The vulnerability arises from accepting a Raw Public Key (RPK) without proper negotiation, allowing bypass of certificate chain validation. The parsing function ParseCertRelative() accepts RPKs without trust verification because they have no chain, which should only be accepted if RPK was negotiated. The implemented fix defaults to expecting X.509 certificates per RFC 7250/8446 when no certificate type is negotiated and rejects any certificate type mismatches, including un-negotiated RPKs, with an UNSUPPORTED_CERTIFICATE error. This vulnerability affects only builds with Raw Public Key support (HAVE_RPK) enabled, which is disabled by default in standalone builds but enabled with --enable-all.

An attacker could bypass certificate chain validation by presenting an un-negotiated Raw Public Key, potentially undermining the trust model of TLS connections. This could lead to acceptance of unauthenticated peers, increasing the risk of man-in-the-middle or impersonation attacks. However, this only affects configurations where Raw Public Key support is enabled and not properly negotiated.

The issue is addressed by defaulting to expect X.509 certificates when no certificate type is negotiated and rejecting un-negotiated Raw Public Keys. Users should ensure that Raw Public Key support (HAVE_RPK) is disabled unless explicitly required and properly negotiated. Since this fix is included in the codebase, users should verify they are using a version with this check implemented. Patch status is not explicitly confirmed; check the vendor advisory for current remediation guidance.