Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

Summary

coturn is a free open source implementation of TURN and STUN Server. Versions from 4.6.2-r5 through 4.7.0-r4 have a bad random number generator for nonces and port randomization after refactoring. Additionally, random numbers aren't generated with openssl's "RAND_bytes" but libc's "random()" (if it's not running on Windows). When fetching about 50 sequential nonces (i.e., through sending 50 unauthenticated allocations requests) it is possible to completely reconstruct the current state of the random number generator, thereby predicting the next nonce. This allows authentication while spoofing IPs. An attacker can send authenticated messages without ever receiving the responses, including the nonce (requires knowledge of the credentials, which is e.g., often the case in IoT settings). Since the port randomization is deterministic given the pseudorandom seed, an attacker can exactly reconstruct the ports and, hence predict the randomization of the ports. If an attacker allocates a relay port, they know the current port, and they are able to predict the next relay port (at least if it is not used before).