Pretty nasty if you didn't know about it. What has happened is that the Mint website was hacked and the links on the download page for that particular day were pointed to a hosting site in Bulgaria. On the Bulgarian hosting site were fake, or altered iso images that contained a backdoor program. So unless you downloaded an image yesterday, no worries (if you are a member of their forum and use the same password on other forums, you might want to change it in all instances). Not the first time a website was hacked, and won't be the last.