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User Agent String

A text string sent by web browsers to identify the browser, operating system, and device to web servers.

Web TechnologiesAlso called: "user agent", "browser string"

User agents help servers deliver appropriate content and detect bots.

Format

Mozilla/5.0 (Windows NT 10.0; Win64; x64)
AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/120.0.0.0 Safari/537.36

Information revealed

  • Browser name and version.
  • Operating system.
  • Device type (mobile, tablet, desktop).
  • Rendering engine (WebKit, Gecko, Blink).

Use cases

  • Serve mobile vs desktop layouts.
  • Browser feature detection.
  • Analytics and tracking.
  • Bot detection and blocking.

Security considerations

  • Easily spoofed (not reliable for security).
  • Can be used for fingerprinting.
  • Privacy concern (reveals system details).
  • Should never be sole authentication factor.

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