Mozilla and Microsoft are pointing fingers at each other over a bug that causes problems when Internet Explorer is opened from with Mozilla's Firefox browser.

Security researcher Thor Larholm kicked off the controversy on Tuesday, claiming that he had discovered a flaw that would let an attacker run commands on a victim's PC.

In Larholm's blog posting, he said the bug was similar to a flaw he'd discovered last month in Apple's Safari 3.0 beta software, and he called it an an "input validation flaw in Internet Explorer."

The problem is with a URL protocol handler component of Internet Explorer, he said. This software allows Internet Explorer users to launch applications such as Excel or Firefox by clicking on specially written links on web pages.

When an Internet Explorer user clicks on a link that launches the Firefox browser, however, the software does not properly check its syntax, and that, Larholm said, lets an attacker create a malicious link that could be used in an attack. Security vendor Secunia ApS rates the flaw as "highly critical."

So while the flaw affects Internet Explorer users, it appears to be a risk only to those who already have Firefox installed. And to make matters more complicated, if a Firefox user were to click on one of the specially-written links, he would not be affected.

Microsoft's security program manager Mark Griesi said that the bug was not his company's problem. "We don't feel that there's an issue in IE and therefore there's nothing to be fixed," he said. Mozilla said it would change the way its Firefox URL protocol handler worked in its next Firefox update. That will fix Larholm's bug, but Mozilla security strategist Window Snyder did point out that without Microsoft making changes to IE, other Windows programs may be at risk.

Noted browser bug-hunter Aviv Raff wrote in his own blog that he thought both Microsoft and Mozilla were to blame.

But another security expert said that the responsibility to fix the problem lies with the open-source browser developers. "This is an oversight in how Mozilla decided to construct their protocol registration, and how they do input validation between this handler and their application," said Eric Schultze, chief security architect with Shavlik Technologies. "I think Mozilla needs to write and release a patch for this issue and Microsoft can take the week off."