Linux Flaw Allows Attackers to Hijack Web Connections

Researchers discovered that a Transmission Control Protocol (TCP) specification implemented in Linux creates a vulnerability that can be exploited to terminate connections and conduct data injection attacks.

The flaw, tracked as CVE-2016-5696, is related to a feature described in RFC 5961, which should make it more difficult to launch off-path TCP spoofing attacks. The specification was formulated in 2010, but it has not been fully implemented in Windows, Mac OS X, and FreeBSD-based operating systems. However, the feature has been implemented in the Linux kernel since version 3.6, released in 2012.

A team of researchers from the University of California, Riverside and the U.S. Army Research Laboratory identified an attack method that allows a blind, off-path attacker to intercept TCP-based connections between two hosts on the Internet.

TCP data packets transmitted from one host to another are identified by unique sequence numbers. Since there are nearly 4 billion possible sequences, it should be impossible to determine which number is associated to a specific communication.

However, experts discovered that the flaw in Linux can be leveraged to deduce the sequence numbers associated with a particular connection simply by knowing the IP addresses of the targeted communicating parties. An attacker, who doesn’t need to be able to directly intercept the connection as in a classic man-in-the-middle (MitM) attack, can exploit this weakness to track users’ activity, terminate connections, and inject arbitrary data into a connection.

Researchers noted that data cannot be injected into HTTPS communications, but the connection can still be terminated using this method. One attack scenario described by the experts involves targeting Tor by disrupting connections between certain relays so that users are forced to use attacker-controlled exit relays.

“The unique aspect of the attack we demonstrated is the very low requirement to be able to carry it out,” explained Zhiyun Qian, one of the researchers involved in this project. “Essentially, it can be done easily by anyone in the world where an attack machine is in a network that allows IP spoofing. The only piece of information that is needed is the pair of IP addresses (for victim client and server), which is fairly easy to obtain.”

The experts demonstrated the attack on the USA Today news website, but pointed out that many services hosted on Linux machines could be vulnerable, including video, ad, news and chat services. The condition is for the website or service to have a long-lived TCP connection – between 40 and 60 seconds. The success rate for the attack is between 88% and 97%.

The vulnerability was patched in the Linux kernel in July with the release of version 4.7. The developers of various Linux distributions indicated that they are working on addressing the security hole.

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