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CVE-2026-2673 — OpenSSL TLS 1.3 server may choose unexpected key agreement group

Affected: OpenSSL, Siemens SIMATIC CN 4100

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…

USN-8155-1 — openssl vulnerabilities

Affected: Ubuntu:22.04:LTS: openssl, Ubuntu:24.04:LTS: openssl, Ubuntu:25.10: openssl

Viktor Dukhovni discovered that OpenSSL incorrectly negotiated the expected preferred key exchange group when used as a TLS 1.3 server. This could result in a less preferred key exchange being used, contrary to expectations. This issue only affected Ubuntu 25.10. (CVE-2026-2673)

Igor Morgenstern discovered that OpenSSL incorrectly handled certain memory operations when used as a DANE client. A remote attacker could use this issue to cause OpenSSL to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2026-28387)

Igor Morgenstern discovered that OpenSSL incorrectly handled certain memory operations when processing a delta CRL. A remote attacker could possibly use this issue to cause OpenSSL to crash, resulting in a denial of service. (CVE-2026-28388)

Nathan Sportsman, Daniel Rhea, and Jaeho Nam discovered that OpenSSL incorrectly handled certain memory operations when processing a crafted CMS EnvelopedData message with KeyAgreeRecipientInfo. A remote attacker could possibly use this issue to cause OpenSSL to crash, resulting in a denial of service. (CVE-2026-28389)

Muhammad Daffa, Joshua Rogers, and Chanho Kim discovered that OpenSSL incorrectly handled processing of a crafted CMS EnvelopedData message with KeyTransportRecipientInfo. A remote attacker could possibly use this issue to cause OpenSSL to crash, resulting in a denial of service. (CVE-2026-28390)

Quoc Tran discovered that OpenSSL incorrectly handled hexadecimal conversion on 32-bi…