Adobe's Reader X, last year's upgrade featuring a 'sandbox' designed to protect users from PDF exploits, stymied a recent attack campaign, researchers said.
But they're not sure why.
"I don't want to take anything away from Adobe - after all, a win is a win - but this particular exploit appears to be designed with previous versions of Reader in mind," said Chris Greamo, who heads the research lab at security company Invincea. "What appears to have happened is that the exploit breaks, but we don't have a good sense if the sandbox was able to contain it."
Meanwhile, engineers Sophos poked into the malware - a malicious PDF document that's attached to spam - and found that Reader X stops its execution. The same malware, which first was noticed by security firms last October, runs correctly when aimed at Adobe Reader 8, however.
The malformed PDF tries to exploit an already-patched vulnerability in older versions of Reader, said Chet Wisniewski, a senior security adviser with the antivirus company.
Like Greamo, Wisniewski wasn't able to definitively credit Reader X's sandbox for blocking the attack. "Was it the sandbox or that the malware was designed for Reader 8 and 9, not Reader X? I have a hunch it was both, really," Wisniewski said.
Malware beaten by updated software
It's not unusual for malware to fail when run on, say, a different edition of Windows than what it was meant to attack, or to crash when encountering a newer version of the target application, even if the security vulnerability still exists in the update.
"[This malware] wouldn't have worked in any case, because the flaw it exploits only exists in older versions of Reader," Wisniewski said. "But even if it had, considering what the exploit had to do, the sandbox in Reader X would have provided protection."
At a minimum, Reader X is a win for Adobe, Wisniewski continued, because that version had eliminated the flaw.
Did Sandbox do its job?
Adobe said it had reached out to Sophos for more details about the malware to determine what vulnerability was targeted, and couldn't confirm that Reader X's sandbox had played a part.
"However, based on the description of the exploit, specifically that it attempts to download and run malicious code, we know the sandbox design seeks to prevent this type of action from happening, even if the vulnerability were present in the Adobe Reader X code base," said an Adobe spokeswoman.
The spokeswoman added that no vulnerabilities that could be used to escape the sandbox have been reported to Adobe since Reader X debuted last November . "Nor have we seen any public discussion of sandbox-breaking exploits," she said.
That's good news for Adobe, which even before Reader X's launch had argued that security researchers would be looking for bugs in its sandbox. "Everyone will want bragging rights to be the first to come up with a working exploit of the sandbox," said Brad Arkin, Adobe's director of security and privacy, in an October 2010 interview.
Reader X's sandbox is based on technologies used by Google and Microsoft. The former sandboxes its Chrome browser, for example, while the latter uses similar defences to protect Internet Explorer and Office 2010 on Windows.
Adobe also claimed that a recently addressed bug in Chrome that lets attackers escape the browser's sandbox was not present in Reader X's sandbox code.
Google patched that bug, the first to earn the reporting researcher the company's top bug bounty of $3,133, three weeks ago.
Last week, Adobe alerted customers that it would ship its next quarterly security update for Reader on Tuesday, February 8. The Reader update will patch the Windows and Mac versions, including Reader X.
Sophos' Wisniewski urged Windows users to upgrade as soon as possible to take advantage of the sandbox defense. "Reader X is the most secure version of the program, and everyone should be using it," he said.