OpenSSL address implementation in Socat contains a hard-coded Diffie-Hellman 1024-bit prime number that was not prime. An attacker could listen and recover secrets from a key exchange that uses them. A post to a technical forum discovered that the non-prime prime was introduced more than a year ago. A developer named Zhiang Wang provided a patch with the new prime. It’s unknown how Wang chose the prime, but other commenters on the forum said checks in OpenSSL and other tools used to generate primes cannot be sure if the numbers are prime.
Source: https://threatpost.com/socat-warns-weak-prime-number-could-mean-its-backdoored/116104/