Security Experts:

Code Execution Flaw Found in Transmission BitTorrent App

Google Project Zero researcher Tavis Ormandy has discovered a critical remote code execution vulnerability in the Transmission BitTorrent client. The expert has proposed a fix, but it has yet to be implemented by the application’s developers.

Transmission is a popular open source BitTorrent client that is available for Windows, Mac and Linux. Ormandy has been analyzing several popular torrent clients and found that Transmission has a serious vulnerability.

According to the researcher, an attacker can execute code on a system running Transmission by getting the targeted user to access a specially crafted website.

“The Transmission bittorrent client uses a client/server architecture, the user interface is the client and a daemon runs in the background managing the downloading, seeding, etc,” Ormandy explained in an advisory. “Clients interact with the daemon using JSON RPC requests to a web server listening on port 9091. By default, the daemon will only accept requests from localhost.”

However, the expert showed that the localhost requirement can be bypassed using a type of attack called “DNS rebinding.”

The attacker sets up a website and adds an iframe that points to a subdomain of that site. The DNS server is configured to respond alternatively with an address controlled by the attacker and localhost (, with a short time to live (TTL). When the victim visits the malicious website, the browser resolves to the attacker-controlled DNS server and then switches to localhost.

“Exploitation is simple, you could set script-torrent-done-enabled and run any command, or set download-dir to /home/user/ and then upload a torrent for ‘.bashrc’,” Ormandy explained.

The expert says he has successfully tested his proof-of-concept (PoC) exploit with Chrome and Firefox running on Windows and Linux.

The vulnerability, tracked as CVE-2018-5702, was reported to Transmission developers on November 30, and Ormandy even provided a fix the next day. However, an official patch still has not been released, which the researcher says is highly unusual for open source projects.

Ormandy pressed the developers and last week they agreed to make his patch public on GitHub so that at least downstream distributions such as Debian and Fedora can roll out their own patches. It’s unclear when an official patch will become available, but it should be included in the next 2.93 release.

Transmission developers pointed out that the macOS and Linux versions are only vulnerable if remote access is enabled; the feature is disabled by default.

Sebastian Lekies, who also works for Google, said he reported the same vulnerability to Transmission developers five years ago, but never heard back from them.

Back in 2016, hackers broke into the Transmission website and planted a malicious installer designed to deliver a new OS X ransomware.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.