GnuTLS v3.8.13 does not reject a critical keyUsage extension when it cannot be processed, such as when extnValue is an ASN.1 NULL (05 00) instead of a BIT STRING. NOTE: the Supplier does not consider this an actionable security vulnerability because of its "threat model doesn't consider attacks only possible by modifying anything within the trust boundary (private keys, trusted certificate authorities and their issued certificates including intermediate CAs, etc.)" policy.