U.S. CERT on Friday issued a warning about the technique. Citing a Sept. 15 blog post by Jeremiah Grossman, founder and CTO of WhiteHat Security, U.S. CERT said, "Clickjacking gives an attacker the ability to trick a user into clicking on something only barely or momentarily noticeable. Therefore, if a user clicks on a Web page, they may actually be clicking on content from another page."
The government security agency also said the flaw affects most Web browsers and that no fix is available, but that risks can be mitigated by disabling scripting and plug-ins in one's browser.
For Firefox users, the NoScript Firefox extension can do that. Grossman in a blog comment posting also suggests the use of security-related plug-ins like FlashBlock, Adblock Plus, and CustomizeGoogle. (Presumably, these plug-ins should not be disabled.)
Clickjacking affects Apple Safari, Google Chrome, Microsoft Internet Explorer, Mozilla Firefox, and Opera.
Hansen likens clickjacking to cross-site request forgery, another form of Web attack. "It's a very straightforward, simple attack," he said. "It basically just takes you mouse click and repurposes it for something that it wasn't originally intended for." Grossman and Hansen discovered the vulnerability and planned to discuss it at the 2008 OWASP USA, NYC security conference this week. But they decided not to give their presentation because the issues they discovered were so serious that they felt compelled not to reveal the vulnerability until it can be addressed.
That has meant discussions with Adobe, Microsoft, and Mozilla, and other major browser vendors. The reason Adobe is involved is that its Flash software, installed in almost all of the browsers out there, can be used for a clickjacking exploit.
But the issue is not specific to Adobe Flash. "It is a generic thing," said Hansen. "Adobe is affected but the irony is that we don't think it really has much to do with them at all. They're affected but so are tons and tons of other things, Web sites, plug-ins, all kinds of stuff."
Hansen said this isn't an Internet-breaking bug along the lines of the vulnerability search researcher Dan Kaminsky disclosed in August. "Kaminsky's bug really, truly affected everyone, everywhere," he said. "You could do really nasty things to people without having any interaction with them whatsoever. Our bug, it does require user interaction. It's very point and shoot, very targeted. It works on a one-off basis. ... It is about the same severity as any of your normal buffer overflows that you'll find in modern browsers, the only difference being that, unlike a buffer overflow, you can't fix it quickly."
"I have not iterated through all the possibilities of this exploit, not even a little bit," he said. "I spent probably a week thinking about the problem, maybe two days coming up with exploit code, and another couple of hours looking at various Web sites, plug-ins, and others things, looking to see what might be vulnerable. And in the process of doing that, pretty much everything I poked at broke."
Ironically, Web sites that attempt to be more secure end up being less secure with regard to clickjacking. The reason, says Hansen, is that sites that try to protect against cross-site request forgery end up making themselves vulnerable to this attack.