Reading the comments, looks like bad/old code mixed with a big update rather than anything malicious. I even ran into themes that killed my KDE last night. Had to purge the configs themes to get it working. Damn glad I didn't wipe my entire setup.
You must have heard that old chestnut about how "the weakest security link in the security chain is the user" by now. There is nothing any technology can do if the user decides to install insecure stuff. Even before today, the KDE Store prominently displayed warnings about being careful with the content.