Does anyone know how to fix this CWE vulnerability? I'm coming across different answers online, from windows updates, to code fixes, but I'm not really sure...It's a C# ASP.Net 4.5.1 Webforms site, using ASP.NET Identity for authentication. There's a crawler that's used to scan for security vulnerabilities, and it's come up with:

Using Diffie Hellman group with prime (por small prime) of size 1024-bit or less, leaves the server vulnerable to man-in-the middle attack (MitM). Diffie-Hellman key exchange algorithm uses fixed primes as a base for computing the secret key used to secure the communication channel. The size of the small prime pdeployed dictates the security level of the generated key. This in turn defines the effective security provided by the Diffie-Helman key exchange algorithm. Research indicates that Diffie-Hellman group using prime size of 1024-bit provides only about 77-80 bits of security. Communication channels that are secured using this key are vulnerable to man-in-the-middle attack. All anonymous, ephermeral and fixed Diffie-Hellman key exchange algorithms except for Elliptical-Curve Diffie-Hellman (ECDHE) key exchange are vulnerable to this attack. WebInspect has detected the target server using Diffie-Hellman small prime pof size 1024 bits in ciphersuite: TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x39). The server may thus be vulnerable to eavesdropping and/or man-in-the-middle attacks.