Logjam attack

"This server is vulnerable to Logjam attacks. Make sure you have TLSv1.2 protocol enabled on your server. Disable support for any export suites on your server and disable all insecure ciphers."

Problem

During the 1990s, the US government set up restrictions for exporting encryption systems. These rules aimed at SSL 3.0 and TLS 1.0 enforced support for weaker DHE_EXPORT cipher suites. Eventually the rules were changed, and the export restrictions removed. Unfortunately, some TLS servers still used a two-group configuration: 1024-bit for standard DHE key exchanges and 512-bit for legacy DHE_EXPORT, while all browsers still support the weak DHE_EXPORT ciphers.

A team of researchers revealed (see Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice) that the old DHE export-grade cryptographic suites are still being used. They also discovered that servers with support for these DHE_EXPORT cipher suites enabled could allow a man-in-the-middle (MITM) to trick clients that support the weak DHE_EXPORT cipher suites into downgrading their connection to a 512-bit key exchange. Then MITM attackers could use preconfigured data along with today’s computing power to crack the keys. Once the encryption of the session is cracked, the MITM can steal any ‘secured’ personal information from the session.

To be vulnerable both these conditions must be met:

  • The server must support DHE_EXPORT cipher suites.
  • The client must support DHE_EXPORT cipher suites

Solution

Server-side

  • Use DigiCert Discovery to scan for vulnerable servers and to see a list of enabled ciphers.
  • Disable support for all DHE_EXPORT cipher suites on your servers.
  • Use a 2048-bit or stronger for all private keys for any SSL/TLS certificate.
  • Enable elliptic curve Diffie-Hellman (ECDHE) key exchange on their servers.

Client-side

Vulnerable clients include Internet Explorer, Chrome, Safari, and Mozilla.

  • Update browsers to the most recent version (post 05/20/2015).
  • Watch for updates to other client-side software to address security concerns.

As part of the research team's disclosure, they created a comprehensive guide to mitigate this vulnerability for both Servers and Clients. See Guide to Deploying Diffie-Hellman for TLS.