CVE-2020-12886: Critical severity arm mbed vulnerability

A buffer over-read was discovered in the CoAP library in Arm Mbed OS 5.15.3. The CoAP parser is responsible for parsing received CoAP packets. The function sncoapparseroptionsparse() parses the CoAP packet header starting from the message token. The length of the token in the received message is provided in the first byte parsed by the sncoapparseroptionsparse() function. The length encoded in the message is not validated against the actual input buffer length before accessing the token. As a result, memory access outside of the intended boundary of the buffer may occur.