Update: Hackers launch PDF attacks, exploit just-patched Reader bug

Antivirus defenses worthless, says researcher; users urged to patch ASAP

Attackers are exploiting one of the vulnerabilities in Adobe Reader that was patched earlier this week, security researchers warned today.

Adobe confirmed the attacks, and urged users to update the software to version 9 immediately, or if that's not possible, to deploy the patched version of Reader 8.

According to Bojan Zdrnja, an analyst at the SANS Institute's Internet Storm Center (ISC), malicious PDF (Portable Document Format) files are circulating that infect systems by exploiting one of the eight bugs Adobe Systems Inc. patched Tuesday.

"This is not surprising, though, as a fully working [proof-of-concept] has been recently published," said Zdrnja in an alert posted to the ISC site early Friday. "But it's interesting to see that the attackers modified the proof-of-concept little bit, probably in order to evade anti-virus detection."

Shortly before Adobe patched the older Reader 8.1.2, Core Security Technologies, a security company that had reported the bug to Adobe in May, published sample attack code as part of its own advisory about the danger posed by rogue PDF files.

The malicious PDFs examined by Zdrnja contained a variation of the Core Security code. "Parts of the publicly-posted proof-of-concept are visible, but the attackers also modified certain parts," he said. "This was probably enough to fool the [anti-virus] vendors."

In fact, that was the result, Zdrnja argued. As of 11 a.m. EST, no anti-virus company had yet released a detection signature that could finger the malformed PDF files.

Kevin Haley, a director of security response at Symantec Corp., seconded Zdrnja's findings a little later on Friday. "We've seen this out in the wild as well," said Haley, "although it doesn't seem to be widespread at the moment." Haley added that Symantec had issued anti-virus signatures to its customers since Zdrnja posted his alert.

A second attack PDF, published to the milw0rm.com site on Wednesday, is slightly more visible to anti-virus vendors. The proof-of-concept PDF is currently identified by about 14% of anti-virus vendors, according to VirusTotal, a free scanning and reporting service.

"If you haven't patched your Adobe Reader installations, do it ASAP as the attacks are in the wild," Zdrnja urged.

Adobe echoed that. "We strongly recommend users update to Adobe Reader 9, which is not vulnerable to this issue," the company said in an e-mail Friday. "Users of Acrobat 8 and Adobe Reader 8 who can't update to Adobe Reader 9 should install the 8.1.3 update to protect themselves."

The nimbleness demonstrated by the attacks is not new. "It's not unusual at all," Haley said, talking about the short span between Adobe's patching and exploits executing. And the fact that Core Security published proof-of-concept attack code certainly played a part, he added. "Once the code is out there, anyone can pick it up, modify it and use it," said Haley.

He also expects to see attack code against this same PDF vulnerability, or if not that, one of the other seven patched earlier this week, to appear in the multi-exploit kits that have become an important weapon in hackers' arsenals. "They'll continue to go after the most popular software, because they're looking to exploit as many systems as possible," Haley said.

The patches released Tuesday bring Adobe Reader and Adobe Acrobat up to version 8.1.3. Although that edition has been superseded by Reader and Acrobat 9, which debuted in June, most users continue to run older software long after updates or upgrades are available, according to statistics published earlier this year by Secunia ASP, a Danish vulnerability tracking company. Secunia offers a free utility called Personal Software Inspector (PSI) that scans Windows machines for unpatched and out-of-date software.

As Adobe said, users running Reader 9 or Acrobat 9 are safe from the current round of PDF attacks.

Copyright © 2008 IDG Communications, Inc.

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