What is Tor Browser? How it works and what it protects

Tor Browser is a free browser, built from a modified Firefox, that sends your traffic through the Tor network: three volunteer-run relays, encrypted in layers. Websites see a Tor relay instead of your IP address, your internet provider can’t see which sites you visit, and the browser resists tracking and fingerprinting.^[1]

Tor is the network; Tor Browser is how most people use it. The browser is built on Firefox ESR, the Extended Support Release of Firefox, with changes for privacy.^[1] Here’s how it works, what it protects and where its limits are.

How does Tor work?

Tor sends your traffic along a path through the network called a circuit, made of randomly chosen relays: servers run by volunteers around the world. Most circuits have three: a guard, where you enter; a middle relay; and an exit, which connects to the website for you.^[1]

Before your data leaves your device, it’s wrapped in three layers of encryption, and each relay removes one layer before passing it on, like peeling an onion.^[1] This is called onion routing, and it means no relay sees the whole picture. The guard sees your IP address but not what you’re doing; the exit sees the traffic you send but not who sent it.^[2] The website sees only the exit relay’s address.^[1]

What is an onion site?

An onion site is a website you can reach only through Tor, at an address ending in .onion. It’s what people often mean by the “dark web”.^[1] Onion services hide where the site is hosted and encrypt the connection end to end. Their addresses are also self-authenticating: the address itself is tied to the site’s encryption key, so you know you reached the right site without relying on a certificate authority, the companies that vouch for ordinary web addresses.^[1]

Traffic to an onion site never leaves the network through an exit relay; the connection usually takes six hops instead. Some ordinary websites run onion versions too, and when a site advertises one, Tor Browser shows a “.onion available” prompt in the address bar.^[1]

What does Tor Browser protect?

• Your IP address, from websites. Sites see a connection from the Tor network, not your real IP address, and don’t know who you are unless you tell them.^[1]
• Your browsing, from your network. Your internet provider, or whoever runs the Wi-Fi, can’t see the names or addresses of the sites you visit. It can see that you’re using Tor.^[1][3]
• Your trail, from trackers. By default, Tor Browser keeps no browsing history, and cookies last only until you close it or choose New Identity, which also switches to new circuits.^[1] It keeps each site’s data apart from other sites’, a defence called first-party isolation.^[4]
• Your device, from fingerprinting. Fingerprinting means recognising a browser from details like its window size and settings, without cookies. Tor Browser tries to make its users look alike. It sizes windows in standard steps, adding margins when needed (a technique called letterboxing), and reports the same operating system version for everyone on a platform, such as Windows 10 for all Windows users.^[4]
• Your connection, from blocking. Where an internet provider or government blocks Tor, Tor Browser includes tools to get around the block.^[1]

Is Tor Browser 100% safe?

No, and the Tor Project doesn’t claim it is. It says perfect anonymity is impossible, even with Tor.^[3] These are the main gaps:

• Exit relays can read unencrypted traffic. Tor’s encryption ends at the exit. If a site doesn’t use HTTPS, the exit can see what you send; with HTTPS, it learns only the destination.^[2] On desktop, Tor Browser’s HTTPS-Only Mode, which insists on encrypted connections, is on by default.^[1]
• Logging in identifies you. If you sign in, or give your name, email or phone number, that site knows who you are.^[3]
• Downloaded files can leak. Documents such as PDFs and Word files can load content outside Tor when you open them in another app, which reveals your real IP address.^[3]
• Add-ons and torrents can undo the protection. Extra add-ons may bypass Tor, and file-sharing apps often send out your real IP address.^[3]
• Fingerprinting is reduced, not removed. The Tor Project says it’s practically impossible to make every user identical, and some details, such as your operating system and language, can’t be fully hidden.^[4]
• Software bugs. Like any browser, Tor Browser can have security holes, and attackers have used them against people running outdated versions.^[5] Updates and higher security levels cut that risk.

What are Tor Browser’s security levels?

Tor Browser has three security levels, the same on desktop and Android. Higher levels switch off web features that can be used in attacks, at the cost of breaking some sites.^[6]

JavaScript is the code that makes pages interactive, and it can also be used to attack a browser and unmask its user.^[1] You change the level in Tor Browser’s settings, the browser restarts to apply it, and you can switch back whenever you like.^[6]

Can the FBI track Tor Browser?

Tor is designed so that no single relay can tie you to what you do online.^[2] Even so, investigators have identified Tor users. In the documented cases below, they didn’t break Tor’s encryption. They went after the weak points around it.

• Outdated software (2013). The FBI took control of Freedom Hosting, a host for onion sites, whose pages then served malware that exploited a Firefox flaw in old versions of the Tor Browser Bundle. It sent each victim’s computer details to a server outside Tor, exposing their real IP address. People who had updated were safe.^[5]
• Personal mistakes (2013). In the Silk Road case, court documents said the site’s alleged operator had used his real name and Gmail address in forum posts connected with setting it up.^[7]
• An outdated app (2019 to 2021). A 2024 news investigation described a law-enforcement attack that identified a user of Ricochet, a long-retired app that ran over Tor’s onion services. The Tor Project said the user ran an old version without protections added since, and that nothing it had learned suggested Tor Browser was attacked or exploited.^[8]

The lessons are dull but useful: keep Tor Browser updated, use maintained software, and keep your real identity away from anything you want to stay anonymous.

Is Tor 100% untraceable?

No. Tor makes tracing you much harder, not impossible.^[3] Besides mistakes and outdated software, there’s timing analysis: matching when traffic appears at different points in the network to link a user to their activity. The 2024 case used connection times this way, and the Tor Project notes that someone who doesn’t keep a connection open for a long time is less exposed to it.^[8]

Can Tor get around censorship?

Often, yes. Some internet providers and governments block Tor.^[1] To get past that, Tor Browser can connect through a bridge, a relay that isn’t publicly listed and so is harder to block, using a “pluggable transport” that disguises the traffic.^[1] The main ones, as of November 2026:

• obfs4 makes Tor traffic look random.
• Snowflake runs through a large pool of short-lived volunteer proxies.
• WebTunnel makes the connection look like ordinary HTTPS browsing.
• meek connects through a big cloud provider.^[1]

Tor Browser’s Connection Assist can pick the bridge setting that works best where you are.^[1] Bridges also help if you’d rather your network not see that you use Tor.^[3]

Is it illegal to browse with Tor?

Not in the US or many other countries. Using Tor is legal in the US, and the Electronic Frontier Foundation, a digital-rights group whose legal FAQ the Tor Project hosts, believes even running a Tor relay, which carries other people’s traffic, is legal under US law.^[9] Some governments block Tor, though, and laws differ, so check whether Tor is legal where you live.^[1] Breaking the law over Tor is still breaking the law.

Tor vs a VPN: what’s the difference?

A VPN (virtual private network) sends your traffic through one company’s server. That’s usually faster than Tor, and it typically covers every app on your device. But the Tor Project’s point about single-server proxies applies: the provider knows both who you are and what you browse, so you have to trust it.^[2] Tor splits that knowledge across three relays, at the cost of speed, and it protects only apps set up to use it, such as Tor Browser.^[2][3]

Neither is better at everything. A VPN suits hiding your traffic from a local network; Tor suits not wanting any single company to hold both your identity and your browsing.

Why do some websites block Tor?

Tor relays are publicly listed, and websites see the exit relay’s address, so a site can tell when a visit comes through Tor.^[1] Because so many requests arrive through the same relays, sites often struggle to tell people from bots, so Tor users see a lot of CAPTCHAs.^[1] Tor Browser’s disguised browser details can also look inconsistent to anti-fraud systems, which may treat Tor users as bots and turn them away.^[4]

If a site won’t load, Tor Browser’s “New Tor Circuit for this Site” option reloads it over a new circuit, which can help when an exit relay is the problem.^[1] If a site shows a message such as “access via proxy, VPN or Tor is not permitted”, it has chosen to refuse those connections.

What is Tails?

Tails is a portable operating system, part of the Tor Project, that you start from a USB stick instead of Windows, macOS or Linux. It uses the Tor network to protect your privacy online and leaves no trace on the computer when you shut it down.^[10] It’s worth a look if you need more protection than a browser alone can give.

Using Tor on iPhone and Android brings its own choices and limits. Mantis plans to build a Tor-routed browser into its app, but it isn’t built or tested yet.