The Tor Project has flagged a server in Russia after a security researcher found it slipped in malware when users were downloading files.

Tor is short for The Onion Router, which is software that offers users a greater degree of privacy when browsing the Internet by routing traffic through a network of worldwide servers. The system is widely used by people who want to conceal their real IP address and mask their web browsing.

The suspicious server was an "exit node" for Tor, which is the last server in the winding chain used to direct web browsing traffic to its destination.

Roger Dingledine, Tor Project's project leader and director, wrote the Russian server has been labeled a bad exit node, which should mean Tor clients will avoid using the server.

The Russian server was found by Josh Pitts, who does penetration testing and security assessments with Leviathan Security Group. He wrote he wanted to find out how common it was to find attackers modifying the binaries of legitimate code in order to deliver malware.

Binaries from large software companies have digital signatures that can be verified to make sure the code hasn't been modified. But Pitts wrote most code isn't signed, and even further, most don't employ TLS (Transport Layer Security) during downloading. TLS is the successor to SSL (Secure Sockets Layer), which encrypts connections between a client and a server.

He suspected attackers were "patching" binaries during man-in-the-middle attacks and took a look at more than 1,110 Tor exit nodes.

Pitts only found one Tor exit node that was patching binaries. The node would modify only uncompressed portable executables, he wrote.

"This does not mean that other nodes on the Tor network are not patching binaries; I may not have caught them, or they may be waiting to patch only a small set of binaries," he wrote.

The broad lesson for users is that they should be wary of downloading code that is not protected by SSL/TLS, even if the binary itself is digitally signed, Pitts wrote.

"All people, but especially those in countries hostile to 'Internet freedom,' as well as those using Tor anywhere, should be wary of downloading binaries hosted in the clear -- and all users should have a way of checking hashes and signatures out of band prior to executing the binary," he wrote.

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