Cybersecurity glossary
What is Logjam (CVE-2015-4000)?
Learn what the Logjam attack (CVE-2015-4000) is, how TLS export-grade Diffie-Hellman enabled downgrade and decryption risks, and how strong DH parameters and modern TLS stop it.
Definition
Logjam, tracked as CVE-2015-4000, is a TLS vulnerability class in which a man-in-the-middle can downgrade connections to export-grade Diffie-Hellman key exchange with weak parameters, enabling computation of session keys and decryption of affected TLS traffic.
Why Logjam mattered
Diffie-Hellman (DH) key exchange underpins forward secrecy in many TLS configurations. In 2015, researchers showed that export-grade DH and surprisingly common weak groups made that exchange brittle. Logjam (CVE-2015-4000) demonstrated that a MITM could downgrade connections to those weak parameters and then compute session keys.
Together with FREAK, Logjam forced operators to delete the last remnants of 1990s export cryptography from production TLS.
How Logjam works
Attacker becomes MITM
The adversary sits on the path and manipulates the TLS handshake.
Force export-grade DHE
Cipher negotiation is altered toward DHE_EXPORT or similarly weak Diffie-Hellman options.
Server uses a weak DH group
Small prime groups—historically 512-bit export parameters—are used for key exchange.
Attacker computes the discrete log
With enough precomputation against common groups, the attacker recovers the shared secret.
Session traffic is decrypted
HTTPS content and credentials on the downgraded connection become readable.
Ecosystem removes weak DH
Clients reject small groups; servers disable export suites and upgrade parameters.
Logjam vs FREAK
| Property | Logjam | FREAK |
|---|---|---|
| CVE | CVE-2015-4000 | CVE-2015-0204 |
| Weak primitive | Export / weak Diffie-Hellman | Export-grade RSA |
| Primary mitigation | Disable export DHE; use strong ECDHE/DHE | Disable export RSA; patch clients/servers |
Mitigation checklist
Disable export suites
Remove all EXPORT cipher suites from TLS terminators and clients.
Prefer ECDHE
Modern elliptic-curve ephemeral key exchange avoids finite-field DH pitfalls.
If DHE remains, use strong groups
Follow current guidance (for example, RFC 7919 groups) and reject tiny primes.
Patch TLS stacks
Update libraries and appliances that still accept weak DH parameters.
- Scan public endpoints for DHE_EXPORT and other obsolete key-exchange options.
- Enforce TLS 1.2+ with an allowlist of modern cipher suites.
- Verify CDNs and load balancers do not reintroduce weak DH for compatibility.
- Retire appliances that cannot disable export cryptography.
- Document approved key-exchange algorithms for engineering teams.
- Monitor TLS telemetry for unexpected downgrades where available.
- Treat shared/common DH primes as a risk factor; prefer ECDHE.
- Re-test after every TLS configuration change.
The practical takeaway
Logjam (CVE-2015-4000) showed that export-grade and weak Diffie-Hellman parameters could be forced and broken by a MITM, undermining TLS confidentiality. The fix is to refuse weak DH entirely and run modern key exchange.
If a server still offers export DHE or tiny DH groups, it is recreating a solved 2015 incident. Disable them and move on to ECDHE with current TLS baselines.
Related security terms
FREAK (CVE-2015-0204)
A related 2015 export-grade attack targeting RSA instead of Diffie-Hellman.
SSL/TLS
The protocols whose key-exchange negotiation Logjam abused.
Man-in-the-Middle (MITM)
Network position required to force the Logjam downgrade.
HTTPS
The common TLS deployment surface affected by Logjam.
Frequently asked questions
What is Logjam in simple terms?
Logjam is an attack where a network attacker forces a TLS connection to use a very weak Diffie-Hellman setup left over from old export rules, then computes the keys and reads the traffic.
What is CVE-2015-4000?
CVE-2015-4000 identifies the Logjam-related TLS Diffie-Hellman export downgrade vulnerability class disclosed in 2015.
How is Logjam different from FREAK?
FREAK focuses on export-grade RSA. Logjam focuses on export-grade Diffie-Hellman and weak DH groups.
Does Logjam still matter?
Modern configurations that disable export ciphers and use strong DH/ECDHE parameters are safe from classic Logjam. Legacy servers that still allow weak DHE remain risky.
What Diffie-Hellman size is considered weak?
Historically, 512-bit export DH was the core problem; even 1024-bit groups were debated as insufficient against well-resourced attackers. Prefer modern elliptic-curve key exchange.
How do you mitigate Logjam?
Disable export cipher suites, prefer ECDHE, use strong parameters if DHE is required, patch TLS libraries, and enforce modern TLS versions.
Did browsers change behavior after Logjam?
Yes. Client and library updates rejected weak DH groups and export suites, shrinking the practical attack surface.
References
Explore authoritative guidance and frameworks related to logjam (cve-2015-4000).
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