Microsoft has announced plans to release an out-of-band patch today tackling a critical zero-day hole in Internet Explorer.

The update will almost certainly tackle an unpatched remote-code execution flaw in earlier versions of IE (detailed in Microsoft Security Advisory 2794220) that has become the target of hacker attacks since late December.

For now, Redmond only says the flaw is critical, as per its standard practice of not going into details ahead of actually publishing a security patch. Microsoft advises customers to apply the critical patch immediately, a piece of advice echoed by security watchers such as Sophos.

Several websites have already been compromised to spread malware exploits based on the vulnerability in IE 6,7 and 8. Users could safeguard themselves by either updating to IE 9 and 10 or using an alternative browser. Microsoft published a temporary FixIt tool to protect against this vulnerability but security researchers found this defence was far from bullet-proof.

IE 9 has been available since March 2011. Although the vulnerability attacks old, arguably obsolete browser software, it still presents a huge risk - not least because it affects 90 per cent of the Internet Explorer installed base, according to cloud security firm Qualys.

Sites booby-trapped to serve exploits based on the attack include an Iranian oil company, a website serving the Uyghur people of East Turkistan, the Council on Foreign Relations website and others.

The attacks bear the hallmarks of previous infections spread by the so-called Elderwood Project. Although a different vulnerability was abused in those earlier attacks, the ultimate aim was geared towards delivering the same malicious payload.

Emergency (out-of-sequence) patches for security flaws in Microsoft software are a rare but far from unprecedented occurrence. Previous examples include a fix for a security bug in ASP.Net applications that allowed attackers to decrypt password files, cookies, and other sensitive data in September 2010. You might also recall the August 2010 patch for a flaw in Windows shortcut, also associated with malware attack, and a March 2010 update to tackle a security bug in IE, also linked with distributing malware.

Patches outside the regular Patch Tuesday update are a pain for administrators and Redmond has done a good job in cutting down their frequency over the last three or four years. Microsoft has been battle-hardened from years of combating Windows bugs and its security practices have become an example to the rest of the industry. Some also argue that it encourages hackers to divert their attention away from Redmond and towards exploiting vulnerabilities in third-party software, most particularly Java and Adobe applications. ®