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Researchers identify serious threat to many internet users

Researchers at the University of California, Riverside have identified a weakness in the Transmission Control Protocol (TCP) of all Linux operating systems since late 2012 that enables attackers to hijack users’ internet communications completely remotely.

Such a weakness could be used to launch targeted attacks that track users’ online activity, forcibly terminate a communication, hijack a conversation between hosts or degrade the privacy guarantee by anonymity networks such as Tor.

While most users do not interact directly with the Linux operating system, the software runs behind-the-scenes on internet servers, android phones and a range of other devices. To transfer information from one source to another, Linux and other operating systems use the TCP to package and send data, and the Internet Protocol (IP) to ensure the information gets to the correct destination.

For example, when two people communicate by email, TCP assembles their message into a series of data packets – identified by unique sequence number associated with any particular communication by chance.

The researchers did not rely on chance, however. They identified a subtle flaw (in the form of ‘side channels’) in the Linux software that enables attackers to infer the TCP sequence numbers associated with a particular connection with no more information than the IP address of the communicating parties.

This means that given any two arbitrary machines on the internet, a remote blind attacker (without being able to eavesdrop on the communication), can track users’ online activity, terminate connections with others and inject false material into their connections. Encrypted connections (e.g. HTTPS) are immune to data injection, but they are still subject to being forcefully terminated by the attacker. The weakness would allow the attackers to degrade the privacy of anonymity networks, such as Tor, by forcing the connections to route through certain relays. The attack is fast and reliable, often taking less than a minute and showing a success rate of about 90 per cent. The researchers created a short video showing how the attack works:

Project advisor Zhiyun Qian said that unlike conventional cyber attacks, users could become victims without doing anything wrong, such as downloading malware or clicking on a link in a phishing email.

“The unique aspect of the attack we demonstrated is the very low requirement to be able to carry it out. 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,” said Qian.

He added that the researchers have alerted Linux about the vulnerability, which has resulted in patches applied to the latest Linux version. Until then, Qian has recommended the following temporary patch that can be applied to both client and server hosts. It raises the ‘challenge ACK limit’ to an extremely large value to make it practically impossible to exploit the side channel. This can be done on Ubuntu, for instance, as follows:

  1. Open /etc/sysctl.conf, append a command “/net.ipv4/tcp_challenge_ack_limit = 999999999”.
  2. Use “sysctl -p” to update the configuration.
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