The sites that you visit can know that you are using Tor, because the <a href="https://metrics.torproject.org/exonerator.html">list of exit nodes of the Tornetwork</a>is public.
Parental controls, Internet service providers, and countries with heavy censorship can identify and block connections to the Tornetwork that don't use Tor bridges.
[[!img doc/anonymous_internet/tor/tor.svg size="600x" link="no" alt="A Tor connection goes through 3 relays with the last one establishing the actual connection to the final destination"]]
Pretend to be the destination server, a technique known as <i>machine-in-the-middle</i> attack (MitM). That is why you should pay even more attention to the security warnings in <i>TorBrowser</i>. If you get such a warning, use the [[New Identity|doc/anonymous_internet/Tor_Browser#new-identity]] feature of <i>TorBrowser</i> to change exit node.
Tor exit nodes have been used in the past to collect sensitive information from unencrypted connections. Malicious exit nodes are regularly identified and removed from the Tornetwork. For an example, see <a href="https://arstechnica.com/information-technology/2007/09/security-expert-used-tor-to-collect-government-e-mail-passwords/">Ars Technica: Security expert used Tor to collect government e-mail passwords</a>.
The Tornetwork has more than 6 000 relays. Organizations running Torrelays include universities like the MIT, activist groups like Riseup, nonprofits like Derechos Digitales, Internet hosting companies like Private Internet Access, and so on. The huge diversity of people and organizations running Torrelays makes it more secureand more sustainable.
A powerful adversary, who could analyze the timing and shape of the traffic entering and exiting the Tornetwork, might be able to deanonymize Tor users.These attacks are called <i>end-to-end correlation</i> attacks, because the attacker has to observe both ends of a Tor circuit at the same time.
No anonymity network used for rapid connections, like browsing the webor instant messaging, can protect 100% from end-to-end correlation attacks. In this case, VPNs (Virtual Private Networks) are less secure than Tor, because they do not use 3 independent relays.