The TLS 1.3 Certificate-message parser in NetX Secure decrements the message length to skip a context byte without first establishing that the message has a byte to skip. A zero-length Certificate message therefore wraps the unsigned length to 0xFFFFFFFF, which makes the bounds check that follows vacuous and sends the parser reading past the session packet buffer.
The trigger is a four-byte handshake header and requires no cryptographic validity. It is reachable pre-authentication.