Diffie-Hellman algorithm does not contain any backdoor itself, but it has been intentionally weakened in an undetectable way by hiding the fact how various applications generate prime numbers. Researchers from University of Pennsylvania, INRIA, CNRS and Universit.. de Lorraine have practically proved how the NSA broke the most widespread encryption used on the Internet. It took researchers just two months and as many as 3,000 CPUs to break one of the 1024-bit keys, which could have allowed them to passively decrypt hundreds of millions of HTTPS-based communications.
Source: https://thehackernews.com/2016/10/nsa-crack-encryption.html