Microsoft is taking the unusual step of rushing out two emergency security patches ahead of its regularly scheduled updates on 11 August.
The patches will include a critical fix for Internet Explorer as well as a related Visual Studio patch rated "moderate" urgency by Microsoft.
"The Internet Explorer bulletin will provide defence-in-depth changes to Internet Explorer to help provide additional protections for the issues addressed by the Visual Studio bulletin," Microsoft said in a blog posting late Friday.
The patches are set to be released on Tuesday.
Microsoft did not say exactly what it was fixing. The company typically doesn't rush out these "out-of-band" emergency updates unless the bug is being exploited by cyber criminals; however, in this case the flaws being patched are not being leveraged in attacks, according to Microsoft.
The problem appears to lie in a widely used Windows component called the Active Template Library (ATL). According to security researcher Halvar Flake, this flaw is also to blame for an ActiveX bug that Microsoft identified earlier this month.
Microsoft issued a kill-bit patch for the problem on July 14, but after looking into the bug, Flake determined that the patch didn't fix the underlying vulnerability, so new attacks are possible.
Whatever the issue, the new patch should be a top priority for IT staff next week. "When Microsoft goes to an out-of-band patch, I think it's sensible for people to apply it," said Roger Thompson, chief research officer with AVG Technologies.
Microsoft didn't give a reason for the rushed update but it may be trying to stay ahead of any public disclosure at next week's Black Hat security conference in Las Vegas.
The emergency updates are set to be released the day before the Black Hat Briefings, where researchers Mark Dowd, Ryan Smith and David Dewey will talk about browser security issues.
According to security experts, thousands of Web sites have been used to launch on-line attacks that exploit the ActiveX vulnerability patched in July. The flaw was first reported to Microsoft more than a year ago.