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Improper Certificate ValidationWeakness · CWE-295

CVE-2026-42013

HIGH · 8.2 CVSS v3.1 Published 2026-05-26
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
91/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
A flaw was found in gnutls. When validating certificates, an oversized Subject Alternative Name (SAN) could cause the validation process to incorrectly fall back to checking the Common Name (CN) field. This could allow a remote attacker to bypass proper certificate validation, potentially leading to spoofing or man-in-the-middle attacks.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In gnutls, when processing X.509 certificates with an oversized Subject Alternative Name (SAN) extension, the validation logic incorrectly falls back to checking the Common Name (CN) field instead of properly rejecting the malformed certificate. This allows a remote attacker with a crafted certificate to bypass proper hostname validation and potentially perform spoofing or man-in-the-middle attacks.

MitigationUpdate gnutls to the patched version provided by your distribution or vendor. Until patched, consider implementing additional certificate validation checks at the application layer or using an alternative TLS library as a compensating control.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed GnuTLS version
    Run 'gnutls-cli --version' or check the library file version with 'dpkg -l | grep gnutls' (Debian) or 'rpm -qa | grep gnutls' (RHEL). Also check libgnutls.so version with 'ldconfig -p | grep gnutls'.
    Affected if The installed version matches or precedes the unpatched version that contains this flawed SAN validation logic.
  2. Find applications using GnuTLS
    Search for linked libraries with 'ldd <binary> | grep gnutls' on critical binaries, or check TLS library usage with 'openssl s_client -connect <host:port> 2>&1 | grep -i gnutls' for remote services.
    Affected if Applications or services linked against GnuTLS are used for TLS certificate validation.
  3. Determine certificate validation flow
    Review application configuration or code to confirm it relies on GnuTLS for certificate chain validation, particularly SAN field checking.
    Affected if The application relies on GnuTLS to validate the SAN extension in X.509 certificates.
  4. Check for oversized SAN handling
    Test certificate validation with a certificate containing an oversized SAN extension (value exceeding reasonable size limits) and observe whether validation falls back to CN or properly rejects the certificate.
    Affected if The validation incorrectly accepts certificates with oversized SANs by falling back to CN checking instead of rejecting them.

You are affected if your system runs any GnuTLS version that contains the flawed SAN validation logic and uses it to validate certificates where an attacker could present a certificate with an oversized SAN to bypass intended validation checks.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Update gnutls to the patched version provided by your distribution or vendor. Until patched, consider implementing additional certificate validation checks at the application layer or using an alternative TLS library as a compensating control.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,440
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-42013 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
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  • Version or environment caveats, and links to real fixes
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