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WYCIWYG is an acronym that stands for What You Cache Is What You Get, commonly displayed in the address bar of Gecko-based Web browsers like Mozilla Firefox as wyciwyg:// when the Web browser is retrieving cached information.
The term WYCIWYG is a play on the acronym WYSIWYG ("What You See Is What You Get").
Mozilla Firefox implements a registered, strictly internal
wyciwyg uniform resource identifier (URI) scheme to sort and later reference locally cached pages that were generated or modified by a script on the client side (a common practice for Web 2.0 sites).
In 2007 Michał Zalewski reported that it was possible to bypass the same-origin checks and read from cached (wyciwyg) documents. It was possible at that time to access wyciwyg:// documents without proper same domain policy checks. This could have enabled an attacker to steal sensitive data,  This was fixed in Firefox 22.214.171.124 and SeaMonkey 1.1.3.
- Firefox wyciwyg:// cache vulnerability demo – Michal Zalewski
- Mozilla Foundation Security Advisory 2007-24: Unauthorized access to wyciwyg:// documents