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They are often used to allow users on a computer system to run programs with temporarily elevated privileges in order 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 setuid and setgid flags have an entirely different meaning depending whether they are set on a file or a directory. 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 own account does not have the required privilege for sending packets.
The setuid and setgid bits are normally set with the command chmod by setting the high-order octal digit to 4 for setuid or 2 for setgid. When a user other than the owner executes the file, the process will run with user and group permissions set upon it by its owner. For example, if the file is owned by user root and group wheel , it will run as root: Most implementations of the chmod command also support finer-grained, symbolic arguments to set these bits.
The numeric way of settings these stick permissions as used above " chmod file " doesn't allow one to withdrawn these same permissions as one would expect as " chmod file ". 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 programs that are not carefully designed.
Due to potential security issues,  many operating systems ignore the setuid attribute when applied to executable shell scripts. The presence of setuid executables explains why the chroot system call is not available to non- root users on Unix. See limitations of chroot for more details.
As is stated in open 2 , "When a new file is created it is given the group of the directory which contains it. 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 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. A user named 'torvalds' creates a file named 'tekken' under the directory named 'videogames'. A user named 'wozniak' 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 user named 'torvalds' who belongs 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 GUID 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 umask allows new files to be edited by group.
Sticky bit and GUID could be combined with something such as a read-only umask or an append only attribute. Developers should 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.
The setuid bit was invented by Dennis Ritchie  and included in su. The patent was later placed in the public domain. From Wikipedia, the free encyclopedia. Changing password for thompson. Retrieved 30 March A Research Unix reader: Retrieved from " https: Computer security procedures Unix file system technology Patents placed into the public domain.