Screen is the traditional terminal multiplexer software used on Linux and Unix systems. We found a local root exploit in Screen 5.0.0 affecting Arch Linux and NetBSD, as well as a couple of other issues that partly also affect older Screen versions, which are still found in the majority of distributions.
In July 2024, the upstream Screen maintainer asked us if we could have a look at the current Screen code base. We treated this request with lower priority, since we already had a cursory look at Screen a few years earlier, without finding any problems. When we actually found time to look into it again, we were surprised to find a local root exploit in the Screen 5.0.0 major version update affecting distributions that ship it as setuid-root (Arch Linux and NetBSD). We also found a number of additional, less severe issues that partly also affect older Screen versions still found in the majority of distributions.

We offer two sets of patches for the issues described in this report, one for screen-4.9.1 and another for screen-5.0.0. These patch sets apply against the screen-4.9.1 and screen-5.0.0 release tarballs, respectively. Due to difficulties in the communication with upstream we do not currently have detailed information about bugfixes and releases published on their end.

The next section provides an overview of the Screen configurations and versions found on common Linux and UNIX distributions. Section 3) discusses each security issue we discovered in detail. Section 4) takes a look at possible further issues in Screen’s setuid-root implementation. Section 5) gives general recommendations for the improvement of Screen’s security posture. Section 6) points out problems we encountered during the coordinated disclosure process for these issues. Section 7) provides an affectedness matrix which gives a quick overview of the situation on various Linux and UNIX systems.