Is CVE-2026-42013 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-42013 being exploited in the wild.

How severe is CVE-2026-42013?

High severity, with a CVSS score of 8.2, published 26 May 2026.

Is there a fix for CVE-2026-42013?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-42013 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-42013?

Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-42013: GnuTLS certificate validation bypass via oversized SAN field

CVE-2026-42013 · Severity: high · CVSS 8.2 · Published 2026-05-26

A security flaw has been identified in GnuTLS, a widely used library that enables secure communications for applications and operating systems. When checking a website's security certificate, the library may fail to properly verify the identity of the server if the certificate contains an unusually large name field. This could allow a malicious actor to impersonate a legitimate service, potentially leading to the interception of sensitive data or unauthorized access to communications.

Quick answers

Is CVE-2026-42013 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-42013 being exploited in the wild.
How severe is CVE-2026-42013?: High severity, with a CVSS score of 8.2, published 26 May 2026.
Which products and versions are affected?: GnuTLS gnutls.
Is there a fix for CVE-2026-42013?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-42013 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-42013?: Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A certificate validation bypass vulnerability exists in GnuTLS due to improper validation of specified quantities in input (CWE-1284). When the library encounters a certificate with an oversized Subject Alternative Name (SAN) field, the validation logic incorrectly fails over to checking the legacy Common Name (CN) field instead of rejecting the malformed or oversized SAN. A remote, unauthenticated attacker can exploit this by presenting a specially crafted certificate during a TLS handshake. Successful exploitation allows the attacker to bypass modern SAN-based identity verification, facilitating man-in-the-middle (MitM) attacks and server spoofing. Red Hat has acknowledged the issue, and fixes involve suppressing the CN fallback mechanism when SAN processing fails due to size constraints.

Affected products

GnuTLS gnutls

Timeline

2026-05-06: disclosed: Initial report in Red Hat Bugzilla
2026-05-26: advisory: NVD publication date

References

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