Adobe Systems Inc. Tuesday confirmed that it's investigating reports that its popular PDF viewing software, Adobe Reader, contains a critical vulnerability.

A security researcher said that the bug is another in a long line of flaws in Adobe's implementation of JavaScript.

The bug was first disclosed Monday on the SecurityFocus site, which posted a link to proof-of-concept attack code. "An attacker can exploit this issue to execute arbitrary code with the privileges of the user running the application," said the advisory.

According to SecurityFocus, the most up-to-date versions, Reader 9.1 and Reader 8.1.4, are vulnerable. The Linux versions definitely have the bug, and other platforms -- Adobe also provides Reader for Windows and the Mac -- may be at risk as well.

For its part, Adobe's acknowledgement was brief. "Adobe is aware of reports of a potential vulnerability in Adobe Reader 9.1 and 8.1.4," said David Lenoe, the company's security program manager, in a security blog entry. "We are currently investigating, and will have an update once we get more information."

An Adobe spokesman declined to answer questions about the bug, saying that all the company had to offer at the moment was Lenoe's comments. More information, he promised, would be published to the blog, or to Adobe's security advisory page.

The short confirmation was reminiscent of Adobe's admission two months ago when it said Reader contained one or more bugs. In that incident, attackers had been exploiting the vulnerability for as many as six weeks, possibly more.

Adobe patched that flaw, and several others, starting on March 10, fixing first Reader 9, then working its way down to Reader 8 and even older versions at one-week intervals.

"This is very similar to February," said Andrew Storms, director of security operations at nCircle Network Security Inc. "They've had a long rash of problems with JavaScript, and again we're seeing what looks like a JavaScript vulnerability."

Storms, who has been critical of Adobe -- last year he said the company suffered from an "epidemic" of JavaScript vulnerabilities -- again blasted its security efforts.

"It's time that Adobe stepped up to the challenge," he said, adding that Adobe was a good example of a company that should try to emulate Microsoft Corp.'s approach to security. "This is another proof-of-concept that's been made public," Storms said. "It's in stark contrast to Microsoft, which somehow manages to get most people to report vulnerabilities responsibly."

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