June 20, 2005
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
Sudo versions affected:
Sudo versions 1.3.1 up to and including 1.6.8p8.
This vulnerability has been assigned CVE-2005-1993
in the Common
Vulnerabilities and Exposures
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
file and not directly from the user, Sudo should
be safe from race conditions involving symbolic links. However,
if a sudoers
entry containing the pseudo-command
follows the user's sudoers
contents of 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
another user access to the ALL
follow the user's sudoers
entry for the race to exist.
For example, the following sudoers file is not affected
by the bug:
Whereas this one would be:
The bug is fixed in sudo 1.6.8p9.
The administrator can order the sudoers file such that all entries
granting Sudo 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 realpath() function
to find the true path. Sudo does not user realpath()
because that function is not present in all operating systems and
is often vulnerable to race conditions where it does exist.