BitTorrent can be exploited for large-scale DoS attacks

Researchers have discovered a weakness in the popular BitTorrent file sharing protocol that could be abused to anonymously generate large-scale denial of service attacks against networks.

Florian Adamsky and Muttukrishnan Rajaran from the City University of London, Rudolf Jäger from Technichesche Hochschule Mittelhesssen and Syed Ali Khayam of security vendor PLUMgrid discovered that a single attacker can exploit BitTorrent peers to generate up to 50 times the amount of traffic originally initiated.

In the case of the BTSync file synchronisation application, an attacker could amplify initial traffic 120 times by sending out specifically crafted requests to peers which exploit weaknesses in their data transmission protocols.

As BitTorrent operates with the user datagram protocol (UDP) - which does not have address-spoofing defences like transmission control protocol (TCP) - attackers can hide their origins and send a large amount of data via other users' clients.

This is a technique called reflection, and is increasingly used in denial of service attacks on the internet, with attackers taking advantage of vulnerable protocols to disrupt networks.

The most popular BitTorrent clients such as Vuze and uTorrent are vulnerable to the exploit, and can be made to take part in DoS attacks, the researchers found.

The IP address spoofing makes it difficult for internet providers to deploy effective countermeasures against BitTorrent reflective DoS attacks.

They suggest a two-pronged strategy comprising global internet provider coordination to prevent IP spoofing, as well as adding defence mechanisms in the protocols used by BitTorrent clients to avoid their exploitation.

The researchers' paper, P2P File-Sharing in Hell: Exploiting BitTorrent Vulnerabilities to Launch Distributed Reflective DoS Attacks [pdf] was presented at a recent Usenix forum. The group crawled more than 2.1 million internet protocol (IP) addresses for analysis.

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