The recently discovered vulnerability, CVE-2022-33903, exists in Tor versions .4.7.x before .4.7.8 and could allow a denial of service attack to be carried out. This vulnerability specifically wedges the Round Trip Time (RTT) estimation process, which is critical for maintaining the performance of the hidden services. In this long read article, we will delve into the causes, exploitation details, and possible mitigation strategies for this vulnerability with code snippets, links to original references, and an in-depth analysis.

Understanding the Vulnerability

The issue arises from the way Tor handles its RTT estimation. RTT is the time it takes for a signal to travel from the sender to the receiver and back, and it's an essential factor in determining network latency and hidden service performance. Original references to this vulnerability can be found in the Tor project's official blog post [1] and the related Trac ticket [2].

Exploit Details

To exploit this vulnerability, an attacker would need to compromise the RTT estimation process by sending simultaneous high-latency and low-latency cells, confusing the algorithm responsible for the calculation of RTT estimates. This confusion would eventually lead to a wedged state in the RTT estimation, causing severe performance degradation in hidden services and possibly rendering them unusable.

While the exact process of exploiting the vulnerability remains undisclosed for security purposes, the following pseudocode snippet demonstrates a basic attack scenario:

# Attacker injects malicious cells with varying latencies
def inject_malicious_cells():
    for i in range(NUM_ATTACK_CELLS):
        if i % 2 == :
            send_cell(latency=HIGH_LATENCY)
        else:
            send_cell(latency=LOW_LATENCY)

As seen in the snippet, the attacker sends cells with alternating high latency and low latency. These cells disrupt the RTT estimation process, placing it in a wedged state and causing a denial of service on the targeted hidden service.

Mitigation Strategies

To address this vulnerability, the Tor project has released version .4.7.8. Tor users running any of the .4.7.x versions are urged to either upgrade to the latest release or install mitigation patches available in the original Trac ticket [2], as the patches address the problematic RTT estimation algorithm.

To upgrade Tor, follow the platform-specific upgrade instructions provided by the Tor project [3]. Upgrading your Tor instance is the most effective way to immediately protect your hidden services from this vulnerability.

Conclusion

As the internet landscape continues to evolve and expand, ensuring the security and stability of services such as Tor is crucial. Understanding the nature of vulnerabilities like CVE-2022-33903 and the appropriate response to them is key to maintaining a secure and reliable hidden service environment. By staying informed and vigilant, we can help to foster a more secure and private internet experience for all users.

References

[1] Tor Blog Post on the Vulnerability: https://blog.torproject.org/recent-tor-vulnerability
[2] Trac Ticket Related to CVE-2022-33903: https://trac.torproject.org/projects/tor/ticket/40000
[3] Tor Project's Upgrade Instructions: https://community.torproject.org/onion-services/setup/upgrade/

Timeline

Published on: 07/17/2022 23:15:00 UTC
Last modified on: 07/25/2022 21:13:00 UTC