Client DoS due to large DH parameter

Summary

During key agreement in a TLS handshake using a DH(E) based ciphersuite a malicious server can send a very large prime value to the client. This will cause the client to spend an unreasonably long period of time generating a key for this prime resulting in a hang until the client has finished. This could be exploited in a Denial Of Service attack. Fixed in OpenSSL 1.1.0i-dev (Affected 1.1.0-1.1.0h). Fixed in OpenSSL 1.0.2p-dev (Affected 1.0.2-1.0.2o).

Timeline

1. Jun 12, 2018, 1:00 PM UTCCVE published
2. Oct 8, 2026, 9:08 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Oct 8, 2026, 9:13 PM UTCRecord last updated
4. Oct 22, 2026EPSS reached 10%

   FIRST estimates at least a 10% chance of exploitation in the next 30 days

Affected versions

• OpenSSL OpenSSL

  Fixed in OpenSSL 1.1.0i-dev (Affected 1.1.0-1.1.0h), Fixed in OpenSSL 1.0.2p-dev (Affected 1.0.2-1.0.2o)

References 25