Adobe admits PDF exploit, posts workaround

Expect Reader, Acrobat updates by end of the month to block Petkov's URI flaw

Adobe Systems has confirmed that there's a critical bug in its most popular programs, but it doesn't yet have a patch that protects Windows XP users against attacks arriving as PDF files.

In an advisory posted last week, Adobe admitted that the flaw first disclosed by Petko Petkov, a U.K.-based security researcher, was real. The company also provided a multiple-step work-around in lieu of a permanent fix to its Adobe Acrobat software and its free Adobe Reader application.

Last month, Petkov claimed in a blog posting that he had found a critical vulnerability that could be leveraged using PDF files, Adobe's popular document format. "Adobe Acrobat/Reader PDF documents can be used to compromise your Windows box," Petkov said. "Completely!!! Invisibly and unwillingly!!! All it takes is to open a PDF document or stumble across a page [that] embeds one."

At the time, Petkov declined to provide proof-of-concept code, telling users: "You have to take my word for it." He recommended steering clear of all PDFs until a fix was available.

Adobe's work-around requires editing the Windows registry, a daunting chore for most users, but it will protect against malicious PDF documents that exploit the "mailto:" URI (universal resource identifier) to trick users into downloading attack code. Mailto:, one of the most-frequently used URIs, launches the default e-mail client and opens a pre-address message when a link is clicked inside a Web browser.

The terse description indicates that the PDF vulnerability is yet another protocol-handling bug. Those flaws have been a hot topic in security circles since July, when another researcher, Norwegian Thor Larholm, showed how Internet Explorer and rival Firefox could be used to run malicious code by exploiting invalid URIs. In fact, the debate over patching responsibility resumed on Friday, when a German analyst said IE7 brought new bugs to Windows XP.

Juergen Schmidt of Heise Security specifically called out Adobe's software in a warning to a security mailing list, while Heise's Web site provided a proof-of-concept attack that used the mailto: URI to inject malicious code via a PDF. "This critical security problem is probably the same as recently detected and described by Petko Petkov," Heise said before Adobe published its advisory.

Adobe said it would update Adobe Reader 8.1 and Adobe Acrobat 8.1, as well as Adobe 3D, by the end of the month, but did not give a more specific date.

Microsoft, which has been criticized for not fixing the protocol-handing capabilities in Windows and Internet Explorer, has repeatedly said that the responsibility for making sure third-party software properly processes URIs such as mailto: falls to other vendors' application developers, not its engineers. In July, IE program manager Markellos Diorinos claimed that it would be "very difficult" for Windows to check for possibly invalid URIs.

If users cannot or will not use the work-around, Adobe's advice was essentially the same as Petkov's of two weeks ago. "Adobe recommends that Acrobat and Reader customers use caution when receiving unsolicited e-mail communications requesting user action, such as opening attachments or clicking Web links," the company said in its advisory.

Only Windows XP users running Internet Explorer 7 are at risk, Adobe said. Owners of Windows Vista, which sports its own version of IE 7, are safe from the mailto:-based attacks.

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Gregg Keizer

Computerworld (US)

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