Multiple security issues in GNU Screen

Nice write-up.

> Screen offers a multi-user mode which allows to attach to Screen sessions owned by other users in the system (given the proper credentials). These multi-user features are only available when Screen is installed with the setuid-root bit set. This configuration of Screen results in highly increased attack surface, because of the complex Screen code that runs with root privileges in this case

I wasn't aware of such a feature but I guess it's what makes stuff like tmate possible. Speaking of which, I wonder if tmux is affected by the same kind of vulnerability.

No, tmux uses unix domain sockets. I have no idea why screen chose to take the setuid approach instead here; it seems totally unnecessary to have root privileges.

EDIT: Further down, TFA gives a plausible explanation: the current screen devs are not fully familiar with the code base. If so, the setuid-root approach was probably the easiest way to make the feature work in lieu of such familiarity.

screen has a lot of architectural baggage that can be traced back to its initial 1987 comp.sources.unix/mod.sources versions in some cases. Being set-UID to the superuser is one of them. See the doco for screen as it was posted in volume 10:

https://sources.vsta.org/comp.sources.unix/volume10/screen/

It's a combination of factors:

* The original author of the project has not been involved in it since 1990. The people who took it over and made it a GNU project then largely stopped working on it in 2004. The people who are now working on it are something like its 3rd or 4th wave of developers.

* Learning the internals of screen now from scratch is a lot harder than when I did it in 1987. There's an awful lot more operating system historical and portability factors, now. In 1987, it was works-on-4.3BSD-might-not-on-your-Unix.

* If you deal with pseudo-terminals cross-platform, there are still huge variations on how pseudo-terminals work and how the long-standing security issues of pseudo-terminals, identified in the 1990s, have been addressed in operating systems.

* screen is encumbered by a lot of 1980s Think. It still today diligently manages, and puts quite a lot of effort into constructing, a generated-on-the-fly TERMCAP environment variable, for example.

* Attitudes to security have changed. At least one security hole in the headlined report was actually a neat-o feature of terminals in Unix in the 1970s and 1980s.