Microsoft updated a security advisory that addressed a broad flaw in Windows and said it is working around the clock to fix the bug. But it may be too late for many. Security researchers said hackers had amped up attacks using malicious PDF files that exploit the vulnerability..

Finland-based F-Secure called the surge in spam carrying the rigged PDF documents "massive" and said the run is ongoing. Ken Dunham, director of response at iSight Partners, confirmed that the number of messages hitting mailboxes with rogue PDFs soared just before the weekend. "PDF exploits are ramping up just in time for the weekend," he said in.

The attacks, which began last week, exploit bugs in the Windows versions of Adobe Systems' Reader and Acrobat software; Adobe patched the newest editions of those programs, but has not yet updated older variants.

According to Dunham and other researchers, the infamous Russian Business Network (RBN), a collective of cybercriminals, is behind the PDF assault. When recipients open an attack PDF, a combination of Trojan Horses, downloaders and rootkits strike, knocking out the Windows firewall and installing code that captures all information entered into any SSL-secured form on a Web page. That information is then transmitted back to RBN.

Microsoft updated its security advisory because it detected what it called "fairly limited" attacks using PDFs, said Bill Sisk, a member of the Microsoft security response team.

"This week we became aware of publicly disclosed exploit code being used in limited attacks on customers," said Sisk in a posting to a Microsoft company blog. "This change in the threat landscape has triggered our Software Security Incident Response Plan." Microsoft's SSIRP coordinates its investigations with researchers from other vendors. Sisk said Microsoft had developers around the world "working around the clock" to devise a fix.

The reason Microsoft is involved is that while the current attacks are based on malformed PDFs, the real vulnerability lies in Windows XP and Windows Server 2003 code, not in Adobe's, Sisk acknowledged. "The vulnerability mentioned in this advisory is in the Microsoft Windows ShellExecute function," he said. "These third-party updates [such as Adobe's fix] do not resolve the vulnerability, they just close an attack vector."

His admission is the clearest yet from Microsoft that the updates produced by Adobe and similar fixes issued by Mozilla for Firefox and Skype for its flagship VoIP software would have been unnecessary if Windows had been patched against problems in URI protocol handlers, which let browsers run other programs via commands in a URL.