CVE-2026-6331 - HMAC zero-length tag forgery MAC verification bypass

CVE ID + Title + Severity + Publication Date

  • CVE ID: CVE-2026-6331
  • Title: HMAC zero-length tag forgery MAC verification bypass
  • Severity: High (CVSS v3.1 score 7.5)
  • Publication date: 2026-06-25
  • Affected tech stack: application/source
  • Revenue tags: sellable_to_fintech, enterprise_blocker, zero_day_gold

One-sentence business risk

High MAC verification bypass in HMAC zero-length tag forgery can turn a routine dependency or application endpoint into data theft, account takeover, service outage, or code execution risk that blocks production releases and customer renewals.

Root cause and affected versions

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.

  • Vulnerable range: See vendor advisory and package manager resolution for the affected range.
  • Fixed or mitigated range: Upgrade to the first vendor-fixed release or apply the referenced patch/backport.
  • Public exploit/PoC status: Treat as public or reproducible when the advisory references a GitHub issue, exploit repository, VulnCheck entry, Wordfence entry, or public PoC. Validate only in isolated test environments.

Exact vulnerable code pattern

/* CVE-2026-6331: validation accepts mismatched or truncated security context. */
if (provided_len <= expected_len && memcmp(provided, expected, provided_len) == 0) {
    return VERIFY_OK;
}

Fixed / mitigated code pattern

if (provided_len != expected_len || expected_len == 0) {
    return VERIFY_FAIL;
}
if (CRYPTO_memcmp(provided, expected, expected_len) != 0) {
    return VERIFY_FAIL;
}
return verify_context_binding(session, sni, alpn, certificate_constraints);

Step-by-step integration guide

  1. Inventory every direct and transitive use of HMAC zero-length tag forgery with package manifests, lockfiles, SBOMs, container images, vendored source, firmware manifests, and deployment overlays.
  2. Confirm whether the vulnerable path is reachable: HMAC zero-length tag forgery and attacker-controlled parameter input are the first review anchors.
  3. Upgrade to the vendor-fixed release or apply the referenced patch/backport; regenerate lockfiles, image digests, SBOMs, and deployment manifests.
  4. Patch owned code so untrusted input is validated before it reaches the vulnerable sink; use the fixed pattern above as the minimum implementation bar.
  5. Add a regression test that sends the advisory-shaped payload and proves the operation is rejected without corrupting memory, crossing trust boundaries, or changing privileged state.
  6. Run unit tests, integration tests for the affected route/parser/protocol, dependency audit, SAST rules for the sink class, and container or firmware build validation.
  7. Deploy through staged rollout with telemetry on rejected exploit-shaped inputs and a rollback plan that does not restore the vulnerable version.

Alternative mitigations

  • Disable the affected endpoint, parser, protocol feature, plugin, decoder, runner option, or integration until the fixed build is live.
  • Put a gateway/WAF rule in front of exposed HTTP paths to block advisory-shaped parameters while application code is patched.
  • For native parsers and protocol libraries, isolate processing in a sandboxed worker with seccomp/AppArmor, memory limits, ASAN canaries in staging, and crash restart rate limits.
  • For authz/authn flaws, require an additional server-side role check at the route/service layer and invalidate sessions or tokens touched during the vulnerable window.
  • For supply-chain tooling, pin the fixed version in CI images and block vulnerable versions with dependency policy.

Detection signature

rg -n "HMAC zero-length tag forgery|CVE-2026-6331|HMAC zero-length tag forgery|input" . and add an integration test for the advisory-shaped input.

Copy-paste skill

You are remediating CVE-2026-6331: HMAC zero-length tag forgery MAC verification bypass.

Goal: produce a reviewer-ready PR that removes exposure to CVE-2026-6331, adds regression coverage, and documents deployment/operator checks.

Rules:
- Scope only CVE-2026-6331 and directly related `HMAC zero-length tag forgery` usage.
- Do not run public PoCs against production, shared staging, customer systems, or third-party infrastructure.
- Treat credentials, tokens, session data, private files, tenant IDs, and exploit samples as sensitive.
- Prefer the vendor-fixed release. Use a temporary mitigation only when upgrade is blocked and document the owner/date for removal.
- If this repository does not own an affected runtime, write `TRIAGE.md` with evidence instead of making unrelated edits.

Steps:
1. Search for `HMAC zero-length tag forgery`, `CVE-2026-6331`, vulnerable package names, `affected handlers`, and parameter `input`.
2. Identify every resolved vulnerable version in manifests, lockfiles, images, SBOMs, vendored code, and deployment templates.
3. Upgrade or patch to the fixed version: Upgrade to the first vendor-fixed release or apply the referenced patch/backport.
4. Replace the vulnerable code shape with input validation, parameterized APIs, strict bounds checks, canonical path checks, or explicit authz as appropriate.
5. Add a negative regression test for the advisory-shaped payload and a positive test for legitimate behavior.
6. Add detection from the signature section and document operator review for suspicious requests, crashes, privilege changes, or file writes.
7. Run the relevant test/build/audit commands and include outputs in the PR.

Stop and write `TRIAGE.md` if the affected runtime is not present, the fix requires production probing, or ownership of the vulnerable deployment is outside this repo.

Keywords and tags

  • Keywords: CVE-2026-6331, HMAC zero-length tag forgery, MAC verification bypass, c/cpp, application/source
  • Revenue tags: sellable_to_fintech, enterprise_blocker, zero_day_gold

References