A flaw in exists in sudo’s environment sanitizing prior to sudo version 1.6.8p10 that could allow a malicious user with permission to run a shell script that utilized the bash shell to run arbitrary commands. The /bin/sh shell on most (if not all) Linux and Mac OS X systems is bash.
All versions prior to 1.6.8p10.
The bash shell uses the value of the PS4 environment variable (after expansion) as a prefix for commands run in execution trace mode. Execution trace mode (xtrace) is normally set via bash’s -x command line option or interactively by running “set -o xtrace”. However, it may also be enabled by placing the string “xtrace” in the SHELLOPTS environment variable before bash is started.
A malicious user with sudo access to a shell script that uses bash can use this feature to run arbitrary commands for each line of the script.
Exploitation of the bug requires that the bash shell be installed on the machine and that users be granted sudo access to run scripts written in bash. On most (if not all) Linux and Mac OS X systems, /bin/sh is bash so /bin/sh scripts are affected by this as well.
The bug is fixed in sudo 1.6.8p10.
The administrator can add a line at the top of the sudoers file:
Defaults env_delete+="PS4 SHELLOPTS"
which will cause sudo to strip the
variables without requiring a recompile.
Alternately, the administrator can add a line to the top of sudoers file:
which will reset the environment to only contain the variables
preventing this attack.
This problem was found by Tavis Ormandy.