Source: https://weakdh.org/
Diffie-Hellman key exchange is a popular cryptographic algorithm that allows Internet protocols to agree on a shared key and negotiate a secure connection. It is fundamental to many protocols including HTTPS, SSH, IPsec, SMTPS, and protocols that rely on TLS.
We have uncovered several weaknesses in how Diffie-Hellman key exchange has been deployed
Here is a work-around for sysadmins: https://weakdh.org/sysadmin.html