Extension

Overview
An extension is native code modules or scripts that work in conjunction with software applications to enhance their capabilities. Extensions have also been devised for email clients and other desktop software. They can be downloaded from either the browser vendor’s site or a third-party site. Although extensions allow browsers to support new types of content and functionality, they are not active content in and of themselves, but simply executables that enable active content technologies. Extensions are usually browser-specific, offering a variety of functionality from FTP clients to automatic form fill-out.

Security concerns
There are security concerns with extensions. Extensions can bypass a browser’s underlying security model. For example, the Firefox browser extension called GreaseMonkey allows users to install scripts to make dynamic changes to HTML Web pages as they are downloaded, often by augmenting their functionality through other Web resources. To accomplish its tasks, it provides a means to bypass the same origin policy of the browser and interact with any domain, which could be exploited by a malicious website.