Threat Actor Compromised More than 25 Percent of Tor Network Relays, Research Shows

Unknown actors took control over a quarter of all Tor network relays to launch man-in-the-middle attacks, target bitcoin addresses and much more.

Tor is a software that lets users obfuscate their network traffic by routing it automatically through numerous volunteer-operated relays worldwide. That traffic is typically encrypted, so intercepting it is not really an option, but the attacker did something more subtle.

Security researcher ‘nusenu’ published an extensive analysis of the threat actors’ actions in 2021, saying that it’s likely the most significant relay compromise to date, covering around 27 percent of Tor Network Relays, a conservative estimate.

One problem the researcher found relates to how the Tor browser deals with unsecure links. It turns out that the Tor browser is not HTTPS-only, which means that it can also display HTTP. Showing websites in plain text is a gold mine for attackers looking for valuable information.

The researcher also says the full nature of the attacks is not known, with a few exceptions.

“We know about mitmproxy, sslstrip, bitcoin address rewrites and download modification attacks but it is not possible to rule out other types of attacks. Imagine an attacker runs 27% of the tor network’s exit capacity and a firefox exploit affecting Tor Browser gets published before all users got their (auto)updates,” said nusenu.

The sslstrip and bitcoin address rewrite are interesting because it likely means that the attackers perform something called SSL stripping, forcing victims to use an HTTP version for cryptocurrency mixing service, exposing the addresses to the attacker. This leaves them open to redirect funds to their wallets.

The researcher also published several possible mitigations, such as moving Tor to an HTTPS-only version, but that isn’t easy to achieve right now. He also proposed a few measures that would make it easier to verify when the relays are tainted.

The few available indicators seem to show that the attacker is operating out of Russia, but it’s difficult to verify that information.

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