A Linux local privilege escalation vulnerability made public last week allows a Root level shell from a standard or guest account.
Last week an exploit was revealed that affected Linux Kernel versions 3.3 through 3.8. Successful use of the exploit allows the attacker to gain root level access on Linux machines.
This is a known issue and Ubuntu has released a Security Bulletin regarding it. Even better they have already supplied a patch to fix the exploit. All you need to do is run Ubuntu updates and the fix will automatically be installed.