TLS used in web browsers is vulnerable to cookie grabbing

I heard something really worrying yesterday – someone’s got a proof-of-concept that defeats TLS (previously known as SSL) encryption. Security researchers Thai Duong and Juliano Rizzo are planning to demonstrate this at Ekoparty in Argentina this week.

Fortunately this isn’t as bad as it sounds as it doesn’t actually involve breaking TLS, but it’s still pretty bad. It only applies to some web browsers, but it does allow the theft of supposedly encrypted login cookies and it seems to me a very practical method, although details aren’t officially published as yet. Basically, it involves putting some Javascript on a web site which causes the browser to fetch something from the site being targeted – say Paypal. The browser sends the request, encrypted along with the login cookie – compressed and then encrypted using TLS. You can’t read what’s in the TLS packets, but you can see how long they are.

Fundamentally, compression works by removing repeated information found in the uncompressed data. Therefore if you have repetition, the data compresses better. By making a number of requests for differing data (like bogus image file names) you’ll know by the size of the compressed packet if data in the unknown login cookie contains data repeated in the file requested simply because the combined encrypted packet will get shorter. In other words, because the unknown cookie and known file request are compressed into the same packet, you can determine whether there is any repetition simply by comparing the size of the compressed data – when it gets shorter you’ve got a match. Apparently you need make as few a six bogus requests for each letter in the login cookie to work out its contents.

You obviously need to be eavesdropping on the line, and your victim must be running your Javascript on their browser, but as TLS is there to prevent eavesdropping then this is a serious failure. It’s not the fault of TLS, but that of browser protocol writers, hoping that implementing TLS gives them security without further consideration.

Some people have suggested that this attack would be difficult to implement in practice, but I disagree. Why not simply hijack the DNS at an Internet Cafe (with a fake DCHP server) and force everyone to run the Javascript from the first web site they tried to open, and either snoop the WiFi or sniff the packets off the wire using traditional methods of circumventing switches.

Apparently this flaw doesn’t affect IE, but the others were vulnerable until tipped off about it. Make sure you’re running a current version

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