`_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.