Security researchers have confirmed that a flaw fixed by a recent "covert" update to Adobe Reader has been exploited to distribute Trojans for at least a fortnight prior to its publication last Thursday (7 February).

Adobe confirmed that version 8.1.2 of Adobe Reader fixes a number of critical flaws that might be remotely exploited by hackers, leaving users (if not hackers) in the dark about what's inside the tin. The update is designed to resolved stability and performance issues as well as a number of unspecified security vulnerabilities.

iDefense Labs said it reported the bug to Adobe in October 2007. Other vendors, including Google, Fortinet and an anonymous contributor to TippingPoint's Zero Day Initiative, are also credited in Adobe's advisory.

The Adobe update materialised on 7 February, a day after grey hat hackers posted a proof of concept demo for the bug. Proof of concept demos validate security warnings, and according to some, act as an incentive for vendors to address security vulnerabilities. On the down side, demonstrations of vulnerabilities could provide hackers with clues about how to run attacks using live exploit ammo.

That's a moot point in the case of the Reader vulnerability, because black hat hackers were already ahead of the game. Since January 20, banner ads have actively served malicious PDF files that exploit the vulnerability to install the Zonebac Trojan, iDefense reports. The Trojan modifies search results and banner ads, presumably in an attempt to rack up illicit affiliate payments.

iDefense reckons the Adobe exploiters are from the same group that used a RealPlayer zero-day exploit to install the same Zonebac Trojan.

Reports of exploitation of the more recent Adobe Reader flaw surfaced on an Italian forum on 20 January, but was largely unnoticed at the time. It could be that this was a probing attack solely designed to test the efficacy of the exploit.

Things changed last week with the publication of Adobe advisory; a steady trickle of exploit reports has flowed in since then. A timeline on the vulnerability can be found in a posting by the Internet Storm Centre here. ®

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