A variant of a bug that uses an Internet Explorer 6 vulnerability exposes Microsoft Windows systems patched with Service Pack 2.
A security researcher claims to have crafted a simpler version of a bug first identified in late October.
That used two vulnerabilities in Internet Explorer 6, in combination with an inappropriate behaviour in the ActiveX Data Objects model, to compromise an SP2-patched system.
The October vulnerability was itself a variant of a bug dating back to August.
Both earlier flaws were ranked "highly critical" by Danish security firm Secunia, the second most serious of its ratings.
The October exploit required a user to drag an image from one part of a web page to another, and then to click a button. At the time, Microsoft said the bug required too much user interaction to be considered serious.
The new version, discovered by the Greyhats Security Group, eliminates the need to click a button. Like the earlier exploit, the new attack could lead to the execution of HTML and script code in the context of a trusted site, Greyhats said.
The researcher posted a proof-of-concept demonstration which, when an image is moved from one part of a web page to another, is designed to create a file on the user's machine, bypassing the "Local Computer" zone lockdown security feature in SP2.
Secunia has confirmed that the exploit is effective against SP2-patched systems.
While no patch is available, users can protect themselves by disabling the "Drag and Drop or copy and paste files" option in Internet Explorer, according to Microsoft.
Secunia said users could also set the security level for the internet zone to "high" and disable active content.
A few vulnerabilities in SP2 have begun to surface. Earlier this month, Finjan Software claimed to have discovered 10 major security flaws in SP2, bypassing many of the security measures the update puts into place and allowing the execution of malicious code on a system by luring users to a specially crafted web page.
However, the service pack has blocked some new attacks that are effective against earlier versions of Windows XP, for example the still-unpatched Iframe vulnerability that allowed attackers to plant malicious code on PCs via a hacked banner ad server.
Matthew Broersma writes for Techworld.com