2026-03-13 13:23:00
openssl
PUBLISHED
Issue summary: An OpenSSL TLS 1.3 server may fail to negotiate the expected preferred key exchange group when its key exchange group configuration includes the default by using the DEFAULT keyword. Impact summary: A less preferred key exchange may be used even when a more preferred group is supported by both client and server, if the group was not included among the clients initial predicated keyshares. This will sometimes be the case with the new hybrid post-quantum groups, if the client chooses to defer their use until specifically requested by the server. If an OpenSSL TLS 1.3 servers configuration uses the DEFAULT keyword to interpolate the built-in default group list into its own configuration, perhaps adding or removing specific elements, then an implementation defect causes the DEFAULT list to lose its tuple structure, and all server-supported groups were treated as a single sufficiently secure tuple, with the server not sending a Hello Retry Request (HRR) even when a group in a more preferred tuple was mutually supported. As a result, the client and server might fail to negotiate a mutually supported post-quantum key agreement group, such as X25519MLKEM768, if the clients configuration results in only classical groups (such as X25519 being the only ones in the clients initial keyshare prediction). OpenSSL 3.5 and later support a new syntax for selecting the most preferred TLS 1.3 key agreement group on TLS servers. The old syntax had a single flat list of groups, and treated all the supported groups as sufficiently secure. If any of the keyshares predicted by the client were supported by the server the most preferred among these was selected, even if other groups supported by the client, but not included in the list of predicted keyshares would have been more preferred, if included. The new syntax partitions the groups into distinct tuples of roughly equivalent security. Within each tuple the most preferred group included among the clients predicted keyshares is chosen, but if the client supports a group from a more preferred tuple, but did not predict any corresponding keyshares, the server will ask the client to retry the ClientHello (by issuing a Hello Retry Request or HRR) with the most preferred mutually supported group. The above works as expected when the servers configuration uses the built-in default group list, or explicitly defines its own list by directly defining the various desired groups and group tuples. No OpenSSL FIPS modules are affected by this issue, the code in question lies outside the FIPS boundary. OpenSSL 3.6 and 3.5 are vulnerable to this issue. OpenSSL 3.6 users should upgrade to OpenSSL 3.6.2 once it is released. OpenSSL 3.5 users should upgrade to OpenSSL 3.5.6 once it is released. OpenSSL 3.4, 3.3, 3.0, 1.0.2 and 1.1.1 are not affected by this issue.