We present a simple yet potentially devastating and hard-to-detect threat, called Gummy Browsers, whereby the browser fingerprinting information can be collected and spoofed without the victim's awareness, thereby compromising the privacy and security of any application that uses browser fingerprinting. The idea is that attacker A first makes the user U connect to his website (or to a well-known site the attacker controls) and transparently collects the information from U that is used for fingerprinting purposes. Then, A orchestrates a browser on his own machine to replicate and transmit the same fingerprinting information when connecting to W, fooling W to think that U is the one requesting the service rather than A. This will allow the attacker to profile U and compromise U's privacy. We design and implement the Gummy Browsers attack using three orchestration methods based on script injection, browser settings and debugging tools, and script modification, that can successfully spoof a wide variety of fingerprinting features to mimic many different browsers (including mobile browsers and the Tor browser). We then evaluate the attack against two state-of-the-art browser fingerprinting systems, FPStalker and Panopticlick. Our results show that A can accurately match his own manipulated browser fingerprint with that of any targeted victim user U's fingerprint for a long period of time, without significantly affecting the tracking of U and when only collecting U's fingerprinting information only once. The TPR (true positive rate) for the tracking of the benign user in the presence of the attack is larger than 0.9 in most cases. The FPR (false positive rate) for the tracking of the attacker is also high, larger than 0.9 in all cases. We also argue that the attack can remain completely oblivious to the user and the website, thus making it extremely difficult to thwart in practice.