Medium

OpenSSL TLS 1.3 server may choose unexpected key agreement group

IdentifiersCVE-2026-2673CWE-670

CVE-2026-2673 is a low-severity flaw in OpenSSL TLS 1.3 server-side key agreement group selection affecting OpenSSL 3.5 and 3.6. When a server configures its TLS 1.3 supported groups using the 'DEFAULT' keyword to interpolate the built-in default group list into a custom configuration, an implementation defect causes the default list to lose its intended tuple structure. Instead of preserving distinct tuples of roughly equivalent security, OpenSSL treats all configured server-supported groups as a single sufficiently secure tuple. As a result, when a client supports a more preferred mutually supported group but does not include a corresponding keyshare in its initial ClientHello, the server may fail to send the expected Hello Retry Request (HRR) and may negotiate a less preferred group from the client's predicted keyshares instead. This can interfere with intended negotiation of hybrid post-quantum groups such as X25519MLKEM768 when clients initially predict only classical groups such as X25519. The issue is outside the OpenSSL FIPS boundary, and OpenSSL FIPS modules are not affected.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation does not provide code execution or memory corruption. The impact is a silent downgrade from the server's intended TLS 1.3 key agreement preference policy: the connection may complete using a less preferred, though still mutually supported, key exchange group instead of a more preferred group that should have been selected after HRR. In practice this can prevent negotiation of intended hybrid post-quantum groups and weaken the server's configured cryptographic posture relative to policy, particularly in deployments relying on tuple-based preference ordering introduced in OpenSSL 3.5. The primary consequence is cryptographic policy degradation rather than compromise of confidentiality or integrity through direct exploitation.

Mitigation

If you can’t patch tonight, do this now.

Avoid using the 'DEFAULT' keyword in custom TLS 1.3 server key agreement group configurations on affected versions. As a workaround, either use the built-in default group list directly without interpolation or explicitly define the desired groups and tuple structure in the server configuration. After changes, retest TLS 1.3 handshakes to confirm that HRR is issued when expected and that preferred groups, including hybrid post-quantum groups where applicable, are negotiated correctly.

Remediation

Patch, then assume compromise.

Upgrade affected versions to a fixed release. OpenSSL 3.6 users should upgrade to OpenSSL 3.6.2, and OpenSSL 3.5 users should upgrade to OpenSSL 3.5.6. OpenSSL states that 3.4, 3.3, 3.0, 1.0.2, and 1.1.1 are not affected. Where immediate package upgrades are not yet available, apply the upstream fixes from the published branch commits referenced by OpenSSL for 3.6 and 3.5.

PUBLIC EXPLOITS

Exploits

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VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

OpenSSL Software FoundationOpensslapplication

SiemensSimatic Cn 4100 Firmwareoperating_system

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

33 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

33 SOURCESView more in app

help net securityNews

Apr 8, 2026

OpenSSL 3.6.2 lands with eight CVE fixes - Help Net Security

Loss of key agreement group tuple structure when the DEFAULT keyword is used in server-side configuration of the key-agreement group list in OpenSSL.

cyberthroneNews

Apr 8, 2026

OpenSSL 3.6.2: The Moderate Severity Wave - TheCyberThrone

A configuration validation flaw where using the DEFAULT keyword in server-side key-agreement group configuration can silently degrade negotiated key groups by losing tuple structure.

openssl libraryNews

Mar 13, 2026

[no-title]

A low-severity OpenSSL TLS 1.3 vulnerability where servers using the "DEFAULT" keyword in key exchange group configuration may select a less preferred mutually supported key agreement group and fail to trigger Hello Retry Request behavior as intended, potentially preventing negotiation of preferred post-quantum groups.

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Exposure mapping

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Threat actor evidence

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Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity29