Summary

SSL version 3.0 is no longer secure. Browsers and websites need to turn off SSLv3 and use more modern security protocols as soon as possible, in order to avoid compromising users’ private information.

We have a plan to turn off SSLv3 in Firefox. This plan was developed with other browser vendors after a team at Google discovered a critical flaw in SSLv3, which can allow an attacker to extract secret information from inside of an encrypted transaction. SSLv3 is an old version of the security system that underlies secure Web transactions and is known as the “Secure Sockets Layer” (SSL) or “Transport Layer Security” (TLS).

Issue

In late September, a team at Google discovered a serious vulnerability in SSL 3.0 that can be exploited to steal certain confidential information, such as cookies. This vulnerability, known as “POODLE”, is similar to the BEAST attack. By exploiting this vulnerability, an attacker can gain access to things like passwords and cookies, enabling him to access a user’s private account data on a website.

Any website that supports SSLv3 is vulnerable to POODLE, even if it also supports more recent versions of TLS. In particular, these servers are subject to a downgrade attack, in which the attacker tricks the browser into connecting with SSLv3. This relies on a behavior of browsers called insecure fallback, where browsers attempt to negotiate lower versions of TLS or SSL when connections fail.

Today, Firefox uses SSLv3 for only about 0.3% of HTTPS connections. That’s a small percentage, but due to the size of the Web, it still amounts to millions of transactions per day.

Impact

The POODLE attack can be used against any browser or website that supports SSLv3. This affects all current browsers and most websites. As noted above, only 0.3% of transactions actually use SSLv3. Though almost all websites allow connections with SSLv3 to support old browsers, it is rarely used, since there are very few browsers that don’t support newer versions of TLS.

Sites that require SSLv3 will remain vulnerable until they upgrade to a more recent version of TLS. According to measurements conducted by Mozilla and the University of Michigan, approximately 0.42% of the Alexa top million domains have some reliance on SSLv3 (usually due to a subdomain requiring SSLv3).

Status

SSLv3 will be disabled by default in Firefox 34, which will be released on Nov 25. The code to disable it is landing today in Nightly, and will be promoted to Aurora and Beta in the next few weeks. This timing is intended to allow website operators some time to upgrade any servers that still rely on SSLv3.

As an additional precaution, Firefox 35 will support a generic TLS downgrade protection mechanism known as SCSV. If this is supported by the server, it prevents attacks that rely on insecure fallback.

Additional Precautions

For Firefox users, the simplest way to stay safe is to ensure that Firefox is configured to automatically update. Look under Preferences / Advanced / Update and make sure that “Automatically install updates” is checked.

For users who don’t want to wait till November 25th (when SSLv3 is disabled by default in Firefox 34), we have created the SSL Version Control Firefox extension to disable SSLv3 immediately.

Website operators should evaluate their traffic now and disable SSLv3 as soon as compatibility with legacy clients is no longer required. (The only remaining browser that does not support TLSv1.0 is Internet Explorer 6). We recommend following the intermediate configuration level from Mozilla’s Server Site TLS guidelines.

We realize that many sites still receive traffic from IE6 and cannot disable SSLv3 entirely. Those sites may have to maintain SSLv3 compatibility, and should actively encourage their users to migrate to a more secure browser as soon as possible.