# CVE-2026-6331: wolfssl: HMAC zero-length tag forgery in EVP_DigestVerifyFinal, where a zero-length tag could be accepted as valid during HMAC...

CVE-2026-6331 is fixed in wolfssl on 1 platform, first by DEBIAN-CVE-2026-6331, released July 18, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-6331
- Priority: High
- CVSS: 7.5 (high)
- EPSS: 0.0019
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 25, 2026

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | wolfssl Debian 12.15 (bookworm) | 5.5.4-2+deb12u3 | [DEBIAN-CVE-2026-6331](https://patchblox.com/security/data/fix/linux/debian-cve-2026-6331.md) | Jul 18, 2026 |
| Linux | wolfssl Debian 13.5 (trixie) | 5.7.2-0.1+deb13u2 | [DEBIAN-CVE-2026-6331](https://patchblox.com/security/data/fix/linux/debian-cve-2026-6331.md) | Jul 18, 2026 |

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Data as of 2026-11-07T18:46:35Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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