![]() ![]() erAgent should equal the header sent over HTTP, for instance. Most spoofing extensions update the values here but many do not. Window.navigator provides information about the operating system and browser of the client. Window.InstallTrigger != undefined -> true Window.mozInnerScreenX != undefined -> true Window.webkitCancelAnimationFrame != undefined -> false We can test the presence of these using CSS.supports. CSS Queriesīrowsers often have their own experimental/non-standard CSS features. For the sake of brevity, I compare Firefox and Chrome, but the ideas contained in this section are applicable to any browser that supports JS. There are many ways JavaScript can be used to detect browser spoofing. This is another vector that can be used to detect spoofing. ![]() Accept-Encoding appears before Accept-Language in Chrome too. In Firefox, the User-Agent header appears at the top, whereas in Chrome it is at the bottom. Furthermore, the order of headers is also different. The values of other headers are different too, like Accept, and Accept-Language. Notice that Firefox is missing many headers Chrome has, mostly the ones beginning with sec. For instance, look at this sample output from p0f The p0f tool, released over 2 decades ago, was the first to do this. Given that there are variations in TCP implementations across operating systems and operating system versions, TCP can be used as a fingerprinting vector. HTTP uses TCP as the underlying transport protocol. This post will begin at the lower levels of the web technology stack and move up from there. This string is important because it can be used by websites for platform detection, bot detection, etc. We can see that the Chrome user agent string reveals OS and browser engine information. For instance, when making a request to, curl sends the following data:Īccept: text/html,application/xhtml+xml,application/xml q=0.9,image/avif,image/webp,image/apng,*/* q=0.8,application/signed-exchange v=b3 q=0.9 ![]() This string is communicated via HTTP and can be accessed through Javascript as well. Each (or most) user agents have a User-Agent string that reveals information about them. Firefox is a user agent, Chrome is a user agent, Opera is a user agent. In computing, a user agent is any software, acting on behalf of a user, which “retrieves, renders and facilitates end-user interaction with Web content.” This post will reveal some of the techniques they use and illustrate the futility of many of these browser extensions. Bot detection and fingerprinting vendors like Distil Networks, Imperva, WhiteOps, etc are all getting smarter about detecting this kind of spoofing. There are some other reasons to change a browser’s user agent, for example to test the mobile version of a website, or to bypass rate limits while scraping. Unfortunately, due to the abundance of other methods to detect browser and operating system information (as will be discussed in this article), these extensions do not meaningfully enhance privacy. On the Chrome Store, there are dozens of extensions allowing you to switch your user agent. ![]() Spoofing a browser’s user agent is often hailed as a privacy enhancing technique. ![]()
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