A user agent is a computer program representing a person, for example, a
browser
in a
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
User-Agent
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
JavaScript
on the client side using the
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"
.