Security watchers are warning of a serious unpatched vulnerability in Adobe's Reader program that's actively being exploited to install malware on the PCs of unsuspecting users.
The vulnerability has been confirmed in versions 8.1.3 and 9.0.0 of Adobe Reader running on Windows XP Service Pack 3 and is presumed to work on other versions of Windows as well, according to this advisory from Shadowserver. Adobe for machines running Linux and Apple's OS X were not tested, but may also be vulnerable, Shadowserver's Steven Adair said.
There are multiple variants of the exploit that are actively circulating, one of which installs a remote access trojan known as Gh0st RAT.
"Right now we believe these files are only being used in a smaller set of targeted attacks," Shadowserver's advisory read. "However, these types of attacks are frequently the most damaging and it is only a matter of time before this exploit ends up in every exploit pack on the internet."
Several anti-virus programs are already detecting the booby-trapped PDFs. Trend Micro and Symantec flag the attack as TROJ_PIDIEF.IN and Trojan.Pidief.E respectively. Both companies rate the threat as low, but those analyses appeared to be a week old, so it's likely attackers have stepped up the exploit since then.
Adobe has issued this advisory aknowledging a "critical vulnerability" in Reader. Updates won't be available until March 11 for version 9 and a later date for earlier versions. InsecureWeb has also issued details here.