors 0-2). The closefrom option can be used to specify a different file descriptor at which to start closing. The default is3
. - passwd_tries
- The number of tries a user gets to enter his/her password before
sudo
logs the failure and exits. The default is3
.
Integers that can be used in a boolean context:
- loglinelen
- Number of characters per line for the file log. This value is used to
decide when to wrap lines for nicer log files. This has no effect on the
syslog log file, only the file log. The default is
80
(use 0 or negate the option to disable word wrap). - passwd_timeout
- Number of minutes before the
sudo
password prompt times out, or0
for no timeout. The timeout may include a fractional component if minute granularity is insufficient, for example2.5
. The default is5
. - timestamp_timeout
- Number of minutes that can elapse before
sudo
will ask for a passwd again. The timeout may include a fractional component if minute granularity is insufficient, for example2.5
. The default is5
. Set this to0
to always prompt for a password. If set to a value less than0
the user's time stamp will never expire. This can be used to allow users to create or delete their own time stamps via “sudo -v
” and “sudo -k
” respectively. - umask
- Umask to use when running the command. Negate this option or set it to
0777 to preserve the user's umask. The actual umask that is used will be
the union of the user's umask and the value of the
umask option,
which defaults to
0022
. This guarantees thatsudo
never lowers the umask when running a command. Note: on systems that use PAM, the default PAM configuration may specify its own umask which will override the value set in sudoers.
Strings:
- badpass_message
- Message that is displayed if a user enters an incorrect password. The
default is
Sorry, try again.
unless insults are enabled. - editor
- A colon (‘
:
’) separated list of editors allowed to be used withvisudo
.visudo
will choose the editor that matches the user'sEDITOR
environment variable if possible, or the first editor in the list that exists and is executable. The default is vi. - iolog_dir
- The directory in which to store input/output logs when the
log_input or log_output options are
enabled or when the
LOG_INPUT
orLOG_OUTPUT
tags are present for a command. The default is /var/log/sudo-io. - mailsub
- Subject of the mail sent to the mailto user. The escape
%h
will expand to the host name of the machine. Default is “*** SECURITY information for %h ***
”. - noexec_file
- The noexec option specifies the the fully-qualified path
to a shared library containing dummy versions of the
execv
(),execve
() andfexecve
() library functions that just return an error. This is used to implement the noexec functionality on systems that supportLD_PRELOAD
or its equivalent. Defaults to /usr/local/libexec/sudo_noexec.so. - passprompt
- The default prompt to use when asking for a password; can be overridden
via the
-p
option or theSUDO_PROMPT
environment variable. The following percent (‘%
’) escape sequences are supported:%H
- expanded to the local host name including the domain name (only if the machine's host name is fully qualified or the fqdn option is set)
%h
- expanded to the local host name without the domain name
%p
- expanded to the user whose password is being asked for (respects the rootpw, targetpw and runaspw flags in sudoers)
%U
- expanded to the login name of the user the command will be run as (defaults to root)
%u
- expanded to the invoking user's login name
%%
- two consecutive
%
characters are collapsed into a single%
character
The default value is “
Password:
”. - role
- The default SELinux role to use when constructing a new security context
to run the command. The default role may be overridden on a per-command
basis in sudoers or via command line options. This
option is only available when
sudo
is built with SELinux support. - runas_default
- The default user to run commands as if the
-u
option is not specified on the command line. This defaults toroot
. - syslog_badpri
- Syslog priority to use when user authenticates unsuccessfully. Defaults to
alert
.The following syslog priorities are supported: alert, crit, debug, emerg, err, info, notice, and warning.
- syslog_goodpri
- Syslog priority to use when user authenticates successfully. Defaults to
notice
.See syslog_badpri for the list of supported syslog priorities.
- sudoers_locale
- Locale to use when parsing the sudoers file, logging commands, and sending
email. Note that changing the locale may affect how sudoers is
interpreted. Defaults to “
C
”. - timestampdir
- The directory in which
sudo
stores its time stamp files. The default is /var/adm/sudo. - timestampowner
- The owner of the time stamp directory and the time stamps stored therein.
The default is
root
. - type
- The default SELinux type to use when constructing a new security context
to run the command. The default type may be overridden on a per-command
basis in sudoers or via command line options. This
option is only available when
sudo
is built with SELinux support.
Strings that can be used in a boolean context:
- askpass
- The askpass option specifies the fully qualified path to
a helper program used to read the user's password when no terminal is
available. This may be the case when
sudo
is executed from a graphical (as opposed to text-based) application. The program specified by askpass should display the argument passed to it as the prompt and write the user's password to the standard output. The value of askpass may be overridden by theSUDO_ASKPASS
environment variable. - env_file
- The env_file option specifies the fully qualified path
to a file containing variables to be set in the environment of the program
being run. Entries in this file should either be of the form
“
VARIABLE=value
” or “export VARIABLE=value
”. The value may optionally be surrounded by single or double quotes. Variables in this file are subject to othersudo
environment settings such as env_keep and env_check. - exempt_group
- Users in this group are exempt from password and PATH requirements. The
group name specified should not include a
%
prefix. This is not set by default. - lecture
- This option controls when a short lecture will be printed along with the
password prompt. It has the following possible values:
- always
- Always lecture the user.
- never
- Never lecture the user.
- once
- Only lecture the user the first time they run
sudo
.
If no value is specified, a value of once is implied. Negating the option results in a value of never being used. The default value is once.
- lecture_file
- Path to a file containing an alternate
sudo
lecture that will be used in place of the standard lecture if the named file exists. By default,sudo
uses a built-in lecture. - listpw
- This option controls when a password will be required when a user runs
sudo
with the-l
option. It has the following possible values:- all
- All the user's sudoers entries for the current host
must have the
NOPASSWD
flag set to avoid entering a password. - always
- The user must always enter a password to use the
-l
option. - any
- At least one of the user's sudoers entries for the
current host must have the
NOPASSWD
flag set to avoid entering a password. - never
- The user need never enter a password to use the
-l
option.
If no value is specified, a value of any is implied. Negating the option results in a value of never being used. The default value is any.
- logfile
- Path to the
sudo
log file (not the syslog log file). Setting a path turns on logging to a file; negating this option turns it off. By default,sudo
logs via syslog. - mailerflags
- Flags to use when invoking mailer. Defaults to
-t
. - mailerpath
- Path to mail program used to send warning mail. Defaults to the path to sendmail found at configure time.
- mailfrom
- Address to use for the “from” address when sending warning
and error mail. The address should be enclosed in double quotes
("") to protect against
sudo
interpreting the@
sign. Defaults to the name of the user runningsudo
. - mailto
- Address to send warning and error mail to. The address should be enclosed
in double quotes ("") to protect against
sudo
interpreting the@
sign. Defaults toroot
. - secure_path
- Path used for every command run from
sudo
. If you don't trust the people runningsudo
to have a sanePATH
environment variable you may want to use this. Another use is if you want to have the “root path” be separate from the “user path”. Users in the group specified by the exempt_group option are not affected by secure_path. This option is not set by default. - syslog
- Syslog facility if syslog is being used for logging (negate to disable
syslog logging). Defaults to
auth
.The following syslog facilities are supported: authpriv (if your OS supports it), auth, daemon, user, local0, local1, local2, local3, local4, local5, local6, and local7.
- verifypw
- This option controls when a password will be required when a user runs
sudo
with the-v
option. It has the following possible values:- all
- All the user's sudoers entries for the current host
must have the
NOPASSWD
flag set to avoid entering a password. - always
- The user must always enter a password to use the
-v
option. - any
- At least one of the user's sudoers entries for the
current host must have the
NOPASSWD
flag set to avoid entering a password. - never
- The user need never enter a password to use the
-v
option.
If no value is specified, a value of all is implied. Negating the option results in a value of never being used. The default value is all.
Lists that can be used in a boolean context:
- env_check
- Environment variables to be removed from the user's environment if the
variable's value contains ‘
%
’ or ‘/
’ characters. This can be used to guard against printf-style format vulnerabilities in poorly-written programs. The argument may be a double-quoted, space-separated list or a single value without double-quotes. The list can be replaced, added to, deleted from, or disabled by using the=
,+=
,-=
, and!
operators respectively. Regardless of whether theenv_reset
option is enabled or disabled, variables specified byenv_check
will be preserved in the environment if they pass the aforementioned check. The default list of environment variables to check is displayed whensudo
is run by root with the-V
option. - env_delete
- Environment variables to be removed from the user's environment when the
env_reset option is not in effect. The argument may be a
double-quoted, space-separated list or a single value without
double-quotes. The list can be replaced, added to, deleted from, or
disabled by using the
=
,+=
,-=
, and!
operators respectively. The default list of environment variables to remove is displayed whensudo
is run by root with the-V
option. Note that many operating systems will remove potentially dangerous variables from the environment of any setuid process (such assudo
). - env_keep
- Environment variables to be preserved in the user's environment when the
env_reset option is in effect. This allows fine-grained
control over the environment
sudo
-spawned processes will receive. The argument may be a double-quoted, space-separated list or a single value without double-quotes. The list can be replaced, added to, deleted from, or disabled by using the=
,+=
,-=
, and!
operators respectively. The default list of variables to keep is displayed whensudo
is run by root with the-V
option.
FILES
- /etc/sudoers
- List of who can run what
- /etc/group
- Local groups file
- /etc/netgroup
- List of network groups
- /var/log/sudo-io
- I/O log files
EXAMPLES
Below are example sudoers entries. Admittedly, some of these are a bit contrived. First, we allow a few environment variables to pass and then define our aliases:
# Run X applications through sudo; HOME is used to find the # .Xauthority file. Note that other programs use HOME to find # configuration files and this may lead to privilege escalation! Defaults env_keep += "DISPLAY HOME"# User alias specification User_Alias FULLTIMERS = millert, mikef, dowdy User_Alias PARTTIMERS = bostley, jwfox, crawl User_Alias WEBMASTERS = will, wendy, wim
# Runas alias specification Runas_Alias OP = root, operator Runas_Alias DB = oracle, sybase Runas_Alias ADMINGRP = adm, oper
# Host alias specification Host_Alias SPARC = bigtime, eclipse, moet, anchor :
SGI = grolsch, dandelion, black :
ALPHA = widget, thalamus, foobar :
HPPA = boa, nag, python Host_Alias CUNETS = 128.138.0.0/255.255.0.0 Host_Alias CSNETS = 128.138.243.0, 128.138.204.0/24, 128.138.242.0 Host_Alias SERVERS = master, mail, www, ns Host_Alias CDROM = orion, perseus, hercules# Cmnd alias specification Cmnd_Alias DUMPS = /usr/bin/mt, /usr/sbin/dump, /usr/sbin/rdump,
/usr/sbin/restore, /usr/sbin/rrestore Cmnd_Alias KILL = /usr/bin/kill Cmnd_Alias PRINTING = /usr/sbin/lpc, /usr/bin/lprm Cmnd_Alias SHUTDOWN = /usr/sbin/shutdown Cmnd_Alias HALT = /usr/sbin/halt Cmnd_Alias REBOOT = /usr/sbin/reboot Cmnd_Alias SHELLS = /usr/bin/sh, /usr/bin/csh, /usr/bin/ksh,
/usr/local/bin/tcsh, /usr/bin/rsh,
/usr/local/bin/zsh Cmnd_Alias SU = /usr/bin/su Cmnd_Alias PAGERS = /usr/bin/more, /usr/bin/pg, /usr/bin/less
Here we override some of the compiled in default values. We want
sudo
to log via syslog(3) using the auth facility in all
cases. We don't want to subject the full time staff to the
sudo
lecture, user millert need
not give a password, and we don't want to reset the
LOGNAME
, USER
or
USERNAME
environment variables when running commands
as root. Additionally, on the machines in the SERVERS
Host_Alias
, we keep an additional local log file and
make sure we log the year in each log line since the log entries will be
kept around for several years. Lastly, we disable shell escapes for the
commands in the PAGERS Cmnd_Alias
(/usr/bin/more, /usr/bin/pg
and /usr/bin/less).
# Override built-in defaults Defaults syslog=auth Defaults>root !set_logname Defaults:FULLTIMERS !lecture Defaults:millert !authenticate Defaults@SERVERS log_year, logfile=/var/log/sudo.log Defaults!PAGERS noexec
The User specification is the part that actually determines who may run what.
root ALL = (ALL) ALL %wheel ALL = (ALL) ALL
We let root and any user in group wheel run any command on any host as any user.
FULLTIMERS ALL = NOPASSWD: ALL
Full time sysadmins (millert, mikef, and dowdy) may run any command on any host without authenticating themselves.
PARTTIMERS ALL = ALL
Part time sysadmins
bostley,
jwfox,
and
crawl)
may run any command on any host but they must authenticate themselves first
(since the entry lacks the NOPASSWD
tag).
jack CSNETS = ALL
The user
jack may run any
command on the machines in the CSNETS alias (the networks
128.138.243.0
,
128.138.204.0
, and
128.138.242.0
). Of those networks, only
128.138.204.0
has an explicit netmask (in CIDR
notation) indicating it is a class C network. For the other networks in
CSNETS, the local machine's netmask will be used during
matching.
lisa CUNETS = ALL
The user
lisa may run any
command on any host in the
CUNETS
alias (the class B network 128.138.0.0
).
operator ALL = DUMPS, KILL, SHUTDOWN, HALT, REBOOT, PRINTING,\ sudoedit /etc/printcap, /usr/oper/bin/
The operator user may run commands limited to simple maintenance. Here, those are commands related to backups, killing processes, the printing system, shutting down the system, and any commands in the directory /usr/oper/bin/.
joe ALL = /usr/bin/su operator
The user joe may only su(1) to operator.
pete HPPA = /usr/bin/passwd [A-Za-z]*, !/usr/bin/passwd root%opers ALL = (: ADMINGRP) /usr/sbin/
Users in the
opers group may run
commands in /usr/sbin/ as themselves with any group
in the
ADMINGRP
Runas_Alias
(the
adm and
oper
groups).
The user pete is allowed to change anyone's password except for root on the HPPA machines. Note that this assumes passwd(1) does not take multiple user names on the command line.
bob SPARC = (OP) ALL : SGI = (OP) ALL
The user
bob may run anything
on the
SPARC
and SGI
machines as any user listed in the
OP
Runas_Alias
(root and
operator.)
jim +biglab = ALL
The user
jim may run any
command on machines in the
biglab
netgroup. sudo
knows that “biglab” is
a netgroup due to the ‘+
’ prefix.
+secretaries ALL = PRINTING, /usr/bin/adduser, /usr/bin/rmuser
Users in the secretaries netgroup need to help manage the printers as well as add and remove users, so they are allowed to run those commands on all machines.
fred ALL = (DB) NOPASSWD: ALL
The user
fred can run
commands as any user in the
DB
Runas_Alias
(oracle
or
sybase)
without giving a password.
john ALPHA = /usr/bin/su [!-]*, !/usr/bin/su *root*
On the ALPHA machines, user john may su to anyone except root but he is not allowed to specify any options to the su(1) command.
jen ALL, !SERVERS = ALL
The user
jen may run any
command on any machine except for those in the SERVERS
Host_Alias
(master, mail, www and ns).
jill SERVERS = /usr/bin/, !SU, !SHELLS
For any machine in the SERVERS
Host_Alias
,
jill may run any
commands in the directory /usr/bin/ except for those
commands belonging to the SU and SHELLS
Cmnd_Aliases
.
steve CSNETS = (operator) /usr/local/op_commands/
The user steve may run any command in the directory /usr/local/op_commands/ but only as user operator.
matt valkyrie = KILL
On his personal workstation, valkyrie, matt needs to be able to kill hung processes.
WEBMASTERS www = (www) ALL, (root) /usr/bin/su www
On the host www, any user in the
WEBMASTERS
User_Alias
(will, wendy, and wim), may run any
command as user www (which owns the web pages) or simply
su(1) to www.
ALL CDROM = NOPASSWD: /sbin/umount /CDROM,\ /sbin/mount -o nosuidnodev /dev/cd0a /CDROM
Any user may mount or unmount a CD-ROM on the machines in the
CDROM Host_Alias
(orion, perseus, hercules) without
entering a password. This is a bit tedious for users to type, so it is a
prime candidate for encapsulating in a shell script.
SECURITY NOTES
Limitations of the ‘!’ operator
It is generally not effective to “subtract” commands
from ALL using the
‘!
’ operator. A user can trivially
circumvent this by copying the desired command to a different name and then
executing that. For example:
bill ALL = ALL, !SU, !SHELLS
Doesn't really prevent bill from running the commands listed in SU or SHELLS since he can simply copy those commands to a different name, or use a shell escape from an editor or other program. Therefore, these kind of restrictions should be considered advisory at best (and reinforced by policy).
In general, if a user has sudo ALL there is
nothing to prevent them from creating their own program that gives them a
root shell (or making their own copy of a shell) regardless of any
‘!
’ elements in the user
specification.
Security implications of fast_glob
If the fast_glob option is in use, it is not possible to reliably negate commands where the path name includes globbing (aka wildcard) characters. This is because the C library's fnmatch(3) function cannot resolve relative paths. While this is typically only an inconvenience for rules that grant privileges, it can result in a security issue for rules that subtract or revoke privileges.
For example, given the following sudoers entry:
john ALL = /usr/bin/passwd [a-zA-Z0-9]*, /usr/bin/chsh [a-zA-Z0-9]*,\ /usr/bin/chfn [a-zA-Z0-9]*, !/usr/bin/* root
User john can still run
/usr/bin/passwd root
if fast_glob
is enabled by changing to /usr/bin and running
./passwd root
instead.
Preventing shell escapes
Once sudo
executes a program, that program
is free to do whatever it pleases, including run other programs. This can be
a security issue since it is not uncommon for a program to allow shell
escapes, which lets a user bypass
sudo
's access control and
logging. Common programs that permit shell escapes include shells
(obviously), editors, paginators, mail and terminal programs.
There are two basic approaches to this problem:
- restrict
- Avoid giving users access to commands that allow the user to run arbitrary
commands. Many editors have a restricted mode where shell escapes are
disabled, though
sudoedit
is a better solution to running editors viasudo
. Due to the large number of programs that offer shell escapes, restricting users to the set of programs that do not is often unworkable. - noexec
- Many systems that support shared libraries have the ability to override
default library functions by pointing an environment variable (usually
LD_PRELOAD
) to an alternate shared library. On such systems,sudo
's noexec functionality can be used to prevent a program run bysudo
from executing any other programs. Note, however, that this applies only to native dynamically-linked executables. Statically-linked executables and foreign executables running under binary emulation are not affected.The noexec feature is known to work on SunOS, Solaris, *BSD, Linux, IRIX, Tru64 UNIX, MacOS X, HP-UX 11.x and AIX 5.3 and above. It should be supported on most operating systems that support the
LD_PRELOAD
environment variable. Check your operating system's manual pages for the dynamic linker (usually ld.so, ld.so.1, dyld, dld.sl, rld, or loader) to see ifLD_PRELOAD
is supported.On Solaris 10 and higher, noexec uses Solaris privileges instead of the
LD_PRELOAD
environment variable.To enable noexec for a command, use the
NOEXEC
tag as documented in the User Specification section above. Here is that example again:aaron shanty = NOEXEC: /usr/bin/more, /usr/bin/vi
This allows user aaron to run /usr/bin/more and /usr/bin/vi with noexec enabled. This will prevent those two commands from executing other commands (such as a shell). If you are unsure whether or not your system is capable of supporting noexec you can always just try it out and check whether shell escapes work when noexec is enabled.
Note that restricting shell escapes is not a panacea. Programs
running as root are still capable of many potentially hazardous operations
(such as changing or overwriting files) that could lead to unintended
privilege escalation. In the specific case of an editor, a safer approach is
to give the user permission to run sudoedit
.
SEE ALSO
ssh(1), su(1), fnmatch(3), glob(3), mktemp(3), strftime(3), sudoers.ldap(5), sudo(8), visudo(8)
CAVEATS
The sudoers file should
always
be edited by the visudo
command which locks the file
and does grammatical checking. It is imperative that
sudoers be free of syntax errors since
sudo
will not run with a syntactically incorrect
sudoers file.
When using netgroups of machines (as opposed to users), if you
store fully qualified host name in the netgroup (as is usually the case),
you either need to have the machine's host name be fully qualified as
returned by the hostname
command or use the
fqdn option in sudoers.
BUGS
If you feel you have found a bug in sudo
,
please submit a bug report at http://www.sudo.ws/sudo/bugs/
SUPPORT
Limited free support is available via the sudo-users mailing list, see http://www.sudo.ws/mailman/listinfo/sudo-users to subscribe or search the archives.
DISCLAIMER
sudo
is provided “AS IS” and
any express or implied warranties, including, but not limited to, the
implied warranties of merchantability and fitness for a particular purpose
are disclaimed. See the LICENSE file distributed with
sudo
or http://www.sudo.ws/sudo/license.html for
complete details.