Secure your Apache server against LOGJAM

Some time ago I wrote a post about the dismaying history of US government attempts to regulate encryption out of existence. I had to omit quite a bit; it was a post and not a book after all. One of the details left out of the story was the DHE_EXPORT cipher suites. During the 90's, developers were forced by the US government to us deliberately insecure ciphers when communicating with entities in foreign countries (readers will remember from the last post that law makers were convinced that encryption should fall under the same rules as weapons technology, and thus could not be shared with anyone outside the Father Land). These insecure ciphers became DHE_EXPORT. The DH stands for Diffie-Hellman; the key exchange system that bears their name was first published in 1976.

Along with the cipher suite was a mechanism to force a normal encrypted transaction to downshift to a lower-bit DHE_EXPORT cipher. As so many short-sighted technology regulations have done in the past, this silly bit of Washington DC-brand programming has come back to haunt us in the form of the LOGJAM vulnerability. Until just a few days ago, all major browsers continued to support these deprecated DHE_EXPORT ciphers, as have a variety of applications as fundamental to web infrastructure as OpenSSL.

The exploit is described in detail on a website hosted by the researchers responsible for its discovery - weakdh.org which also hosts their paper on the same subject (PDF).

Meanwhile, patching your Apache server is simple: Apache HTTP Server (mod_ssl)

SSL parameters can globally be set in httpd.conf or within specific virtual hosts.

Cipher Suites

Disable support for SSLv2 and SSLv3 and enable support for TLS, explicitly allow/disallow specific ciphers in the given order :

SSLProtocol             all -SSLv2 -SSLv3

SSLCipherSuite          ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA

SSLHonorCipherOrder     on

DH Parameters

In newer versions of Apache (2.4.8 and newer) and OpenSSL 1.0.2 or later, you can directly specify your DH params file as follows:

SSLOpenSSLConfCmd DHParameters "{path to dhparams.pem}"

If you are using Apache with LibreSSL, or Apache 2.4.7 and OpenSSL 0.9.8a or later, you can append the DHparams you generated earlier to the end of your certificate file.

Reload configuration

sudo service apache2 reload