CVE-2026-6331
HMAC zero-length tag forgery in EVP_DigestVerifyFinal, where a zero-length tag could be accepted as valid during HMAC verification. In the OpenSSL-compatibility HMAC verify path the supplied signature length was only checked as not exceeding the MAC length, so a zero-length or otherwise truncated tag could pass verification. The fix requires the supplied tag length to exactly equal the MAC length and rejects a zero-length MAC, so a forged short or empty tag is no longer accepted.
CSIRTS triage
- What
- There is a forgery vulnerability in HMAC zero-length tags in EVP_DigestVerifyFinal.
- Who is affected
- Deployments using affected cryptographic libraries.
- Urgency
- Remediation is high urgency due to the high severity rating.
- Action
- Patch the cryptographic library to address the forgery vulnerability.
AI-assisted analysis generated from the source advisory — verify against the original.
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Exploitation outlook
- Low exploitation risk0.15% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 4% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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