Matomo is a product that allows website owners to accurately spy on their userbase. Marketed as an open source analytics program with loads of privacy posturing, Matomo, going beyond simple information such as IP Addresses and User Agents, has the ability to allow webmasters to easily collect a vast amount of information about users using functions in Javascript, ranging from screen resolutions all the way to a users geographical location. Matomo can also be used for what they call "session recording", which means that a website can record everythingyou do with Matomo, such as keypresses and clicks, the same behavior of a Remote Access Trojan. This can be used to build a psychological profile on you.
Unlike websites that use third party tracking scripts such as Google Analytics, which you can easily detect and blacklist, you have no way of knowing for certain if Matomo is installed on a website or not since its a first party script, the only way to be certain is to disable javascript even doe Ghostery can alert you of its usage and you can just look at page source or network requests in devtools.
WARNING: Insufficient security clearance. This interaction has been reported to the nearest NSA supernode. Terminating session...
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ERROR: Session termination failed. Please disregard all information within this portal until the advanced infohazard task force arrives at your location.