There have been several high profile plugins lately that have been found to be posting spam and deceptive links on user’s blogs lately.
One such is the “Display Widgets” plugin. You can read Wordfence’s detailed breakdown of the spam. It turns out the original developer of the plugin sold it, and the new owner started to place spammy backlinks and other bad code into the plugin. This gave this “developer” access to tens of thousands of blogs and the site owner’s never knew it was happening.