The
Unix
Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
and
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
access rights flags setuid and setgid (short for ''set user identity'' and ''set group identity'') allow users to run an
executable
In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), in ...
with the
file system permissions
Typically, a file system maintains permission settings for each stored item commonly files and directories that either grant or deny the ability to manipulate file system items. Often the settings allow controlling access based on function s ...
of the executable's owner or group respectively and to change behaviour in directories. They are often used to allow users on a computer system to run programs with temporarily elevated privileges to perform a specific task. While the assumed user id or group id privileges provided are not always elevated, at a minimum they are specific.
The flags
setuid
and
setgid
are needed for tasks that require different privileges than what the user is normally granted, such as the ability to alter system files or databases to change their login password.
Some of the tasks that require additional privileges may not immediately be obvious, though, such as the
ping
command, which must send and listen for
control packets on a network interface.
File modes
The
setuid
and
setgid
bits are normally represented as the values 4 for
setuid
and 2 for
setgid
in the high-order octal digit of the file mode. For example,
6711
has both the
setuid
and
setgid
bits () set, and also the file read/write/executable for the owner (7), and executable by the group (first 1) and others (second 1). Most implementations have a symbolic representation of these bits; in the previous example, this could be
u=rwx,go=x,ug+s
.
Typically,
chmod
does not have a recursive mode restricted to directories, so modifying an existing directory tree must be done manually, with a command such as .
Effects
The
setuid
and
setgid
flags have different effects, depending on whether they are applied to a file, to a directory or binary executable or non-binary executable file. The
setuid
and
setgid
flags have an effect only on binary executable files and not on scripts (e.g., Bash, Perl, Python).
When set on an executable file
When the
setuid
or
setgid
attributes are set on an
executable
In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), in ...
file, then any users able to execute the file will automatically execute the file with the privileges of the file's owner (commonly
root
In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...
) and/or the file's group, depending upon the flags set.
This allows the system designer to permit trusted programs to be run which a user would otherwise not be allowed to execute. These may not always be obvious. For example, the
ping command may need access to networking privileges that a normal user cannot access; therefore it may be given the setuid flag to ensure that a user who needs to ping another system can do so, even if their account does not have the required privilege for sending packets.
Security impact
For security purposes, the invoking user is usually prohibited by the system from altering the new process in any way, such as by using
ptrace
,
LD_LIBRARY_PATH
or sending signals to it, to exploit the raised privilege, although signals from the terminal will still be accepted.
While the
setuid
feature is very useful in many cases, its improper use can pose a security risk
if the
setuid
attribute is assigned to
executable
In computer science, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), in ...
programs that are not carefully designed. Due to potential security issues, many operating systems ignore the
setuid
attribute when applied to executable ''
shell script
A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be command languages. Typical operations performed by shell scripts include file manipu ...
s''.
The presence of
setuid
executables explains why the
chroot
chroot is a shell (computer), shell command (computing), command and a system call on Unix and Unix-like operating systems that changes the apparent root directory for the current running process and its Child process, children. A program that i ...
system call is not available to non-
root
In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often bel ...
users on Unix. See
limitations of chroot
for more details.
When set on a directory
Setting the
setgid
permission on a directory causes files and subdirectories created within to inherit its group ownership, rather than the primary group of the file-creating process. Created subdirectories also inherit the
setgid
bit. The policy is only applied during creation and, thus, only prospectively. Directories and files existing when the
setgid
bit is applied are unaffected, as are directories and files moved into the directory on which the bit is set.
Thus is granted a capacity to work with files amongst a group of users without explicitly setting permissions, but limited by the security model expectation that existing files permissions do not implicitly change.
The
setuid
permission set on a directory is ignored on most
UNIX
Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
and
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
systems. However
FreeBSD
FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...
can be configured to interpret
setuid
in a manner similar to
setgid
, in which case it forces all files and sub-directories created in a directory to be owned by that directory's owner - a simple form of inheritance. This is generally not needed on most systems derived from
BSD
The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley, beginni ...
, since by default directories are treated as if their
setgid
bit is always set, regardless of the actual value. As is stated in
open(2)
, "When a new file is created it is given the group of the directory which contains it."
Examples
Checking permissions
Permissions of a file can be checked in octal form and/or alphabetic form with the command line tool
stat
torvalds ~ $ stat -c "%a %A" ~/test/
1770 drwxrwx--T
SUID
4701 on an executable file owned by 'root' and the group 'root'
A user named 'thompson' attempts to execute the file. The executable permission for all users is set (the '1') so 'thompson' can execute the file. The file owner is 'root' and the SUID permission is set (the '4') - so the file is executed as 'root'.
The reason an executable would be run as 'root' is so that it can modify specific files that the user would not normally be allowed to, without giving the user full root access.
A default use of this can be seen with the
/usr/bin/passwd
binary file.
/usr/bin/passwd
needs to modify
/etc/passwd
and
/etc/shadow
which store account information and password hashes for all users, and these can only be modified by the user 'root'.
thompson ~ $ stat -c "%a %U:%G %n" /usr/bin/passwd
4701 root:root /usr/bin/passwd
thompson ~ $ passwd
passwd: Changing password for thompson
The owner of the process is not the user running the executable file but the owner of the executable file
SGID
2770 on a directory named 'music' owned by the user 'root' and the group 'engineers'
A user named 'torvalds' who belongs primarily to the group 'torvalds' but secondarily to the group 'engineers' makes a directory named 'electronic' under the directory named 'music'. The group ownership of the new directory named 'electronic' inherits 'engineers.' This is the same when making a new ''file'' named 'imagine.txt'
Without SGID the group ownership of the new directory/file would have been 'torvalds' as that is the primary group of user 'torvalds'.
torvalds ~ $ groups torvalds
torvalds : torvalds engineers
torvalds ~ $ stat -c "%a %U:%G %n" ./music/
2770 root:engineers ./music/
torvalds ~ $ mkdir ./music/electronic
torvalds ~ $ stat -c "%U:%G %n" ./music/electronic/
torvalds:engineers ./music/electronic/
torvalds ~ $ echo 'NEW FILE' > ./music/imagine.txt
torvalds ~ $ stat -c "%U:%G %n" ./music/imagine.txt
torvalds:engineers ./music/imagine.txt
torvalds ~ $ touch ~/test
torvalds ~ $ stat -c "%U:%G %n" ~/test
torvalds:torvalds ~/test
Sticky bit
1770 on a directory named 'videogames' owned by the user 'torvalds' and the group 'engineers'.
A user named 'torvalds' creates a file named 'tekken' under the directory named 'videogames'. A user named 'wozniak', who is also part of the group 'engineers', attempts to delete the file named 'tekken' but he cannot, since he is not the owner.
Without sticky bit, 'wozniak' could have deleted the file, because the directory named 'videogames' allows read and write by 'engineers'. A default use of this can be seen at the
/tmp
folder.
torvalds /home/shared/ $ groups torvalds
torvalds : torvalds engineers
torvalds /home/shared/ $ stat -c "%a %U:%G %n" ./videogames/
1770 torvalds:engineers ./videogames/
torvalds /home/shared/ $ echo 'NEW FILE' > videogames/tekken
torvalds /home/shared/ $ su - wozniak
Password:
wozniak ~/ $ groups wozniak
wozniak : wozniak engineers
wozniak ~/ $ cd /home/shared/videogames
wozniak /home/shared/videogames/ $ rm tekken
rm: cannot remove ‘tekken’: Operation not permitted
Sticky bit with SGID
3171 on a directory named 'blog' owned by the group 'engineers' and the user 'root'
A user named 'torvalds' who belongs primarily to the group 'torvalds' but secondarily to the group 'engineers' creates a file or directory named 'thoughts' inside the directory 'blog'. A user named 'wozniak' who also belongs to the group 'engineers' cannot delete, rename, or move the file or directory named 'thoughts', because he is not the owner and the sticky bit is set. However, if 'thoughts' is a file, then 'wozniak' can edit it.
Sticky bit has the final decision. If sticky bit and SGID had not been set, the user 'wozniak' could rename, move, or delete the file named 'thoughts' because the directory named 'blog' allows read and write by group, and wozniak belongs to the group, and the default 0002
umask allows new files to be edited by group. Sticky bit and SGID could be combined with something such as a read-only umask or an append only attribute.
torvalds /home/shared/ $ groups torvalds
torvalds : torvalds engineers
torvalds /home/shared/ $ stat -c "%a %U:%G %n" ./blog/
3171 root:engineers ./blog/
torvalds /home/shared/ $ echo 'NEW FILE' > ./blog/thoughts
torvalds /home/shared/ $ su - wozniak
Password:
wozniak ~/ $ cd /home/shared/blog
wozniak /home/shared/blog/ $ groups wozniak
wozniak : wozniak engineers
wozniak /home/shared/blog/ $ stat -c "%a %U:%G %n" ./thoughts
664 torvalds:engineers ./thoughts
wozniak /home/shared/blog/ $ rm thoughts
rm: cannot remove ‘thoughts’: Operation not permitted
wozniak /home/shared/blog/ $ mv thoughts /home/wozniak/
mv: cannot move ‘thoughts’ to ‘/home/wozniak/thoughts’: Operation not permitted
wozniak /home/shared/blog/ $ mv thoughts pondering
mv: cannot move ‘thoughts’ to ‘pondering’: Operation not permitted
wozniak /home/shared/blog/ $ echo 'REWRITE!' > thoughts
wozniak /home/shared/blog/ $ cat thoughts
REWRITE!
Security
Developers design and implement programs that use this bit on executables carefully in order to avoid security vulnerabilities including
buffer overruns and
path injection. Successful buffer-overrun attacks on vulnerable applications allow the attacker to execute arbitrary code under the rights of the process exploited. In the event that a vulnerable process uses the
setuid
bit to run as
root
, the code will execute with root privileges, in effect giving the attacker root access to the system on which the vulnerable process is running.
Of particular importance in the case of a
setuid
process is the
environment of the process. If the environment is not properly sanitized by a privileged process, its behavior can be changed by the unprivileged process that started it. For example,
GNU libc was at one point vulnerable to an
exploit using
setuid
and an environment variable that allowed executing code from untrusted
shared libraries.
History
The
setuid
bit was invented by
Dennis Ritchie
Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American computer scientist. He created the C programming language and the Unix operating system and B language with long-time colleague Ken Thompson. Ritchie and Thomp ...
and included in
su
. His employer, then
Bell Telephone Laboratories
Nokia Bell Labs, commonly referred to as ''Bell Labs'', is an American industrial research and development company owned by Finnish technology company Nokia. With headquarters located in Murray Hill, New Jersey, Murray Hill, New Jersey, the compa ...
, applied for a patent in 1972; the patent was granted in 1979 as patent number . The patent was later placed in the
public domain
The public domain (PD) consists of all the creative work to which no Exclusive exclusive intellectual property rights apply. Those rights may have expired, been forfeited, expressly Waiver, waived, or may be inapplicable. Because no one holds ...
.
See also
References
{{Reflist, 30em
External links
* Chen, Hao;
Wagner, David; and Dean, Drew
Setuid Demystified(pdf)
* Tsafrir, Dan;
Da Silva, Dilma; and Wagner, David
The Murky Issue of Changing Process Identity: Revising Setuid Demystified(pdf)
* Pollock, Wayne
Computer security procedures
Unix file system technology
Patents placed into the public domain