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Greasemonkey Hacks
By Mark Pilgrim
November 2005
More Info

HACK
#11
Embed Graphics in a User Script
Add images to web pages without hitting a remote server
The Code
[Discuss (0) | Link to this hack]

The Code

This user script runs on all pages. It uses an XPath query to find web bugs: 1 x 1-pixel img elements that advertisers use to track your movement online. The script filters this list of potential web bugs to include only those images that point to a third-party site, since many sites use 1 x 1-pixel images for spacing in table-based layouts.

There is no way for Greasemonkey to eliminate web bugs altogether; by the time a user script executes, the image has already been fetched. But we can make them more visible by changing the src attribute of the img element after the fact. The image data is embedded in the script itself.

Save the following user script as webbugs.user.js:

	// ==UserScript==
	// @name Web Bug Detector
	// @namespace http://diveintomark.org/projects/greasemonkey/
	// @description make web bugs visible
	// @include *
	// ==/UserScript==

	var snapImages = document.evaluate("//img[@width='1'][@height='1']",
		document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
	for (var i = snapImages.snapshotLength - 1; i >= 0; i--) 
		var elmImage = snapImages.snapshotItem(i);
		var urlSrc = elmImage.src;
		var urlHost = urlSrc.replace(/^(.*?):\/\/(.*?)\/(.*)$/, "$2");
		if (urlHost == window.location.host) continue;
		elmImage.width = '80';
		elmImage.height = '80';
		elmImage.title = 'Web bug detected! src="' + elmImage.src + '"';
		elmImage.src =
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	}


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