What is a User Agent?

By Ljubomir Jakimovski on Mon Jan 30 2023


One concept that often goes overlooked is the user agent. But what exactly is a user agent, and why is it important?

What is a User Agent?

A user agent (UA) is a software component that identifies the device and browser used to access a website or web application. It consists of different parts such as the browser type, browser version, operating system, and device type. This information is sent to the server via the HTTP request header, allowing the server to tailor its response to the specific client's capabilities.

User Agent History

User agents have been around since the early days of the internet, with the first mention of them in the HTTP/0.9 specification in 1991. Initially, the purpose of a user agent was to identify the specific software and device being used to access a website, allowing the website to tailor its content and functionality accordingly.

As the internet and web browsing technology evolved, so did the capabilities of user agents. Today, a user agent (UA) not only identifies the device and software but also provides information about the device's capabilities and preferences to the website or web application.

HTTP and User Agent Header Depreciation

In HTTP requests, user agents are sent via the "User-Agent" header. However, some browsers, such as Chrome, have begun to deprecate this header in favor of other headers that provide more detailed information and improve the security and privacy of the client and the website.

Syntax and Explanations

The User-Agent string is composed of several parts, including the product token, the comment, and the product version.

A user agent string is typically included in the headers of an HTTP request. The format of a user agent string is as follows:

User-Agent: <product token>/<product version> (<comment>)

Let's take a look at an example of a User-Agent string:

User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.82 Safari/537.36

The product token is used to identify the browser and its family. In this example, "Mozilla/5.0" is the product token for the Chrome browser. The comment section, "Windows NT 10.0; Win64; x64," provides information about the operating system and architecture being used. The product version, "89.0.4389.82," indicates the specific version of the browser.

Below, we can see a user agent string that belongs to Safari on an iPhone running iOS 14.2.

Mozilla/5.0 (iPhone; CPU iPhone OS 14_2 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.1 Mobile/15E148 Safari/604.1

Some browsers allow the User-Agent string to be modified, allowing users to change the UA to a different browser, browser version, operating system, or screen resolution. This can be useful for web developers who want to test their website's compatibility with different browsers and devices.

User Agent Types

There are many different user agents, each with a specific purpose and capabilities.

  • Web browsers like Chrome, Firefox, and Safari are the most common type of user agents. They send a user agent string that identifies the browser and its version, as well as the operating system and device.
  • Search engine crawlers, such as Googlebot and Bingbot, are used by search engines to scan and index websites. They send a user agent string that identifies themselves as a crawler and includes information about the search engine and version.
  • Mobile apps, such as Facebook and Twitter, are also considered user agents. They send a user agent string that identifies the app, version, and operating system being used.
  • Web applications, such as PDF readers, media players, and streaming portals, are also considered user agents. They send a user agent string that identifies the application and version being used.

Changing Your User Agent

Each browser has its method for changing the user agent. In Chrome, Safari, and Microsoft Edge, it can be done through the developer tools, while in Firefox, it requires an add-on called User-Agent Switcher. Changing the user agent can be useful for testing how a website appears on different devices and browsers, and for masking the true identity of the user for privacy reasons. However, for more comprehensive privacy and security measures, it is recommended to use a proxy or VPN.

Spoofing and Sniffing

While the user agent provides practical information, it's possible to use it maliciously. Spoofing a user agent is when a user or hacker alters their user agent string to misrepresent themselves as a different browser, version, or device. In turn, they can bypass security measures or access restricted content.

Sniffing means collecting and analyzing user agent strings to gather information about the device, browser, and operating system and target specific vulnerabilities or serve user-tailored ads and content.

To protect yourself from spoofing, you can use a random user agent, a proxy, or a VPN. These options will change your UA string and mask your true identity. Additionally, you can use tools like 'What is my IP' or 'IP Lookup' to check the address, and ensure that it isn't compromised.


User agents are involved with how websites and servers interact and convey information. Each type of user agent serves a unique purpose. Understanding user agents can help developers optimize website performance, improve user experience and protect against spoofing and sniffing.

In summary:

  • User agents are strings that identify a specific browser or device to a website.
  • They were first introduced in the early days of the internet as a way for servers to identify and provide appropriate content to different browsers and devices.
  • The user agent string has several parts, including the browser name, version, and operating system.
  • HTTP headers include the user agent string as a way for servers to identify and provide appropriate content to different browsers and devices.
  • User-agent strings are becoming less reliable as a means of identifying browsers and devices, as more and more users are masking or spoofing their user agents.
  • Different types of user agents include web browsers, search engine crawlers, mobile apps, web applications, SEO tools, and link checkers.
  • User agents play a role in website development and allow developers to test how their site will perform on different browsers and devices.
  • User agents can also be used for privacy and security, as users can change their user agent string to mask their identity or location.