The head of the Linux Mint project Clement Lefebvre said in a blog post that the Linux Mint website had been hacked and that the hackers were modified Linux Mint 17.3 Cinnamon ISO with a backdoor on Saturday 20th, February.To check if your ISO is compromised, just check its MD5 signature.
The valid signatures:
If you are affected just backup your, reset your passwords and reinstall the OS.
The linux mint forums database was also compromised during this attack. just reset the password.