Microsoft has warned users that hackers are exploiting the unpatched bug in ASP.Net to hijack encrypted Web sessions.
In a Monday update to a previously-published security advisory, Microsoft said that it was seeing "limited, active attacks at this time."
Symantec, which has a massive global network of sensors and honey trap-like systems to detect and capture exploits, said it had not seen any attacks, however.
The vulnerability exists in all versions of ASP.Net, the company's Web application framework used to craft millions of sites and applications, and lets attackers access Web applications with full administrator rights; decrypt session cookies or other encrypted data on a remote server; and access and snatch files from a site or Web application that relies on ASP.Net.
Microsoft acknowledged the flaw last Friday, the same day that a pair of researchers demonstrated how the "oracle padding" bug can be exploited by force-feeding cipher text to an ASP.Net application and noting the returned error messages it returns.
The company again promised to patch the vulnerability, but like last week, did not set a delivery date for the fix.
"We will be releasing a patch on Windows Update, so all machines will get it," said Scott Guthrie, the Microsoft executive who runs the ASP.Net development team.
The next regularly-scheduled security updates will ship Oct. 12, a little less than three weeks from today.
Occasionally Microsoft issues emergency patches -- called "out-of-band" updates -- to tackle problems, almost always after attacks appear.
Until the company releases a fix, Guthrie continued to urge Web site and application developers to plug the hole with a temporary workaround that involves editing the "web.config" file.
SharePoint Server 2007 and SharePoint 2010 are also vulnerable to ASP.Net attacks, said Microsoft. The SharePoint team has published different web.config editing instructions for its high-profile, highly-profitable collaboration software.
Microsoft wasn't downplaying the threat to Web sites and applications. "The publicly disclosed exploit can be used against all types of ASP.NET applications," Guthrie said in a FAQ he posted Monday.
"All sites that use ASP.NET are affected by this vulnerability," echoed Kevin Brown, an engineer with the Microsoft Security Response Center.
The MSRC also took its usual shot at researchers who publicly disclose bugs in its products before a patch is ready. In an entry to the group's blog, spokesman Dave Forstrom cited "coordinated vulnerability disclosure," a term Microsoft coined last summer to describe how it wants outside researchers to behave.
"We fundamentally believe, and history has shown, that once vulnerability details are released publicly, the probability of exploitation rises significantly," Forstrom said. "Without coordination in place to provide a security update or proper guidance, risk to customers is greatly amplified."
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is firstname.lastname@example.org.