A serious memory corruption vulnerability in polkit (formerly PolicyKit) has finally been discovered after 12+ years. This program is found in essentially all modern Linux distributions.
The most likely attack scenario is from an internal threat where a malicious user can escalate from no privileges whatsoever to full root privileges. From an external threat perspective, if an attacker has been able to gain foothold on a system via another vulnerability or a password breach, that attacker can then escalate to full root privileges through this vulnerability.
The attack is highly efficient but entirely relies on the fact you need to get a foothold first on the host system.
I updated the OP with more infos.