MediaWiki contains a flaw that allows a remote cross site scripting attack. This flaw, which affects MSIE clients only, exists if MediaWiki has been configured to output through HTML Tidy ($wgUseTidy) and is caused by the application not properly validating user input. This could allow a user to create a specially crafted URL that would execute arbitrary code in a user's browser within the trust relationship between the browser and the server, leading to a loss of integrity.
Upgrade to version 1.3.12, 1.4.2 or higher, as it has been reported to fix this vulnerability. An upgrade is required as there are no known workarounds.