An adversary may abuse configurations where an application has the setuid or setgid bits set in order to get code running in a different (and possibly more privileged) user’s context. On Linux or macOS, when the setuid or setgid bits are set for an application binary, the application will run with the privileges of the owning user or group respectively. Normally an application is run in the current user’s context, regardless of which user or group owns the application. However, there are instances where programs need to be executed in an elevated context to function properly, but the user running them may not have the specific required privileges.
Instead of creating an entry in the sudoers file, which must be done by root, any user can specify the setuid or setgid flag to be set for their own applications (i.e. Linux and Mac File and Directory Permissions Modification). The
chmod command can set these bits with bitmasking,
chmod 4777 [file] or via shorthand naming,
chmod u+s [file]. This will enable the setuid bit. To enable the setgid bit,
chmod 2775 and
chmod g+s can be used.
Adversaries can use this mechanism on their own malware to make sure they're able to execute in elevated contexts in the future. This abuse is often part of a "shell escape" or other actions to bypass an execution environment with restricted permissions.
Alternatively, adversaries may choose to find and target vulnerable binaries with the setuid or setgid bits already enabled (i.e. File and Directory Discovery). The setuid and setguid bits are indicated with an "s" instead of an "x" when viewing a file's attributes via
ls -l. The
find command can also be used to search for such files. For example,
find / -perm +4000 2>/dev/null can be used to find files with setuid set and
find / -perm +2000 2>/dev/null may be used for setgid. Binaries that have these bits set may then be abused by adversaries.
|S0401||Exaramel for Linux|
|M1028||Operating System Configuration||
Applications with known vulnerabilities or known shell escapes should not have the setuid or setgid bits set to reduce potential damage if an application is compromised. Additionally, the number of programs with setuid or setgid bits set should be minimized across a system.
|ID||Data Source||Data Component||Detects|
Monitor for execution of utilities, like chmod, and their command-line arguments to look for setuid or setguid bits being set.
Monitor the file system for files that have the setuid or setgid bits set.
Monitor for changes made to files that may perform shell escapes or exploit vulnerabilities in an application with the setsuid or setgid bits to get code running in a different user’s context.