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Mozilla Foundation Security Advisory 2005-16

Spoofing download and security dialogs with overlapping windows

February 24, 2005
Michael Krax
Firefox, Mozilla Suite
Fixed in
  • Firefox 1.0.1
  • Mozilla Suite 1.7.6


Michael Krax demonstrates that the download dialog and security dialogs can be spoofed by partially covering them with an overlapping window. Some users may not notice the OS window border and browser statusbar bisecting what appears to be a single dialog, and be convinced by the spoofing text of the top-most window to click on the "Allow" or "Open" button of the window below.

If the security dialog button is clicked signed scripts could perform any action on the users machine.

If the download button is clicked a file could be downloaded to the default download location. If the target file MIME type is one handled by a helper app clicking the button could launch that helper app. If there's a known exploit in a popular or default helper app then opening the attackers file in that app could take advantage of that exploit.


Carefully inspect dialogs triggered by web content.

Disable the ability of web pages to resize and position windows:

  1. Select the "Options..." item on the "Tools" menu
  2. Select the "Web Features" icon
  3. Press the "Advanced..." button on the line with the Javascript checkbox
  4. Uncheck "Raise or lower windows"
  5. Uncheck "Move or resize existing windows"
  6. Click OK on both dialogs to save the settings