Only a handful of exploits per decade reveal a vulnerability that is truly significant. Thai Duong and Juliano Rizzo's BEAST (Browser Exploit Against SSL/TLS) attack will rank among them because it compromises the SSL and TLS browser connections hundreds of millions of people rely on every day.
BEAST cannot break the latest version of TLS -- the current standard based on SSL -- but most browsers and nearly all websites that support secure connections rely on earlier versions of the SSL and TLS protocols, which are vulnerable to BEAST attack. Browser vendors and websites that host secure connections are already scrambling to upgrade to TLS 1.1 or 1.2. How quickly that occurs depends on how many attacks occur in the wild.
The BEAST tool, presented last Friday at the 2011 Ekoparty Security Conference in Argentina, made real a theoretical SSL/TLS vulnerability first documented 10 years ago. It allows an attacker with previous MitM (man-the-middle) access to compromise a user's SSL/TLS-protected HTTPS cookie. This would allow an attacker to hijack the victim's active HTTPS-protected session or listen in on the previously cryptographically protected network stream. (Download Duong and Rizzo's paper on the BEAST attack [pdf].)
MitM attacks are fairly easy to do when the attacker and victim are located on the same local network (such as wireless networks, VPNs, or corporate LANs). Some hacking tools, such as Cain & Abel, make MitM attacks and network packet sniffing truly a click of a button.
An old flaw turns critical
BEAST takes advantage of the fact that versions of TLS and SSL prior to TLS 1.1 (often referred to as SSL 3.2) do not use an implicit random IV (initialization vector) with each subsequent data stream initiated in an HTTPS connection. This particular SSL/TLS flaw was first discussed at an OpenSSL development forum in 2002. It's similar to the cryptographic weakness found in the WEP protocol, which significantly weakened the protection of wireless networks.