A security flaw in Microsoft's Internet Explorer (IE) Web browser can completely undermine the supposedly watertight secure sockets layer (SSL) standard for securing online transactions and e-commerce, researchers said last week.
IE's implementation of SSL contains a vulnerability that allows what is described as an active, undetected, man-in-the-middle attack, where no dialogue boxes are shown and no warnings are given.
Security researcher Mike Benham said the problem is that IE fails to check the basic constraints of certificates signed by intermediate Certificate Authorities (CAs). That means that as far as IE is concerned, anyone with a signed certificate for any domain can generate a certificate for any other domain, which will appear to be signed by a valid CA.
Describing the flaw, Internet security Web site Hideaway.net said: "Spoofing a trusted Web site is thus a trivial exploit; when combined with session hijacking, a man-in-the-middle attack is quite feasible. This destroys the whole purpose of SSL certificates in the first place."
Benham said that IE 5 and IE 5.5 are totally vulnerable to this kind of exploit, and IE 6 is vulnerable under most circumstances.
"I would consider this to be incredibly severe," Benham said in a newsgroup thread. "Any of the standard connection hijacking techniques can be combined with this vulnerability to produce a successful man-in-the-middle attack. Since no warnings are given and no dialogues are shown, the attacker has effectively circumvented all security that an SSL certificate provides."
Microsoft has given no indications that it plans to fix this flaw, and Benham said his experience showed it would be difficult to get Microsoft to address the issue.
"After seeing that, I don't feel like wasting time with the Microsoft PR department," Benham said.