Security Update:Another Diffie-Hellman Key Exchange Flaw, Logjam Attack, Discovered
Date and Time
System Administrator and General Computer Users
IT experts uncovered a flaw in the way that some servers handle the Diffie-Hellman key exchange, a bug that’s somewhat similar to the FREAK attack and threatens the security of many Web and mail servers.. The flaw, called LogJam, can significantly weaken the encrypted connection between a user and a Web or email server.
Advanced Users: For a complete description of the vulnerabilities and affected versions visit The Logjam Attack at https://weakdh.org/.
AFFECTED SOFTWARE and DEVICES:
- Websites, mail servers, and other TLS-dependent services that support DHE_EXPORT ciphers are at risk for the Logjam attack.
- Websites that use one of a few commonly shared 1024-bit Diffie-Hellman groups may be susceptible.
WHAT'S THE PROBLEM?
Logjam attack can significantly weaken the encrypted connection between a user and a Web or email server and allow an attacker to read/change any data passed through this connection.
WHAT DO I NEED TO DO?
1. Web Site and/or System Administrators
a. If you have a web or mail server, you should disable support for export cipher suites and generate a unique 2048-bit Diffie-Hellman group.
- Refer to Guide to Deploying Diffie-Hellman for TLS with step-by-step instructions.
b. If you use SSH, you should upgrade both your server and client installations to the most recent version of OpenSSH, which prefers Elliptic-Curve Diffie-Hellman Key Exchange.
2. General User
a. Make sure you have the most recent version of your browser installed, and check for updates frequently. Google Chrome (including Android Browser), Mozilla Firefox, Microsoft Internet Explorer, and Apple Safari are all deploying fixes for the Logjam attack.
- Apple and Google are preparing patches to be released in the near future.
- Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice - https://weakdh.org/imperfect-forward-secrecy.pdf
- Guide to Deploying Diffie-Hellman for TLS - https://weakdh.org/sysadmin.html
- ITS Security & Policy - http://it.ucsf.edu/security