The XMonad.Hooks.DynamicLog module in xmonad-contrib before 0.11.2 allows remote attackers to execute arbitrary commands via a web page title, which activates the commands when the user clicks on the xmobar window title, as demonstrated using an action tag.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2013-1474
http://www.securityfocus.com/bid/61491
http://www.openwall.com/lists/oss-security/2013/07/26/5