A browser extension is a software module for customizing a web browser. Browsers typically allow users to install a variety of extensions, including user interface modifications, cookie management, ad blocking, and the custom scripting and styling of web pages.[1]

Browser plug-ins are a different type of module and no longer supported by the major browsers.[2][3] One difference is that extensions are distributed as source code, while plug-ins are executables (i.e. object code).[2] The most popular browser, Google Chrome,[4] has over 100,000 extensions available[5] but stopped supporting plug-ins in 2020.[6]


Internet Explorer was the first major browser to support extensions, with the release of version 4 in 1999.[7] Firefox has supported extensions since its launch in 2004. Opera and Chrome began supporting extensions in 2009,[8] and Safari did so the following year. Microsoft Edge added extension support in 2016.[9]

API conformity

In 2015, a community working group formed under the W3C to create a single standard application programming interface (API) for browser extensions.[10] While that goal is unlikely to be achieved,[11] the majority of browsers already use the same or very similar APIs due to the popularity of Google Chrome.

Chrome was the first browser with an extension API based solely on HTML, CSS, and JavaScript. Beta testing for this capability began in 2009,[12][13] and the following year Google opened the Chrome Web Store. As of June 2012, there were 750 million total installations of extensions and other content hosted on the store.[14] In the same year, Chrome overtook Internet Explorer as the world's most popular browser,[15] and its usage share reached 60% in 2018.[16]

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.[17] But after three years Edge still had a disappointingly small market share, so Microsoft rebuilt it as a Chromium-based browser.[18][19] (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.[20]

In 2015, Mozilla 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.[21] This change was enacted in 2017.[22][23] Firefox extensions are now largely compatible with their Chrome counterparts.[24]

Until 2020, Apple was the lone major exception to this trend, but with the release of Safari 14 for macOS, the browser added support for extensions conforming to the Chrome API.[25] The following year, extensions were enabled in the iOS version for the first time.[26]

Unwanted behavior

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.[27][28] As a result, there have been instances of malware, so users need to be cautious about what extensions they install.[29][30][31][32]

There have also been cases of applications installing browser extensions without the user's knowledge, making it hard for the user to uninstall the unwanted extension.[33]

Some Google Chrome extension developers have sold their extensions to third-parties who then incorporated adware.[34][35] In 2014, Google removed two such extensions from the Chrome Web Store after many users complained about unwanted pop-up ads.[36] The following year, Google acknowledged that about five percent of visits to its own websites had been altered by extensions with adware.[37][38][39]


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  4. ^ "StatCounter Global Stats". StatCounter. Retrieved 11 November 2020.
  5. ^ Cimpanu, Catalin. "Half of all Google Chrome extensions have fewer than 16 installs". ZDNet. Retrieved 2021-02-19.
  6. ^ "Google Chrome 88 released: RIP Flash Player". Retrieved 29 January 2021.
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  8. ^ Edwards, Lin; Phys.org. "Google Chrome extensions to be officially released". phys.org. Retrieved 2023-04-27.
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  17. ^ "Porting an extension from Chrome to Microsoft Edge". Microsoft. Retrieved 30 December 2018.
  18. ^ "Microsoft Edge: Making the web better through more open source collaboration". Windows Experience Blog. 2018-12-06. Retrieved 2018-12-14.
  19. ^ 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.
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  22. ^ "Upcoming Changes in Compatibility Features". Mozilla Add-ons Blog. 10 August 2017. Retrieved 2018-12-15.
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  29. ^ "Security firm ICEBRG uncovers 4 malicious Chrome extensions - gHacks Tech News". www.ghacks.net. 16 January 2018. Retrieved 2018-12-15.
  30. ^ "Google's bad track record of malicious Chrome extensions continues - gHacks Tech News". www.ghacks.net. 11 May 2018. Retrieved 2018-12-15.
  31. ^ "Chrome Extension Devs Use Sneaky Landing Pages after Google Bans Inline Installs". BleepingComputer. Retrieved 2018-12-15.
  32. ^ "Google Chrome extensions with 500,000 downloads found to be malicious". Ars Technica. 17 January 2018. Retrieved 2018-12-30.
  33. ^ "PUP Criteria". Malwarebytes. Retrieved 13 February 2015.
  34. ^ "Adware vendors buy Chrome Extensions to send ad- and malware-filled updates". Ars Technica. 17 January 2014. Retrieved 20 January 2014.
  35. ^ Bruce Schneier (21 Jan 2014). "Adware Vendors Buy and Abuse Chrome Extensions".
  36. ^ Winkler, Rolfe (19 January 2014). "Google Removes Two Chrome Extensions Amid Ad Uproar". blogs.wsj.com. Wall Street Journal. Retrieved 17 March 2014.
  37. ^ "Ad Injection at Scale: Assessing Deceptive Advertisement Modifications" (PDF). Archived from the original (PDF) on 2015-06-05.
  38. ^ "Superfish injects ads into 5 percent of all Google page views". PC World. IDG.
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