On Friday, a pair of security researchers will present a hacking tool which they claim decrypts secure Web requests to sites using the Transport Layer Security 1.0 protocol and SSL 3.0, allowing a person or program to hijack sessions with financial websites and other services. Juliano Rizzo and Thai Duong are unveiling their Browser Exploit Against SSL/TLS tool, dubbed BEAST, at the Ekoparty security conference in Buenos Aires.

The tool is based on a blockwise-adaptive chosen-plaintext attack, a man-in-the-middle approach that injects segments of plain text sent by the target's browser into the encrypted request stream to determine the shared key. The code can be injected into the user's browser through JavaScript associated with a malicious advertisement distributed through a Web ad service or an IFRAME in a linkjacked site, ad, or other scripted elements on a webpage.

Using the known text blocks, BEAST can then use information collected to decrypt the target's AES-encrypted requests, including encrypted cookies, and then hijack the no-longer secure connection. That decryption happens slowly, however; BEAST currently needs sessions of at least a half-hour to break cookies using keys over 1,000 characters long.

The attack, according to Duong, is capable of intercepting sessions with PayPal and other services that still use TLS 1.0—which would be most secure sites, since follow-on versions of TLS aren't yet supported in most browsers or Web server implementations.

While Rizzo and Duong believe BEAST is the first attack against SSL 3.0 that decrypts HTTPS requests, the vulnerability that BEAST exploits is well-known; BT chief security technology officer Bruce Schneier and UC Berkeley's David Wagner pointed out in a 1999 analysis of SSL 3.0 that "SSL will provide a lot of known plain-text to the eavesdropper, but there seems to be no better alternative." And TLS's vulnerability to man-in-the middle attacks was made public in 2009. The IETF's TLS Working Group published a fix for the problem, but the fix is unsupported by SSL.

PayPal spokesperson Anuj Nayar issued this statement regarding the threat embodied by BEAST: “We’ve seen speculation about new research into the security of the SSL technology used by most websites around the world. This research has not been made public, but we have already been looking into the SSL technology employed on the PayPal website and reinforcing our security. We’ll continue to do so once the research is released in the coming week. In the meantime, we can reassure our customers that PayPal’s top priority is the security of their accounts and their personal and financial information. We have dedicated teams of information security experts who continually review and strengthen our security systems. We’ll further review this once we have details of the research later in the week.”