Widespread exploitation of an unpatched Windows vulnerability involving cursor animation files over the weekend have prompted Microsoft to announce plans to release an out-of-sequence patch on Tuesday.

A stack buffer overflow flaw in Windows' handling of animated cursor (.ANI) files, first reported last week, allows hackers to inject hostile code into unpatched systems. Internet Explorer can process ANI files in HTML documents, so web pages and HTML email messages can also be vectors for the vulnerability.

Hackers were quick to latch on to the potential of the vulnerability, for instance creating worms or launching spam campaigns featuring email messages that sought to exploit the security bug. In response, the Zeroday Emergency Response Team (ZERT) released an unofficial patch to address the vulnerability.

Microsoft responded to reports of widespread abuse by pushing out an emergency fix, to be released on Tuesday, 3 April, a week before its regular Patch Tuesday update.

A posting on Microsoft's Security Response Centre blog reports that Microsoft's security gnomes have been working round the clock to produce and test a fix and explains the rationale for Redmond's unusual (but far from unprecedented) decision to publish an out-of-sequence fix. ®