# CVE-2026-6331: HMAC zero-length tag forgery in EVP_DigestVerifyFinal

> Attackers can bypass HMAC verification by submitting zero-length tags, allowing forged signatures to be accepted as valid.

- **Severity:** Low 2.1 (`CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Jun 2026
- **Updated:** 26 Jun 2026
- **Also known as:** DEBIAN-CVE-2026-6331, GHSA-5hpf-pc4x-3jcf, JLSEC-2026-749, UBUNTU-CVE-2026-6331

## What is CVE-2026-6331?

CVE-2026-6331 is a low-severity vulnerability in Wolfssl Wolfssl, published on 25 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2026-6331?

It affects 1 product.


Products: Wolfssl Wolfssl.

## How do I fix CVE-2026-6331?

Apply the vendor's patch.

## Is CVE-2026-6331 being exploited?

There is no known exploitation of CVE-2026-6331 in the wild and no public exploit code.

## Other vulnerabilities in Wolfssl Wolfssl

- [CVE-2024-5991](https://insights.sbomify.com/CVE-2024-5991/): Buffer overread in domain name matching (critical 10.0)
- [CVE-2021-37155](https://insights.sbomify.com/CVE-2021-37155/): Security feature bypass in n/a (critical 9.8)
- [CVE-2020-36177](https://insights.sbomify.com/CVE-2020-36177/): Memory corruption in n/a (critical 9.8)
- [CVE-2014-2896](https://insights.sbomify.com/CVE-2014-2896/): Memory corruption in n/a (critical 9.8)
- [CVE-2014-2897](https://insights.sbomify.com/CVE-2014-2897/): Memory corruption in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6331 and fix it if it is affected.

What it is: CVE-2026-6331 is a low-severity vulnerability in Wolfssl Wolfssl, published on 25 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-6331 in the wild and no public exploit code.

Affected product: Wolfssl Wolfssl. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-6331/ (Markdown: https://insights.sbomify.com/CVE-2026-6331/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6331
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6331
- OSV: https://osv.dev/vulnerability/CVE-2026-6331

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6331/
Summaries are generated and may contain mistakes; the official description above is the record.