There is a new man-in-the-middle attack that goes against the cryptographic protocol TLS, used to encrypt online banking and shopping, and other sensitive connections, to thwart eavesdroppers, researchers said.

TheTriple Handshake attack can overcome key checks carried out to verify the identity of a user connecting to a server over a secure connection.

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It is possible to for a malicious system to intercept a user’s login credential (a client certificate in this case) and masquerade as that victim with any server that also accepts the same credential, said researchers at the French National Institute for Research in Computer Science and Control (INRIA).

In addition, the attack has implications for the security of SSL (Secure Sockets Layer), the widely used predecessor to TLS (Transport Layer Security), the researchers said.

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“We have discovered a new class of attacks against applications that rely on the TLS Internet Standard for securing their communications,” said the researchers Karthikeyan Bhargavan, Antoine Delignat-Lavaud, Cédric Fournet, Alfredo Pironti and Pierre-Yves Strub in a paper on the subject. “Essentially, if a user connects to a malicious website and presents a TLS client certificate, the malicious website can then impersonate the user at any other website that accepts the same TLS client certificate. The attacks work with all TLS and SSL versions, as well as the DTLS variant.”

Online websites and similar services typically use usernames and passwords rather than TLS client certificates to log users in. This limits the impact of the attack. In practice the flaw becomes more of a problem when it comes to logging into Wi-Fi access points.

“Variants of our attacks apply to specific scenarios of standard authentication protocols that rely on TLS, such as the PEAP Wi Fi authentication mechanism,” the researchers said, adding their exploit is similar to a previously uncovered cryptographic flaw – the 2009 Ray and Rex TLS renegotiation attack.

The researchers advocate short term application-level mitigations as well as long-term changes to the TLS protocol to strengthen the standard and safeguard its users against this latest attack and other assaults along the same lines.

“Let me stress that the attacks we found exploit a protocol-level issue, and not specific implementation bugs,” Pironti said. “We also propose short-term application-level mitigation, but we aim at getting the protocol fixed, which would solve the issue at its root.”

The French team have already notified major vendors of TLS software implementations (at Microsoft, Google and others) as well as the Internet Engineering Task Force (IETF) about their research. The researchers disclosed the attack and possible countermeasures at an IETF meeting in London on Tuesday night.

Click here for an outline of their research.


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