SuidPerl Mail Shell Escape Vulnerability
The interaction between some security checks performed by suidperl, the setuid version of perl, and the /bin/mail program creates a scenario that allows local malicious users to execute commands with root privileges.
The suidperl program performs a number of checks to make sure it can't be fooled into executing a perl script with root privileges when its not suid root. When one of these checks fails the program will compose a message to the root user. The mail message looks like this:
From: Bastard Operator <firstname.lastname@example.org>
User 500 tried to run dev 769 ino 343180 in place of dev 769 ino 343183!
(Filename of set-id script was /some/thing, uid 500 gid 500.)
The name of the script to execute (inserted into the message) is taken from the program's argument list (argv). suidperl executes /bin/mail to inject the message into the mail system. It does so without cleaning the environment or dropping its root privileges. The /bin/mail program has an undocumented feature. By setting the environment variable "interactive" to any value, /bin/mail will interpret the sequence "~!" as an escape sequence to start a shell and execute commands even when the program is not attached to a terminal. The environment variable "interactive" can be also set from ~/.mailrc with a "set interactive" line.
A malicous user can create a file with an escape sequence and commands embedded in the file name, then execute suidperl in such a way that the security check fails. suidperl will send a message to root via /bin/mail with the escape sequence embedded in the message. This will cause /bin/mail to start a root shell and execute the commands.