I was contacted privately about a potential vulnerability in polkitd and pkexec.

Briefly, the problem is that the UID for the parent process of pkexec(1) is read from /proc by stat(2)'ing /proc/PID.

The problem with this is that this returns the effective uid of the process which can easily be set to 0 by invoking a setuid-root binary such as /usr/bin/chsh in the parent process of pkexec(1). Instead we are really interested in the real-user-id.

While there's a check in pkexec.c to avoid this problem (by comparing it to what we expect the uid to be - namely that of the pkexec.c process itself which is the uid of the parent process at pkexec-spawn-time), there is still a short window where an attacker can fool pkexec/polkitd into thinking that the parent process has uid 0 and is therefore authorized. It's pretty hard to hit this window - I actually don't know if it can be made to work in practice.

Either way, if exploitable (which I think it is), this bug is a local root exploit so we should treat it like that. Now that there is no vendor-sec list anymore, I don't know what it means wrt to embargoing? (so far this issue has been kept confidential - and the patches fixing this are not yet publicly available)

I already have patches for polkit master to fix this problem (to look up the right uid) and also avoid having to look up the UID in /proc/PID at all (doing so is generally causes TOCTTOU bugs). These patches should all work in the polkit versions shipped in supported versions of Fedora.

I am right now working on patches for RHEL6.