A new vulnerability that affects many Linux systems has been revealed: Pwnkit. This attack uses a vulnerability in polkitd to allow any user to escalate his privileges to root. There are patches for major Linux systems: see the LowEndTalk thread.
It’s important to note that the vulnerability can only be used by users logged into the system. You can’t launch this attack on just any Linux box you ping on the network.
Patch your systems ASAP!