Mozilla Foundation Security Advisory 2015-95

Add-on notification bypass through data URLs

August 27, 2015
Bas Venis
Firefox, Firefox ESR, SeaMonkey
Fixed in
  • Firefox 40.0.3
  • Firefox ESR 38.2.1
  • SeaMonkey 2.35


Security researcher Bas Venis reported a mechanism where add-ons could be installed from a different source than user expectations. Normally, when a user enters the URL to an add-on directly in the addressbar, warning prompts are bypassed because it is the result of direct user action. He discovered that a data: URL could be manipulated on a loaded page to simulate this direct user input of the add-on's URL, which would result in a bypassing of the install permission prompt. He also reported that in the absence of the permission prompt, it is possible to cause the actual installation prompt to appear above another site's location by causing a page navigation immediately after triggering add-on installation. This could manipulate a user into falsely believing a trusted site (such as has initiated the installation. This could lead to users installing an add-on from a malicious source.