sudo-rs Session File Relative Path Traversal vulnerability
Background
Sudo-rs allows users to not have to enter authentication at every sudo attempt, but instead only requiring authentication every once in a while in every terminal or process group. Only once a configurable timeout has passed will the user have to re-authenticate themselves. Supporting this functionality is a set of session files (timestamps) for each user, stored in /var/run/sudo-rs/ts. These files are named according to the username from which the sudo attempt is made (the origin user).
Impact
An issue was discovered where usernames containing the . and / characters could result in the corruption of specific files on the filesystem. On some operating systems usernames are not limited as to the characters they can contain, so a username appearing to be a relative path can be constructed. For example we could add a user to the system containing the username ../../../../bin/cp. When logged in as a user with that name, that user could run sudo -K to clear their session record file. The session code then constructs the path to the session file by concatenating the username to the session file storage directory, resulting in a resolved path of /bin/cp. The code then clears that file, resulting in the cp binary effectively being removed from the system.
An attacker needs to be able to login as a user with a constructed username. Given that such a username is unlikely to exist on an existing system, they will also need to be able to create the users with the constructed usernames.
Affected systems
Debian bookworm (and earlier) and Ubuntu 24.04 (and earlier) allow creating of these usernames using useradd without warning. On most other systems and later versions an attempt to create such users will result in an error.
Patches
The bug is fixed in version 0.2.1 of sudo-rs. Sudo-rs now uses the uid for the user instead of their username for determining the filename. Note that an upgrade to this version will result in existing session files being ignored and users will be forced to re-authenticate. It also fully eliminates any possibility of path traversal, given that uids are always integer values.
Workarounds
The sudo -K and sudo -k commands can run, even if a user has no sudo access. Make sure that your system does not contain any users with a specially crafted username. While this is the case and while untrusted users do not have the ability to create arbitrary users on your system you should not be able to exploit this issue.
References
This issue was identified by Andrea Jegher from Radically Open Security during an audit of the sudo-rs code. The same problem existed in original sudo and was fixed in version 1.9.15.
Notes
This advisory was updated in 2026 to better reflect the fact that "path-like" user names are normally forbidden from creation, and the CVSS score downgraded accordingly.
