A browser extension is a small software module for customizing a web browser. Browsers typically allow a variety of extensions, including user interface modifications, ad blocking, cookie management, web form recovery, and browsing session exporting.
Browser plug-ins are a separate type of module. The main difference is that extensions are usually just source code, but plug-ins are always executables (i.e. object code). As of 2021, plug-ins have been deprecated by most browsers, while extensions are widely used. The most popular browser, Google Chrome, has over 100,000 extensions available but no longer supports plug-ins.
Internet Explorer was the first major browser to support extensions, with the release of version 4 in 1999. Firefox has supported extensions since its launch in 2004. Opera began supporting extensions in 2009, and both Google Chrome and Safari did so the following year. Microsoft Edge added extension support in 2016.
In 2015, a community working group formed under the W3C to create a single standard application programming interface (API) for browser extensions. While that goal is unlikely to be achieved, the majority of browsers already use the same or very similar APIs due to the popularity of Google Chrome.
Because of Chrome's success, Microsoft created a very similar extension API for its Edge browser, with the goal of making it easy for Chrome extension developers to port their work to Edge. But after three years Edge still had a disappointingly small market share, so Microsoft rebuilt it as a Chromium-based browser. (Chromium is Google's open-source project that serves as the functional core of Chrome and many other browsers.) Now that Edge has the same API as Chrome, extensions can be installed directly from the Chrome Web Store.
With its own market share in decline, Mozilla also decided to conform. In 2015, the organization announced that the long-standing XUL and XPCOM extension capabilities of Firefox would be replaced with a less-permissive API very similar to Chrome's. This change was enacted in 2017. Firefox extensions are now largely compatible with their Chrome counterparts.
Until 2020, Apple was the lone major exception to this trend, as its API for Safari required using the Xcode tool to create extensions. However, Apple announced that Safari 14 would conform to the Chrome API as part of the macOS 11 update.
Browser extensions typically have access to sensitive data, such as browsing history, and they have the ability to alter some browser settings, add user interface items, or replace website content. As a result, there have been instances of malware, so users need to be cautious about what extensions they install.
Some Google Chrome extension developers have sold their extensions to third-parties who then incorporated adware. In 2014, Google removed two such extensions from the Chrome Web Store after many users complained about unwanted pop-up ads. The following year, Google acknowledged that about five percent of visits to its own websites had been altered by extensions with adware.
- "What are extensions?". MDN Web Docs. Retrieved 2019-08-05.
- "StatCounter Global Stats". StatCounter. Retrieved 11 November 2020.
- Cimpanu, Catalin. "Half of all Google Chrome extensions have fewer than 16 installs". ZDNet. Retrieved 2021-02-19.
- "Google Chrome 88 released: RIP Flash Player". Retrieved 29 January 2021.
- "Browser Extensions". Retrieved 2020-09-29.
- Bright, Peter (18 March 2016). "Edge browser now has extensions in the latest Windows 10 preview". Ars Technica. Condé Nast.
- "Browser Extension Community Group Charter — Browser Extension Community Group". browserext.github.io. Retrieved 2018-12-14.
- "Re: One question from Florian Rivoal on 2017-07-29 (firstname.lastname@example.org from July 2017)". lists.w3.org. Retrieved 2018-12-14.
- "Extensions Status: On the Runway, Getting Ready for Take-Off". Chromium Blog. Retrieved 2018-12-14.
- "Extensions beta launched, with over 300 extensions!". Chromium Blog. Retrieved 2018-12-14.
- Vikas SN (2012-06-29). "The Lowdown: Google I/O 2012 Day 2 – 310M Chrome Users, 425M Gmail & More". MediaNama. Retrieved 2013-06-14.
- "Internet Explorer usage to plummet below 50 percent by mid-2012". 3 September 2011. Archived from the original on 20 November 2011. Retrieved 4 September 2011.
- Statcounter. "Browser Market Share Worldwide | StatCounter Global Stats". gs.statcounter.com. Retrieved October 20, 2018.
- "Porting an extension from Chrome to Microsoft Edge". Microsoft. Retrieved 30 December 2018.
- "Microsoft Edge: Making the web better through more open source collaboration". Windows Experience Blog. 2018-12-06. Retrieved 2018-12-14.
- Keizer, Gregg (2018-12-08). "With move to rebuild Edge atop Google's Chromium, Microsoft raises white flag in browser war". Computerworld. Retrieved 2018-12-14.
- "Add or remove extensions in Microsoft Edge". Microsoft. Retrieved 2020-06-08.
- "The Future of Developing Firefox Add-ons". Mozilla Add-ons Blog. Retrieved 2018-12-15.
- "Upcoming Changes in Compatibility Features". Mozilla Add-ons Blog. Retrieved 2018-12-15.
- "How to enable legacy extensions in Firefox 57 - gHacks Tech News". www.ghacks.net. Retrieved 2018-12-14.
- "Porting a Google Chrome extension". Mozilla. Retrieved 30 December 2018.
- "Building a Safari App Extension". Apple. Retrieved 10 February 2019.
- "macOS Big Sur Preview". Retrieved 2 July 2020.
- "Protect User Privacy". Google Chrome Docs. Retrieved 2018-12-15.
- "Add-on Policies". MDN Web Docs. Retrieved 2018-12-15.
- "Security firm ICEBRG uncovers 4 malicious Chrome extensions - gHacks Tech News". www.ghacks.net. Retrieved 2018-12-15.
- "Google's bad track record of malicious Chrome extensions continues - gHacks Tech News". www.ghacks.net. Retrieved 2018-12-15.
- "Chrome Extension Devs Use Sneaky Landing Pages after Google Bans Inline Installs". BleepingComputer. Retrieved 2018-12-15.
- "Google Chrome extensions with 500,000 downloads found to be malicious". Ars Technica. Retrieved 2018-12-30.
- "PUP Criteria". Malwarebytes. Retrieved 13 February 2015.
- "Adware vendors buy Chrome Extensions to send ad- and malware-filled updates". Ars Technica. Retrieved 20 January 2014.
- Bruce Schneier (21 Jan 2014). "Adware Vendors Buy and Abuse Chrome Extensions".
- Winkler, Rolfe. "Google Removes Two Chrome Extensions Amid Ad Uproar". blogs.wsj.com. Wall Street Journal. Retrieved 17 March 2014.
- "Ad Injection at Scale: Assessing Deceptive Advertisement Modifications" (PDF). Archived from the original (PDF) on 2015-06-05.
- "Superfish injects ads into 5 percent of all Google page views". PC World. IDG.
- "Superfish injects ads in one in 25 Google page views". CIO. IDG.