CVE-2026-2673

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 client's 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 server's 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 client's

configuration results in only 'classical' groups (such as 'X25519' being the

only ones in the client's 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 client's 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 server's 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.