Company officials added that the flaw also is not in Microsoft's CryptoAPI (CAPI), which would leave a number of applications and Windows services vulnerable, not just IE.
Microsoft said it is working on patches for Windows 98, ME, NT4, 2000 and XP. It would not say when the patches would be available.
"This SSL flaw has been described as an [Internet Explorer] problem but it is a Windows issue. It's in the crypto of the operating system so we have to patch the OS," said Scott Culp manager of the Microsoft Security Response Center. "IE is a consumer of those crypto services."
He said it is an "implementation problem in the way SSL certificates are processed where information is not available in the certificate or it is available in two places and there is a conflict."
Culp said the flaw does not lie within CAPI and that it lies in code that performs validation of SSL certificate chains, meaning the hierarchy of trust that cascades from certificate authorities such as VeriSign. The OS must be patched because IE does not have its own cryptography code and must rely on the OS for that service, he said.
Konqueror.org was able to patch its open source Konqueror Web browser, which had the same SSL flaw as IE, in under 90 minutes because it uses its own built-in certificate verification library.
Microsoft officials said it makes sense for the OS to provide cryptographic services to any application that needs it instead of each application having to include it's own cryptographic technology.
But Culp said the SSL flaw does not effect any other application outside IE and that it is a client side issue only.
"That's interesting, I'll have to do some more testing," said Mike Benham, an independent researcher who first reported the SSL flaw. "Possibly this is a second can of worms."
Benham reported on Tuesday that Internet Explorer had a security flaw that undermines the security provided by Secure Socket Layer (SSL), a standard for securing online transactions and electronic commerce.
The flaw opens up a vulnerability that is called a man-in-the-middle attack, where the attacker can hijack an SSL session and decrypt messages that could contain credit card numbers or social security numbers.
The IDG News Service contributed to this report.
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