This is a Google Chrome extension that exploits a flaw in JavaScript that allows anchor elements to be changed AFTER the user clicks on them.
How to load unpacked Google Chrome extensions:
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Visit chrome://extensions in your browser (or open up the Chrome menu by clicking the icon to the far right of the Omnibox and select Extensions under the Tools menu to get to the same place).
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Ensure that the Developer Mode checkbox in the top right-hand corner is checked.
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Click Load unpacked extension… to pop up a file-selection dialog.
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Navigate to the directory in which your extension files live, and select it.
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If the extension is valid, it'll be loaded up and active right away! If it's invalid, an error message will be displayed at the top of the page.
(More info: https://developer.chrome.com/extensions/getstarted.html#unpacked)
Based on the work found in http://bilaw.al/2013/03/17/hacking-the-a-tag-in-100-characters.html.