x509: read the ASN.1 tag after the length check, not before - #413
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_nx_secure_x509_asn1_tlv_block_parse() loaded buffer[0] into current_tag one statement before testing *buffer_length < 1, so every caller that runs out of data read one byte past the end of the buffer it was given. It is reachable from the wire. A certificate two bytes long reaches it through _nx_secure_x509_certificate_parse(), and so does the issuer walk in the certificate store, which calls the parser repeatedly as it consumes a chain and hands it whatever remains. Found by a fuzz driver over _nx_secure_tls_process_remote_certificate() with a real DER corpus, and confirmed under AddressSanitizer. The read is moved below the guard. The guard already returned the right status; only the load was in the wrong place.
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_nx_secure_x509_asn1_tlv_block_parse() loaded buffer[0] into current_tag one statement before testing *buffer_length < 1, so every caller that runs out of data read one byte past the end of the buffer it was given.
It is reachable from the wire. A certificate two bytes long reaches it through _nx_secure_x509_certificate_parse(), and so does the issuer walk in the certificate store, which calls the parser repeatedly as it consumes a chain and hands it whatever remains.
Found by a fuzz driver over _nx_secure_tls_process_remote_certificate() with a real DER corpus, and confirmed under AddressSanitizer.
The read is moved below the guard. The guard already returned the right status; only the load was in the wrong place.