How to fix Logjam vulnerability in Apache HTTP & Nginx web server
Logjam vulnerability is found in TLS libraries (EXPORT cipher) on 20th May 2015 by team of computer scientists at CNRS, Inria Nancy-Grand Est, Inria Paris-Rocquencourt, Microsoft Research, Johns Hopkins University, University of Michigan, and the University of Pennsylvania: David Adrian, Karthikeyan Bhargavan, Zakir Durumeric,Pierrick Gaudry, Matthew Green, J. Alex Halderman, Nadia Heninger, Drew Springall, Emmanuel Thomé, Luke Valenta, Benjamin VanderSloot,Eric Wustrow, Santiago Zanella-Beguelin, and Paul Zimmermann.
The Logjam vulnerability helps attacker (main-in-the-middle) to downgrade TLS connections to 512-bit export-grade cryptography. This helps an attacker to read and modify any data transmitted over the network connection. You see, this is dangerous as an attacker can read credit card or sensitive information if your application is vulnerable to Logjam. This reminds me FREAK attack.
Logjam vulnerability can be on any protocols like HTTPS, SSH, IPSec, SMTP that leverage on TLS. As of 24th May, there are 8.4% of top 1 million domains are affected by Logjam vulnerability. Let’s take a look at below stats taken on 24th May.
Test if client (Browser) or Server (HTTPS) is vulnerable of Logjam
The easiest way to test would be to hit https://weakdh.org/ from the browser. If it’s safe you will get Good news like below.
If not, you will get a warning like below.
You can perform a test on below URL against your website to check if it’s safe from Logjam attack.
Ex: tested against google.com
Fix Logjam Attack Vulnerability
You can disable EXPORT cipher suits at respective web server configuration to mitigate this vulnerability.
Apache HTTP Server
Disable export cipher by adding following in SSL configuration file.
Restart Apache and that’s all.
Add following in nginx.conf file
ssl_ciphers '!EXPORT'; #Note: - If you already have ssl_ciphers configured, you just need to add !EXPORT in existing line instead of adding new one.
You may also refer here to fix this in Sendmail, Tomcat, IIS, HAProxy, Lighthttpd, etc.