BitTorrent patches reflective DDoS attack security vulnerability

A vulnerability which could divert traffic to launch cyberattacks has been mitigated two weeks after public disclosure.


BitTorrent has taken rapid steps to mitigate a flaw which could divert user traffic to launch reflective DDoS attacks.

The flaw, reported by Florian Adamsky at the USENIX conference in Washington, D.C., affects popular BitTorrent clients such as uTorrent, Mainline and Vuze, which were known to be vulnerable to distributed reflective denial-of-service (DRDoS) attacks.

According to the researchers from City University London, BitTorrent protocols could be exploited to reflect and amplify traffic from other users within the ecosystem -- which could then be harnessed to launch DRDoS attacks powered up to 120 times the size of the original data request.

Successful distributed denial-of-service (DDoS) and DRDoS attacks launched against websites flood domains with traffic, often leaving systems unable to cope with the influx and resulting in legitimate traffic being denied access to Web resources.

The team said in a paper (.PDF) documenting the vulnerability that BitTorrent protocols Micro Transport Protocol (uTP), Distributed Hash Table (DHT), Message Stream Encryption (MSE) and BitTorrent Sync (BTSync) are exploitable.

On Thursday, Vice President of Communications at BitTorrent Christian Averill said in a blog post no attack using this method has been observed in the wild and as the researchers informed the BitTorrent team of the vulnerability ahead of public disclosure, this has given BitTorrent the opportunity to "mitigate the possibility of such an attack."

Francisco De La Cruz, a software engineer from the uTorrent and BitTorrent team, wrote a detailed analysis of the attack and the steps the company has taken to reduce the risk of this vulnerability.

The vulnerability lies within libµTP, a commonly used tool which can detect network congestion and automatically throttle itself -- a useful feature when BitTorrent clients are being used on home networks. However, the way libµTP handles incoming connections allows reflectors to accept any acknowledgement number when receiving a data packet, which opens the doorway to traffic abuse.

The success of a DRDoS relies on how much traffic an attacker can direct towards a victim, known as the Bandwidth Amplification Factor (BAF). The higher the BAF, the more successful the attack. In order to reduce the BAF ratio and mitigate the security issue, BitTorrent engineers have ensured a unique acknowledgement number is required when a target is receiving traffic.

While this can still be guessed, it would be difficult and time-consuming to do so for a wide pool of victims.

De La Cruz said:

"As of August 4th, 2015 uTorrent, BitTorrent and BitTorrent Sync clients using libµTP will now only transition into a connection state if they receive valid acknowledgments from the connection initiators.

This means that any packets falling outside of an allowed window will be dropped by a reflector and will never make it to a victim. Since the mitigation occurs at the libµTP level, other company protocols that can run over libµTP like Message Stream Encryption (MSE) are also serviced by the mitigation."

Regarding BTSync, BitTorrent says the severity of the vulnerability -- even before recent updates were applied to the protocol -- mitigated the risk of this vulnerability. In order to exploit the security weakness, an attacker would have to know the Sync user, identifiers would have to be made public, and the protocol's design ensures that peers in a share are limited -- keeping the potential attack scale down. According to the BitTorrent executive, the protocol therefore would "not serve as an effective source to mount large-scale attacks."

Averill commented:

"This is a serious issue and as with all security issues, we take it very seriously. We thank Florian for his work and will continue to both improve the security of these protocols and share information on these updates through our blog channels and forums."

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