A flaw in how TLS/DTLS, when CBC-mode encryption is used, communicates was reported. This vulnerability can allow for a Man-in-the-Middle attacker to recover plaintext from a TLS/DTLS connection, when CBC-mode encryption is used.

This flaw is in the TLS specification, and not a bug in a specific implementation (as such, it affects nearly all implementations). As such, it affects all TLS and DTLS implementations that are compliant with TLS 1.1 or 1.2, or with DTLS 1.0 or 1.2. It also applies to implementations of SSL 3.0 and TLS 1.0 that incorporate countermeasures to deal with previous padding oracle attacks. All TLS/DTLS ciphersuites that include CBC-mode encryption are potentially vulnerable.

To perform a successful attack, when TLS is used, a large number of TLS sessions are required (target plaintext must be sent repeatedly in the same position in the plaintext stream across the sessions). For DTLS, a successful attack can be carried out in a single session. The attacker must also be located close to the machine being attacked.

Further details are noted in the paper:

http://www.isg.rhul.ac.uk/tls/TLStiming.pdf

Currently Mozilla is working on final NSS fixes:

https://bugzilla.mozilla.org/showbug.cgi?id=822365

External References:

http://www.isg.rhul.ac.uk/tls/