Security Vulnerability: Vaudenay Timing Attack on CBC mode blockciphers

[Wan-Teh Chang]

Recently a timing-based attack on SSL/TLS implementations
of CBC mode block cipher suites was disclosed 
(http://lasecwww.epfl.ch/memo_ssl.shtml). At present the
implementation of SSL and TLS in NSS is susceptible to this
method. The flaw is exploited on the recipient of sensitive
data, which is normally servers. Servers are vulnerable to
the attack only if they implement all of the following:

    * TLS (supported by NSS 2.8 and later);
    * cipher suites that use block ciphers;
    * application protocols that are likely to receive
      sensitive data (for example, passwords) at exactly
      the same offset in many messages from a client.
We have implemented a countermeasure and will release NSS
patch releases soon. Until updated NSS libraries are
available, we recommend the following action:
    * Netscape/mozilla browser users do not need to take
      any action. They could choose to disable TLS or
      disable CBC mode block ciphersuites as a precaution
      against vulnerable servers.
    * Administrators of servers that are based on NSS 2.8
      or later and that enable TLS need to take action.
      They could disable TLS or disable CBC mode block
      cipher suites.
For more information, please see our article on this security
flaw.
(http://www.mozilla.org/projects/security/pki/nss/news/vaudenay-cbc.html)
Wan-Teh Chang