If you use the plugin W3 Total Cache to speed up the performance of your site, there’s a vulnerability that potentially could expose passwords on your site to a hacker. If you don’t know what W3 Total Cache is, there’s nothing to worry about. The plugin developer is reportedly working on a fix for this problem.
See this short article on The Register for more info on what to do if this vulnerability affects you. Note: I don’t use W3 Total Cache on any of the sites I design, so this vulnerability won’t affect any of my clients.