Key executives behind Internet Explorer and Firefox are each blaming the other for multiple Windows zero-day vulnerabilities.

In an entry on the IE blog, IE program manager Markellos Diorinos said that the spate of protocol handler bugs involving the Microsoft browser are the fault of the other applications, not IE.

"The limitless variety of [third-party] applications and their unique capabilities make it very difficult to have any meaningful automated parameter validation by the hosting (caller) application," said Diorinos. In the vulnerabilities disclosed so far that involve Mozilla's Firefox browser and Cerulean Studios' Trillian instant messaging client, IE has been pegged as the application that calls on those program's unique protocols.

It is, in Diorinos' terminology, the "hosting" application. "It is the responsibility of the receiving (called) application to make sure it can safely process the incoming parameters," Diorinos said.

Diorinos' comments came in response to the ongoing controversy about a flaw that involves both IE and Firefox. Last week, Danish researcher Thor Larholm, among others, put the onus on IE, and said that while Firefox registers the "firefoxurl://" protocol used in his proof-of-concept exploits, Mozilla's browser is an innocent bystander.

This week, other security researchers said a similar problem involved IE and the "aim" protocol used by Trillian, a multi-service instant messaging program, or for that matter, any AIM (AOL Instant Messenger) client.

Microsoft's stance has not changed since last week, when it first denied that IE was at fault. "Microsoft has thoroughly investigated the claim of a vulnerability in Internet Explorer and found that this is not a vulnerability in a Microsoft product," a spokesman said last Thursday.

"URL protocol handlers are one of the ways we enable rich experiences in browsing, however, as with any other program that accepts untrusted data from the web, URL protocol handling applications must be carefully designed based on the threat environment," stressed Diorinos.

Mozilla, meanwhile, turned up the heat Tuesday when it updated Firefox to version 2.0.0.5 and patched the open-source browser so it would not accept out-of-bounds data from other applications, such as IE.

And like its browser rival, Mozilla said Microsoft should shape up.

"[This] does not fix the critical vulnerability in Internet Explorer," Window Snyder, Mozilla's chief security executive, said of the Firefox update. "Microsoft needs to patch Internet Explorer."