CVE-2026-2673 in OpenSSL
Summary
by MITRE • 03/13/2026
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.
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Analysis
by VulDB Data Team • 05/18/2026
The vulnerability described in CVE-2026-2673 affects OpenSSL TLS 1.3 servers that utilize the 'DEFAULT' keyword in their key exchange group configuration. This issue stems from a fundamental flaw in how the DEFAULT keyword is processed when interpolating built-in default group lists into server configurations. The problem manifests when servers attempt to negotiate preferred key exchange groups, particularly in scenarios involving hybrid post-quantum cryptography. When the DEFAULT keyword is used, the implementation defect causes the originally structured 'tuple' organization of key exchange groups to be lost, effectively treating all server-supported groups as a single security tuple instead of maintaining their intended hierarchical structure based on security equivalence. This structural degradation fundamentally undermines the intended security negotiation process, as the server fails to properly evaluate the relative preferences of different group tuples during handshake negotiations.
The operational impact of this vulnerability is significant for TLS 1.3 implementations that rely on the DEFAULT configuration approach, particularly in environments where hybrid post-quantum key exchange groups are supported. When a server configuration includes the DEFAULT keyword and subsequently loses the tuple structure, the server does not issue Hello Retry Requests (HRR) even when more preferred key exchange groups from higher security tuples are mutually supported by both client and server. This behavior is particularly problematic for post-quantum groups such as X25519MLKEM768, where client configurations may initially only predict classical groups like X25519 in their keyshare predictions. The failure to properly execute HRR mechanisms means that clients and servers may settle on less preferred key exchange groups even when more secure alternatives are available and supported by both parties, creating potential security degradation in the cryptographic handshake process.
The technical flaw specifically relates to how OpenSSL handles the transition from legacy flat group list configurations to the newer tuple-based approach that was introduced to better manage security preferences across different cryptographic strength categories. In the old flat list model, all supported groups were treated equally from a security perspective, which led to suboptimal group selection when the client's predicted keyshares did not align with the server's preferred group order. The new tuple-based approach was designed to address this by grouping similar security levels together and implementing proper HRR mechanisms when clients support groups from more preferred tuples that weren't initially predicted. However, when the DEFAULT keyword is used, this tuple structure is corrupted, causing the server to make incorrect security decisions during handshake negotiations and potentially selecting weaker cryptographic parameters than what would be optimal given the mutual capabilities of both parties.
This vulnerability aligns with CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues related to key management. The issue also maps to ATT&CK technique T1592 which involves reconnaissance techniques used to gather information about target systems, and T1593 which covers reconnaissance techniques related to network infrastructure mapping. The vulnerability represents a critical flaw in the TLS cryptographic handshake process that could enable adversaries to downgrade security to less preferred key exchange mechanisms, potentially impacting the confidentiality and integrity of encrypted communications. Organizations should prioritize upgrading to the patched versions of OpenSSL 3.5 and 3.6, as the affected versions contain a fundamental implementation defect that compromises the security of TLS 1.3 connections when using DEFAULT group configurations. The vulnerability does not affect OpenSSL FIPS modules since the problematic code resides outside the FIPS boundary, but standard OpenSSL installations using the DEFAULT keyword syntax remain at risk and require immediate remediation through the recommended version upgrades.