Microsoft said it is working around the clock on a patch for a Windows flaw that is partly responsible for an ongoing attack wave of infected PDFs.

The company has updated a security advisory to reflect the fact that exploit code is in the wild, but it may be too late for many. Security researchers said hackers have ramped up attacks using malicious PDF files that target the vulnerability.

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 rogue PDFs soared on Friday.

The attacks, which began last Tuesday, exploit bugs in the Windows versions of Adobe's 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.

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

While the attacks use 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 a patch for problems in Windows URI protocol handlers would have made recent patches in Acrobat, Firefox and Skype unnecessary.

This summer, researchers argued over who was responsible for URI protocol handler vulnerabilities that were beginning to surface. Microsoft strenuously denied that its software was at fault until earlier this month, when it issued an advisory and said it would create a patch.

"This may be Microsoft's first public acceptance that this bug is in fact a Microsoft vulnerability," said Andrew Storms, director of security operations at nCircle Network Security. Although Microsoft has not said when it will release a patch to plug the hole, Storms bet it would be next month. "It's safe to assume Microsoft will attempt to release a patch in time for the November regular patch cycle," he said.

The next scheduled patch day for Microsoft is 13 November, more than two weeks away.

The newest PDF-based exploits, said researchers, are using different file names and subject headings in the spam that delivers the files. According to F-Secure, spam subjects now include "Your credit report," "Your Credit File" and "Personal Finance Statement."

Another researcher, Don Jackson of SecureWorks, said that the malware eventually planted on PCs by the RBN attacks is a new variant of Gozi, a Trojan he pegged in February as responsible for the theft of at least $2 million from bank and credit card accounts.