Improper DNS Constraint Validation in Go Certificate Verification

Executive Summary

CVE-2026-33810 is a vulnerability in the Go programming language's crypto/x509 package that affects the verification of certificate chains containing excluded DNS constraints.

The issue occurs because excluded DNS constraints are not correctly enforced when applied to wildcard DNS Subject Alternative Names (SANs) if the case of the wildcard SAN differs from that of the excluded DNS constraint.

This means that a certificate with a wildcard DNS name like "*.example.com" may not properly enforce an excluded DNS constraint such as "EXAMPLE.COM" due to case mismatch during validation.

The vulnerability affects the validation of otherwise trusted certificate chains issued by a root Certificate Authority (CA) present in the VerifyOptions.Roots CertPool or the system certificate pool.

This flaw resides in the Certificate.Verify function and was fixed in Go version 1.26.2.

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Impact Analysis

This vulnerability can impact you by allowing a certificate chain that should be excluded due to DNS constraints to be incorrectly validated as trusted.

Specifically, if a wildcard DNS SAN in a certificate uses a different case than the excluded DNS constraint, the constraint may not be enforced, potentially allowing unauthorized or malicious certificates to be accepted.

This could lead to security risks such as man-in-the-middle attacks or unauthorized access if your application relies on Go's crypto/x509 package for certificate validation.

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Detection Guidance

This vulnerability affects the Go programming language's crypto/x509 package during certificate chain verification involving excluded DNS constraints and wildcard DNS SANs with case differences.

To detect this vulnerability on your system, you should first identify if you are running a vulnerable Go version (from 1.26.0-0 up to but not including 1.26.2).

You can check your Go version by running the command: `go version`.

To detect if your applications are affected, you may need to audit or test certificate chain validations that involve excluded DNS constraints and wildcard DNS SANs with differing case sensitivity.

There are no specific commands provided in the resources for scanning network traffic or system logs for this vulnerability.

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Mitigation Strategies

The primary mitigation step is to upgrade the Go programming language to version 1.26.2 or later, where this vulnerability has been fixed.

If upgrading immediately is not possible, consider reviewing and restricting the use of certificate chains that include excluded DNS constraints and wildcard DNS SANs with case differences until the fix can be applied.

Additionally, monitor for updates from the Go project and apply patches as soon as they become available.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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