When sudo is configured to allow a user to run commands as an arbitrary
user via the
ALL keyword in a Runas specification, it is possible
to run commands as root by specifying the user ID -1 or 4294967295.
This can be used by a user with sufficient sudo privileges to run
commands as root even if the Runas specification explicitly disallows
root access as long as the
ALL keyword is listed first in
the Runas specification.
Log entries for commands run this way will list the target user as 4294967295 instead of root. In addition, PAM session modules will not be run for the command.
Sudo versions prior to 1.8.28 are affected.
Exploiting the bug requires that the user have sudo privileges that
allow them to run commands with an arbitrary user ID. Typically,
this means that the user’s sudoers entry has the special value
ALL in the Runas specifier.
Sudo supports running a command with a user-specified user name or
user ID, if permitted by the sudoers policy. For example, the
following sudoers entry allow the id command to be run as any user
because it includes the
ALL keyword in the Runas specifier.
alice myhost = (ALL) /usr/bin/id
Not only is user alice is able to run the id command as any valid user, she is also able to run it as an arbitrary user ID by using the #uid syntax, for example:
sudo -u#1234 id -u
setreuid(2) system calls,
which sudo uses to change the user ID before running the command,
treat user ID -1 (or its unsigned equivalent 4294967295), specially
and do not change the user ID for this value. As a result,
sudo -u#-1 id -u
sudo -u#4294967295 id -u
will actually return
This is because the sudo command itself is already running as user ID 0
so when sudo tries to change to user ID -1, no change occurs.
This results in sudo log entries that report the command as being
run by user ID 4294967295 and not root (or user ID 0).
Additionally, because the user ID specified via the
does not exist in the password database, no PAM session modules will
If a sudoers entry is written to allow the user to run a command as any user except root, the bug can be used to avoid this restriction. For example, given the following sudoers entry:
bob myhost = (ALL, !root) /usr/bin/vi
User bob is allowed to run vi as any user but root. However, due
to the bug, bob is actually able to run vi as root by running
sudo -u#-1 vi, violating the security policy.
Only sudoers entries where the
ALL keyword is present in
the Runas specifier are affected. For example, the following sudoers
entry is unaffected:
alice myhost = /usr/bin/id
In this example, alice is only allowed to run the id command as root. Any attempt to run the command as a different user will be denied.
The bug is fixed in sudo 1.8.28.
Joe Vennix from Apple Information Security found and analyzed the bug.