The vulnerability in Legion of the Bouncy Castle Inc. bc-csharp (before 2.7.0) arises from improper validation of certificate name constraints in the directoryName field during PKIX path validation. Specifically, the PkixNameConstraintValidator.WithinDNSubtree method incorrectly checks the directoryName name-constraint by locating the first Relative Distinguished Name (RDN) of the constraint anywhere within the subject name and then comparing subsequent RDNs from that point. This approach violates RFC 5280 sections 4.2.1.10 and 7.1, which require the constraint to be an initial prefix of the name. Consequently, an attacker who controls or can obtain certificates from a name-constrained intermediate CA can present certificates with subject distinguished names or directoryName subjectAltNames outside the allowed subtrees, bypassing intended restrictions and potentially undermining trust in the certificate chain.

An attacker with control over or the ability to obtain certificates from a name-constrained intermediate CA can issue certificates that appear valid under PKIX path validation despite having subject names outside the permitted subtrees. This undermines the security guarantees of name constraints, potentially allowing unauthorized certificate issuance and trust bypass in systems relying on bc-csharp for certificate validation.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should be cautious when validating certificates using affected versions of bc-csharp and consider additional validation controls if feasible.