How to configure mod_gnutls to use the RC4 cipher to mitigate the SSL/TLS vulnerability

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It’s been a while since the details of an SSL/TLS vulnerability have been released to the public. Since then, security experts have worked on the issue and have released a whitepaper describing how to mitigate the attack, known as BEAST (Browser Exploit Against SSL/TLS).

From the security researchers’ article:

The problem lies in the way that block ciphers are used in SSL/TLS. Block ciphers are generally operated in one of several modes that define how encrypted blocks are manipulated to ensure complete confidentiality. Cipher Block Chaining, or CBC mode, is used in SSL for all block ciphers, including AES and Triple-DES. The BEAST attack relies on a weakness in the way CBC mode is used in SSL and TLS. Non-CBC cipher suites, such as those using the RC4 stream encryption algorithm, are not vulnerable.

There have been several suggested mitigations that can be put into play from the perspective of the client, such as reorganizing the way the data is sent in the encrypted stream. Servers can protect themselves by requiring a non-CBC cipher suite. One such cipher suite is rc4-sha, which is widely supported by clients and servers.

Researchers have concluded that the RC4 (Alleged RC4) based cipher suites are not vulnerable to the BEAST attack, while CBC (Cipher Block Chaining mode) based cipher suites are. This involves both the TLS 1.0 and the SSL 3.0 protocols. On the contrary, TLS 1.1 and 1.2 have not been found to be vulnerable, but their use is very limited since they haven’t been adopted by the majority of HTTP clients and servers yet.

So, the use of RC4 based ciphers is all that is left for the moment. The security experts have released a list of cipher suites that is suitable for use in the configuration of the mod_ssl module for httpd:

SSLHonorCipherOrder on
SSLCipherSuite !aNULL:!eNULL:!EXPORT:!DSS:!DES:RC4-SHA:RC4-MD5:ALL

They have also released a one-liner list of ciphers suitable for use in the relevant fields of the Local Group Policy Editor in Windows Server boxes.

However, there is no info about configuring the mod_gnutls module for apache to use RC4 based ciphers, so, as a dedicated user of mod_gnutls, I decided to release this tip. All you have to do is set the preferred ciphers in the GnuTLSPriorities directive. In this example we use the TLS 1.0 protocol:

GnuTLSPriorities NONE:+VERS-TLS1.0:+ARCFOUR-128:+RSA:+SHA1:+COMP-NULL

Visiting a secure web site that has been configured using any of the methods described above and by checking the information of the secure connection to that website, you should see the following message:

Firefox message about using RC4 encryption cipher

Firefox message about using RC4 encryption cipher

This means that everything is working correctly.

As always, comments and suggestions are welcome and appreciated.

How to configure mod_gnutls to use the RC4 cipher to mitigate the SSL/TLS vulnerability by George Notaras, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Copyright © 2011 - Some Rights Reserved

One response on “How to configure mod_gnutls to use the RC4 cipher to mitigate the SSL/TLS vulnerability

  1. Aeomer Permalink →

    What worries me about replacing the chosen CBC mode cipher with RC4 is the use of the MD5 hash algol. Some of the serious issues with lower versions of SSL came about because of the statistically frequent hash collisions when using MD5. I’d expect everyone to be worried the same thing could happen again with RC4/MD5 – surely memories are not that short!

    What are the options for, say, RC4/SHA256 ?

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