Clickjacking is a type of vulnerability discovered in 2008, which is similar to CSRF.
The attack involves displaying the target webpage in a iframe embedded in a malicious website. Using CSS, the submit button of the form on the target webpage is made invisible, and then overlaid with some button or link on the malicious website that encourages the user to click on it. For more information, see:
Web browsers have standardised on a defence called X-Frame-Options, which puts the onus on the web application to prevent framing of sensitive content.
A fix will be shortly released, in MediaWiki 1.16.1. Our approach is to allow framing of basic page views and search pages, but to deny framing of special pages unless they have explicitly opted out of clickjacking protection. By taking this moderate approach, instead of denying all framing, we hope to avoid having system administrators disable the security feature by patching it out.
Security-conscious system administrators may wish to disable framing entirely using $wgBreakFrames = true. In MediaWiki 1.16.1 or later, this will provide protection against clickjacking vulnerabilities in extensions which opt out incorrectly or embed sensitive forms on pages which are opted out.
Alternatively, you can configure your web server to send an X-Frame-Options header on all pages. For example with Apache mod_headers:
Header always set X-Frame-Options DENY
For MediaWiki 1.15.x or earlier branches, we will provide a simplified security patch which denies all framing.
Extensions which wish to allow framing on a given page should do so by calling $wgOut->allowClickjacking(). This should only be done on pages which do not contain CSRF-protected forms or links. Any extension which embeds a CSRF-protected form in an unprotected article view page should call $wgOut->preventClickjacking(), after Article::view() has called $wgOut->allowClickjacking(), to reset the flag.