The first exploit to target one of last week's Patch Tuesday vulnerabilities has been released.

A security researcher said he has published the first exploit against one of the 14 vulnerabilities patched last week by Microsoft, Symantec has warned its customers.

In a posting to the Full-Disclosure security mailing list, Alla Bezroutchko, a senior security engineer at Brussels-based Scanit, spelled out JavaScript code that crashes Internet Explorer 6.0 on Windows 2000 and Windows XP Service Pack 2. Bezroutchko's proof of concept exploits the critical bug in XML Core Services that was patched by MS07-042.

That update, one of six rated "critical" by Microsoft, affected every current version of Windows, including Vista. An analyst last week pegged MS07-042 as patch that should be deployed immediately. "MS07-042 affects everything," said Don Leatham, director of solutions and strategies at PatchLink. "There's so much going on with XML in enterprises. That's why this is so dangerous."

Symantec warned users of its DeepSight threat-alert service to expect Bezroutchko's crude exploit to be polished soon. "The current proof of concept will crash Internet Explorer; however, it is likely that this code will be modified to produce a code-execution exploit in the near future," Symantec said.

It has become common for exploits to appear within days, if not hours, of Microsoft releasing its security updates, to the point that the practice even has a nickname: Exploit Wednesday.

According to other research, however, Exploit Wednesday is part myth. Hackers don't actually stockpile attack code and synchronise the release with the appearance of patches, Craig Schmugar, a security researcher at McAfee, said in June when he released the results of a survey of 200 zero-day Windows vulnerabilities.

"I don't see Exploit Wednesday as a strategically timed release but that it comes about simply because more information is being made available," Schmugar said