Recently, a new vulnerability in Diffie-Hellman, informally referred to as 'logjam' has been published, for which this page has been put together suggesting how to counter the vulnerability:
We have three recommendations for correctly deploying Diffie-Hellman
for TLS:
- Disable Export Cipher Suites. Even though modern browsers no longer
support export suites, the FREAK and Logjam attacks allow a
man-in-the-middle attacker to trick browsers into using export-grade
cryptography, after which the TLS connection can be decrypted. Export
ciphers are a remnant of 1990s-era policy that prevented strong
cryptographic protocols from being exported from United States. No
modern clients rely on export suites and there is little downside in
disabling them.- Deploy (Ephemeral) Elliptic-Curve Diffie-Hellman
(ECDHE). Elliptic-Curve Diffie-Hellman (ECDH) key exchange avoids all
known feasible cryptanalytic attacks, and modern web browsers now
prefer ECDHE over the original, finite field, Diffie-Hellman. The
discrete log algorithms we used to attack standard Diffie-Hellman
groups do not gain as strong of an advantage from precomputation, and
individual servers do not need to generate unique elliptic curves.- Generate a Strong, Unique Diffie Hellman Group. A few fixed groups are
used by millions of servers, which makes them an optimal target for
precomputation, and potential eavesdropping. Administrators should
generate unique, 2048-bit or stronger Diffie-Hellman groups using
"safe" primes for each website or server.
What are the best-practice steps I should take to secure my server as per the above recommendations?
Best Answer
From the article you linked, there are three recommended steps to protect yourself against this vulnerability. In principle these steps apply to any software you may use with SSL/TLS but here we will deal with the specific steps to apply them to Apache (httpd) since that is the software in question.
Dealt with in the configuration changes we'll make in 2. below (
!EXPORT
near the end of theSSLCipherSuite
line is how we'll disable export cipher suites)For this, you need to edit a few settings in your Apache config files - namely
SSLProtocol
,SSLCipherSuite
,SSLHonorCipherOrder
to have a "best-practices" setup. Something like the following will suffice:Note: as for which
SSLCipherSuite
setting to use, this is always changing, and it is a good idea to consult resources such as this one to check for the latest recommended configuration.To do so, you can run
openssl dhparam -out dhparams.pem 2048
.Note that this will put significant load on the server whilst the params are generated - you can always get around this potential issue by generating the params on another machine and using
scp
or similar to transfer them onto the server in question for use.To use these newly-generated
dhparams
in Apache, from the Apache Documentation:(emphasis mine)
which is then followed by a standard 1024-bit DH parameter. From this we can infer that the custom-generated DH parameters may simply be appended to the relevant
SSLCertificateFile
in question.To do so, run something similar to the following:
cat /path/to/custom/dhparam >> /path/to/sslcertfile
Alternatively, as per the Apache subsection of the article you originally linked, you may also specify the custom dhparams file you have created if you prefer not to alter the certificate file itself, thusly:
SSLOpenSSLConfCmd DHParameters "/path/to/dhparams.pem"
in whichever Apache config(s) are relevant to your particular SSL/TLS implementation - generally in
conf.d/ssl.conf
orconf.d/vhosts.conf
but this will differ depending on how you have configured Apache.It is worth noting that, as per this link,
On Debian Wheezy upgrade apache2 to 2.2.22-13+deb7u4 or later and openssl to 1.0.1e-2+deb7u17. The above SSLCipherSuite does not work perfectly, instead use the following as per this blog:
You should check whether your Apache version is later than these version numbers depending on your distribution, and if not - update it if at all possible.
Once you have performed the above steps to update your configuration, and restarted the Apache service to apply the changes, you should check that the configuration is as-desired by running the tests on SSLLabs and on the article related to this particular vulnerability.