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

Routine

HIGH 7.5REMOTEUPGRADE

CVE-2026-6331 is a high-severity flaw (CVSS 7.5) affecting wolfssl. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 o

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 91%)
CISA exploited list	No
Weakness	CWE-347
Published	2026-06-25
Last modified	2026-06-27

Publisher (CNA): wolfSSL Inc. · Vendor: wolfssl

Last updated 2026-06-27 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-25 22:17 UTCPublished CVE record published
2026-06-27 19:48 UTCUpdated Record last modified

wolfssl wolfssl UPGRADE

Affected: >= 3.15.5 and < 5.9.2

Fixed in: 5.9.2

Safe to downgrade to: a release before 3.15.5

1. Check installed version

apt-cache policy wolfssl || rpm -q wolfssl || wolfssl --version

2. UPGRADE to 5.9.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade wolfssl

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade wolfssl

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.15.5 (Debian / Ubuntu)

apt-cache madison wolfssl sudo apt install wolfssl=<VERSION_BEFORE_3.15.5> sudo apt-mark hold wolfssl

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list wolfssl sudo dnf downgrade wolfssl-<VERSION_BEFORE_3.15.5> sudo dnf versionlock add wolfssl

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-6331?

wolfssl: upgrade to 5.9.2. Exact commands for each OS are on this page.

Is CVE-2026-6331 being exploited?

CVE-2026-6331 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-6331 affect?

CVE-2026-6331 affects wolfssl. Severity is high (CVSS 7.5). 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 exceedin

How urgent is CVE-2026-6331?

Priority 33/100. High severity; Remotely exploitable, no login, no user action

Software pages

OpenSSL vulnerabilities · wolfSSL vulnerabilities

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References

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