Name Constraints bypasses hit multiple TLS libraries

Several open-source TLS and X.509 libraries failed to enforce certificate Name Constraints as specified in RFC 5280, according to three CERT/CC vulnerability notes published in mid-October 2026. Researcher Oleh Konko reported the issues, which span DNS name matching, URI authority parsing, IP address handling, and wildcard semantics.

Name Constraints let a CA limit the hostnames, emails, or other identities a subordinate certificate may assert. Incomplete enforcement can allow a certificate that should have been rejected to validate against a name outside the permitted set, weakening the chain of trust for TLS clients that rely on those libraries.

CVE-2026-34073 affected pyca/cryptography: DNS constraints were applied to the certificate's subject alternative names but not to the peer name the application actually requested. It is fixed in 46.0.6.

CVE-2026-3833 in GnuTLS stemmed from case-sensitive comparison of DNS names and the domain portion of rfc822Name constraints; RFC 5280 requires case-insensitive matching. The fix shipped in 3.8.13.

CVE-2026-5263 covered wolfSSL, which parsed URI constraints but did not enforce them, and separately mishandled certain fragment wildcards. Both are addressed in 5.9.1.

rustls/webpki fixed acceptance of unsupported URI constraints and improper handling of wildcards under permitted subtrees. AWS-LC corrected a wildcard Common Name fallback path. Other libraries and related reports remain in various states of remediation under the CERT notes VU#573195, VU#948176, and VU#410595.

Operators using affected stacks for certificate validation should upgrade to the fixed releases. The gaps do not require a new CA compromise; they matter wherever intermediate or constrained certificates are accepted in automated trust decisions.