Microsoft appears to be reluctant to acknowledge a weakness in Internet Explorer that allows attackers to hijack secure web sessions, with the company insisting it is still evaluating whether the weakness, which was highlighted nearly two months ago, exists.

However, Apple, which bases its Safari for Windows browser on Microsoft code, says Safari for Windows has the same weakness - and the Microsoft code is the reason.

If Microsoft doesn't fix the problem, Apple can't fix it on its own, Apple says. The company says it has fixed the problem for Safari for Macs.

"Microsoft is currently investigating a possible vulnerability in Microsoft Windows. Once our investigation is complete, we will take appropriate action to help protect customers," a Microsoft spokesperson said.

"We will not have any more to share at this time."

Man-in-the-middle attacks

The weakness can be exploited by man-in-the-middle attackers who trick the browser into making SSL sessions with malicious servers rather than the legitimate servers users intend to connect to.

Current versions of Safari for Mac, Firefox and Opera address the problem, which is linked to how browsers read the x.509 certificates that are used to authenticate machines involved in setting up SSL/TLS sessions.

In July two separate talks presented by researchers Dan Kaminski and Moxie Marlinspike at the Black Hat Conference warned about how the vulnerability could be exploited by using what they call null-prefix attacks.

The attacks involve getting certificate authorities to sign certificates for domain names assigned to legitimate domain-name holders and making vulnerable browsers interpret the certificates as being authorised for different domain-name holders.

For instance, someone might register In many x.509 implementations the certificate authority will sign certificates for any request from the root domain, regardless of any sub-domain prefixes that might be appended.

In that case, the authority would sign a certificate for, ignoring the sub-domain bestbank and signing based on the root domain, Marlinspike says.

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