CVE-2018-0732: Client DoS due to large DH parameter

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).

— Ubuntu

OpenSSL is vulnerable to a denial of service, caused by the sending of a very large prime value to the client by a malicious server during key agreement in a TLS handshake. By spending an unreasonably long period of time generating a key for this prime, a remote attacker could exploit this vulnerability to cause the client to hang.

— IBM

OpenSSL versions 1.1.0 to 1.1.0h and 1.0.2 to 1.0.2o allow malicious servers to send very large primes to a client during DH(E) based TLS handshakes. 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.

External Reference:

https://www.openssl.org/news/secadv/20180612.txt

Upstream Patches:

https://github.com/openssl/openssl/commit/ea7abeeabf92b7aca160bdd0208636d4da69f4f4 https://github.com/openssl/openssl/commit/3984ef0b72831da8b3ece4745cac4f8575b19098

— Red Hat