The pstm_reverse function in MatrixSSL before 3.8.4 allows remote attackers to cause a denial of service (invalid memory read and crash) via a (1) zero value or (2) the key's modulus for the secret key during RSA key exchange.
https://blog.fuzzing-project.org/51-Fun-with-Bignums-Crashing-MatrixSSL-and-more.html