October 14, 2014

This POODLE bites: exploiting the SSL 3.0 fallback

Today we are publishing details of a vulnerability in the design of SSL version 3.0. This vulnerability allows the plaintext of secure connections to be calculated by a network attacker. I discovered this issue in collaboration with Thai Duong and Krzysztof Kotowicz (also Googlers).

SSL 3.0 is nearly 18 years old, but support for it remains widespread. Most importantly, nearly all browsers support it and, in order to work around bugs in HTTPS servers, browsers will retry failed connections with older protocol versions, including SSL 3.0. Because a network attacker can cause connection failures, they can trigger the use of SSL 3.0 and then exploit this issue.

Disabling SSL 3.0 support, or CBC-mode ciphers with SSL 3.0, is sufficient to mitigate this issue, but presents significant compatibility problems, even today. Therefore our recommended response is to support TLS_FALLBACK_SCSV. This is a mechanism that solves the problems caused by retrying failed connections and thus prevents attackers from inducing browsers to use SSL 3.0. It also prevents downgrades from TLS 1.2 to 1.1 or 1.0 and so may help prevent future attacks.

Google Chrome and our servers have supported TLS_FALLBACK_SCSV since February and thus we have good evidence that it can be used without compatibility problems. Additionally, Google Chrome will begin testing changes today that disable the fallback to SSL 3.0. This change will break some sites and those sites will need to be updated quickly.

In the coming months, we hope to remove support for SSL 3.0 completely from our client products.

Thank you to all the people who helped review and discuss responses to this issue.

Posted by Bodo Möller, Google Security Team

[Updated Oct 15 to note that SSL 3.0 is nearly 18 years old, not nearly 15 years old.]

5 comments:

  1. I can't wait to see POODLE take over the internet! Thank goodness we have heroes to save us from the evils of SSL 3.0 and such and such, etc.

    ReplyDelete
  2. Not sure how to enable TLS_FALLBACK_SCSV on apache or nginx.

    To test I just disabled SSLv2 and SSLv3 on my personal https web site, so far so good all browsers (modern) traffic goes thru.

    ReplyDelete
  3. nvd still says it is under review. Is there a patch coming?

    ReplyDelete
  4. It's strange, but google.com is also vulnerable to POODLE attack:
    https://www.ssllabs.com/ssltest/analyze.html?d=google.com&s=74.125.239.96&hideResults=on

    ReplyDelete

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