TLS key-exchange downgrade via lost tuple boundaries
Published Mar 13, 2026 · Updated Mar 13, 2026
Algorithm downgrade in OpenSSL 3.5.0 through 3.5.5 and 3.6.0 through 3.6.1 allows remote attackers to negotiate a less-preferred TLS 1.3 key exchange. The DEFAULT expansion path in server-side group-list parsing discards tuple boundaries, treats every supported group as equally secure, and suppresses the required Hello Retry Request. Exposure requires a TLS 1.3 server configuration that interpolates DEFAULT and a client that omits a mutually supported preferred keyshare, allowing a classical group to replace a hybrid post-quantum group.
Summary
What happened
Algorithm downgrade in OpenSSL 3.5.0 through 3.5.5 and 3.6.0 through 3.6.1 allows remote attackers to negotiate a less-preferred TLS 1.3 key exchange. The DEFAULT expansion path in server-side group-list parsing discards tuple boundaries, treats every supported group as equally secure, and suppresses the required Hello Retry Request. Exposure requires a TLS 1.3 server configuration that interpolates DEFAULT and a client that omits a mutually supported preferred keyshare, allowing a classical group to replace a hybrid post-quantum group.
The record
- CVE
- CVE-2026-2673
- Published
- Mar 13, 2026
- Updated
- Mar 13, 2026
- Vendor
- OpenSSL Project
- Product
- OpenSSL
- Classifications
- CWE-757, T1600
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 13, 2026CVE publishedPublication date reported by the CVE source.
- Mar 13, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=3.5.0 <3.5.6
- Affected versionversion=3.6.0 <3.6.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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