"_nx_secure_x509_parse_signature_data" computes the certificate signature's length as "tlv_length - 1" without first checking that "tlv_length" is non-zero. A certificate whose signature is an empty ASN.1 BIT STRING therefore parses successfully and leaves the certificate holding a signature length of "0xFFFFFFFF" and a signature pointer one byte past the end of the buffer. Certificates are parsed from the peer during the TLS handshake, so the input is remote and unauthenticated.