Logjam.

A 2015 TLS attack that downgrades Diffie-Hellman key exchange to weak 512-bit export-grade primes and uses precomputation to break them. It belongs to the Attacks & Threats category of cybersecurity.

A 2015 TLS attack that downgrades Diffie-Hellman key exchange to weak 512-bit export-grade primes and uses precomputation to break them.

Logjam was disclosed in May 2015 by Adrian, Bhargavan, Durumeric, Heninger and others. A man-in-the-middle modifies the TLS handshake so the server selects DHE_EXPORT, forcing a 512-bit Diffie-Hellman group derived from a small set of well-known primes. Because attackers can precompute the discrete-log database for those primes, individual sessions are then broken in real time and the master secret is recovered. Logjam also showed that nation-state adversaries could plausibly precompute common 1024-bit groups. Mitigations include disabling export ciphersuites, using DH groups of at least 2048 bits with named groups or ECDHE, and prioritising TLS 1.3 which removes static export and weak DH support entirely.

Defences for Logjam typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: Logjam Attack, DHE_EXPORT downgrade.