X.509 Name Constraint Bypass in wolfSSL

Detection Guidance

This vulnerability involves a bypass of DNS name constraints via the Subject Common Name (CN) in X.509 certificates when the Subject Alternative Name (SAN) extension is missing. Detection involves verifying whether certificates accepted by your system or network correctly enforce DNS name constraints on the CN field if SAN is absent.

To detect this vulnerability, you can inspect certificates used in your environment to check if any certificate's Subject CN violates the issuing CA's DNS name constraints but is still accepted.

While no specific commands are provided in the resources, a general approach includes using OpenSSL commands to examine certificates for their CN and SAN fields and verify compliance with DNS name constraints.

• Use OpenSSL to inspect a certificate's Subject CN and SAN extensions: openssl x509 -in certificate.pem -noout -text
• Check if the SAN extension is missing and then verify if the CN violates expected DNS constraints.
• Monitor your TLS/SSL validation logs for any warnings or errors related to name constraints enforcement.

For wolfSSL users, ensure you are running a version that includes the fix from pull request #10223, which enforces DNS name constraints on the CN when SAN is missing.

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Impact Analysis

The impact of this vulnerability is that an attacker could present a certificate with a Subject Common Name that violates the DNS name constraints imposed by the issuing CA, yet the certificate might still be accepted as valid. This could lead to trust in unauthorized or malicious certificates, potentially enabling man-in-the-middle attacks or impersonation.

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Executive Summary

This vulnerability involves a bypass of X.509 name constraints through the Subject Common Name (CN) field when it is treated as a DNS-type name. Specifically, a certificate whose Subject CN violates the DNS name constraints set by the issuing Certificate Authority (CA) could still be accepted, potentially allowing unauthorized certificates to be trusted.

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Compliance Impact

This vulnerability allows a certificate whose Subject Common Name (CN) violates an issuing Certificate Authority's DNS name constraints to be accepted, potentially undermining the integrity of certificate validation.

Such a bypass in enforcing DNS name constraints could lead to acceptance of unauthorized or malicious certificates, which may impact the security assurances required by standards and regulations like GDPR and HIPAA that mandate strong identity verification and secure communications.

Therefore, this vulnerability could negatively affect compliance with these regulations by weakening the trust model of certificate-based authentication and encryption.

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Mitigation Strategies

To mitigate this vulnerability, ensure that your system uses the updated wolfSSL version that includes the fix for CVE-2026-6731.

The fix enforces DNS name constraints on the Subject Common Name (CN) when the Subject Alternative Name (SAN) is missing, preventing acceptance of certificates that violate issuing CA's DNS name constraints.

Updating to the patched wolfSSL version merged on April 19, 2026, which includes this fix, is recommended.

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