User agent

A user agent is a computer program representing a person, for example, a {{Glossary("Browser","browser")}} in a {{Glossary("World Wide Web", "Web")}} context.

Besides a browser, a user agent could be a bot scraping webpages, a download manager, or another app accessing the Web. Along with each request they make to the server, browsers include a self-identifying {{HTTPHeader("User-Agent")}} {{Glossary("HTTP")}} header called a user agent (UA) string. This string often identifies the browser, its version number, and its host operating system.

Spam bots, download managers, and some browsers often send a fake UA string to announce themselves as a different client. This is known as user agent spoofing.

The user agent string can be accessed with {{Glossary("JavaScript")}} on the client side using the {{domxref("navigator.userAgent")}} property.

A typical user agent string looks like this: "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:124.0) Gecko/20100101 Firefox/124.0".

See also

• User agent on Wikipedia

• {{domxref("navigator.userAgent")}}

• Browser detection using the user agent

• {{RFC(2616, "", "14.43")}}: The User-Agent header

• Related glossary terms:

  • {{Glossary("Browser")}}

• HTTP Headers

  • {{HTTPHeader("User-agent")}}