A race condition in Sudo’s command pathname handling prior to Sudo version 1.6.8p9 that could allow a user with Sudo privileges to run arbitrary commands.
Sudo versions 1.3.1 up to and including 1.6.8p8.
When a user runs a command via Sudo, the inode and device numbers
of the command are compared to those of commands with the same
basename found in the
sudoers file (see the Background
section for more information). When a match is found, the path to
the matching command listed in the
sudoers file is stored
in the variable
safe_cmnd, which is later used to execute
the command. Because the actual path executed comes from the
sudoers file and not directly from the user, Sudo should
be safe from race conditions involving symbolic links. However,
sudoers entry containing the pseudo-command
ALL follows the user’s
sudoers entry the
safe_cmnd will be overwritten with the path
the user specified on the command line, making Sudo vulnerable to
the aforementioned race condition.
Exploitation of the bug requires that the user be allowed to run
one or more commands via Sudo and be able to create symbolic links
in the filesystem. Furthermore, a
sudoers entry giving
another user access to the
ALL pseudo-command must
follow the user’s
sudoers entry for the race to exist.
For example, the following
sudoers file is not affected
by the bug:
root server=ALL someuser server=/bin/echo
Whereas this one would be:
someuser server=/bin/echo root server=ALL
The bug is fixed in sudo 1.6.8p9.
The administrator can order the sudoers file such that all entries
ALL privileges precede all other entries.
This problem was brought to my attention by Charles Morris.
The reason Sudo uses the inode for command matching is to make relative paths work and to avoid problems caused by automounters where the path to be executed is not the same as the absolute path to the command.
Another possible approach is to use the
to find the true path. Sudo does not user
because that function is not present in all operating systems and
is often vulnerable to race conditions where it does exist.