Petko Petkov, a penetration tester who disclosed a zero-day flaw in Apple Inc.'s QuickTime a week ago and a similarly critical bug in Microsoft Corp.'s Windows Media Player on Tuesday, now says that Adobe System Inc.'s Acrobat Reader files harbor a serious vulnerability.

He explained why he refused to publish code to prove that the PDF vulnerability is real.

"The issue is quite critical given the fact that PDF documents are in the core of today's modern business," Petkov wrote. "This and the fact that it may take a while for Adobe to fix their closed source product are the reasons why I am not going to publish any POCs.

Although Petkov provided no proof of his claim, Symantec Corp. was ready to believe. "Although these claims are currently unverifiable, this researcher has identified several vulnerabilities in the past and is likely credible regarding these findings," the company said in an alert to customers of its DeepSight threat network.

"My advise is for you is not to open any PDF files (locally or remotely)," Petkov said. Petkov said he had tested and verified that the vulnerability exists in the most recent version of Acrobat Reader, Version 8.1, and can be exploited on PCs running Windows XP SP2.