Server impersonation via oversized SAN CN fallback
Published May 26, 2026 · Updated Aug 31, 2026
Certificate hostname validation in GnuTLS before 3.8.13 allows remote attackers to impersonate trusted servers via an oversized DNS SAN. The gnutls_x509_crt_check_hostname2 function stops SAN iteration when its 256-byte buffer receives GNUTLS_E_SHORT_MEMORY_BUFFER, leaves found_dnsname unset, and falls back to Common Name matching. Exploitation requires an application to validate a certificate from an attacker-controlled endpoint; a matching Common Name can then authenticate the wrong host.
Summary
What happened
Certificate hostname validation in GnuTLS before 3.8.13 allows remote attackers to impersonate trusted servers via an oversized DNS SAN. The gnutls_x509_crt_check_hostname2 function stops SAN iteration when its 256-byte buffer receives GNUTLS_E_SHORT_MEMORY_BUFFER, leaves found_dnsname unset, and falls back to Common Name matching. Exploitation requires an application to validate a certificate from an attacker-controlled endpoint; a matching Common Name can then authenticate the wrong host.
The record
- CVE
- CVE-2026-42013
- Published
- May 26, 2026
- Updated
- Aug 31, 2026
- Vendor
- 389 Directory Server
- Product
- Red Hat OpenShift Container Platform 4.16
- Classifications
- CWE-1284, CWE-295, T1557
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- May 26, 2026CVE publishedPublication date reported by the CVE source.
- Aug 31, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Affected versions are unavailable in this record. Check the vendor advisory for version and patch details.
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Oversized dNSName SAN hostname-verification PoCproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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