According to some comments the plugin in question is "amember" but there are several (old) posts on their forums say they don't use plaintext. I'd be surprised if it was, but then again ...
Posts are from spring this year, so it's not "old". The first post also references an upgrade from version 3 to version 4, so I guess they still use version 3 and didn't get around to updating to v4 yet, and now they pay the price.