Authentication bypass when clock is reset

When a user successfully authenticates with sudo, a time stamp file is updated to allow that user to continue running sudo without requiring a password for a preset time period (five minutes by default). The user’s time stamp file can be reset using sudo -k or removed altogether via sudo -K.

A user who has sudo access and is able to control the local clock (common in desktop environments) can run a command via sudo without authenticating as long as they have previously authenticated themselves at least once by running sudo -k and then setting the clock to the epoch (1970-01-01 01:00:00).

The vulnerability does not permit a user to run commands other than those allowed by the sudoers policy.

Sudo 1.6.0 through 1.7.10p6 and sudo 1.8.0 through 1.8.6p6 inclusive.

This vulnerability has been assigned CVE-2013-1775 in the Common Vulnerabilities and Exposures database.

By default, sudo displays a lecture when the user’s time stamp file is not present. In sudo 1.6, the -k option was changed to reset the time stamp file to the epoch rather than remove it to prevent the lecture from being displayed the next time sudo was run. No special case was added for handling a time stamp file set to the epoch since the clock should never legitimately be set to that value.

However, there are two common ways for the clock to be reset to the epoch. The first way is when the clock is reset due to a fully drained battery on some systems. The other way is by a user logged in to a desktop environment that allows changes to the date and time.

As long as the user has successfully run sudo before, they are able to run “sudo -k” to reset the time stamp file. This action does not require a password and is not logged. If the user is also able to reset the date and time to the epoch (1970-01-01 01:00:00), they will be able to run sudo without having to authenticate.

The flaw may allow someone with physical access to a machine that is not password-protected to run sudo commands without knowing the logged in user’s password. On systems where sudo is the principal way of running commands as root, such as on Ubuntu and Mac OS X, there is a greater chance that the logged in user has run sudo before and thus that an attack would succeed.

The bug is fixed in sudo 1.8.6p7 and 1.7.10p7. These versions will ignore a time stamp file that is set to the epoch.

Using sudo -K instead of sudo -k will completely remove the time stamp file instead of just resetting it.

I’d like to thank Marco Schoepl for finding and reporting this long-standing bug.