Tor is one of the world's most famous anonymity networks, designed to route your traffic through volunteer relays all over the world to help keep your online activity private. But just like any complex software, Tor is not immune to critical bugs and security vulnerabilities. In this post, we dive into CVE-2022-33903: a subtle yet serious Denial of Service (DoS) flaw in Tor's .4.7.x family before version .4.7.8.

Let’s walk through what this vulnerability is, why it matters, and what exploitation looks like.

What is CVE-2022-33903?

CVE-2022-33903 is a vulnerability that affects the Tor anonymity network (specifically versions .4.7.x before .4.7.8). The bug causes the round-trip time (RTT) estimation logic inside Tor to become "wedged," which in simpler terms means it gets stuck in a bad state.

Tor uses RTT estimation to figure out how long it takes for information to travel between nodes, ensuring reliable and efficient communication. If this estimation breaks, Tor can end up making poor decisions about circuit handling.

The end result: an attacker can make certain connections inside Tor unresponsive or cause Tor relays to use excessive resources, rendering parts of the service useless—a classic Denial of Service scenario.

Where is the Bug?

RTT estimation is managed in Tor’s codebase by keeping track of how long network transmissions take. If malformed, spoofed, or unexpected network conditions occur, a bad value can get locked in. From the Tor changelog for .4.7.8:

> "Fix a bug where circuit RTT estimation could get 'stuck,' making the circuit appear to have extremely high (or low) latency."

How Does Exploitation Work?

A malicious user (attacker) can purposely trigger this bug by sending crafted network packets or manipulating circuit activity to poison RTT estimation. Once RTT estimation is "wedged," any further data passing through the affected circuits or relays suffers from delays or even total loss of connectivity.

This attack can disrupt Tor circuits—potentially for many users—with little effort.

Example Exploit (Pseudocode)

Suppose you’re an attacker with the ability to interact with a Tor relay. Here’s a simplified exploit flow:

import socket
import time

def send_jammed_cells(tor_relay_ip, tor_relay_port):
    for _ in range(100):
        s = socket.socket()
        s.connect((tor_relay_ip, tor_relay_port))
        # Send a crafted cell (replace with the actual payload affecting RTT)
        s.send(b'\x00\x00\x00...')  # Malformed Tor cell data here
        # Optionally wait to manipulate RTT recording
        time.sleep(.5)
        s.close()

# Replace with real Tor relay's IP and port
send_jammed_cells("10...2", 9001)

This demonstrates the concept: injecting malformed or oddly timed cells to make Tor’s RTT estimator record completely wrong values, wedging it. A real exploit would use Tor’s network protocol directly.

Note: This has been patched in Tor .4.7.8, so use updated relays!


## How to Fix / Mitigate

The fix for CVE-2022-33903 is present in Tor .4.7.8. If you are running any Tor relay or using Tor in a sensitive environment, upgrade immediately to at least .4.7.8.

sudo apt update
sudo apt install tor

Original References

- NVD - CVE-2022-33903
- Tor Changelog .4.7.8
- Tor Project Security Advisories

Final Thoughts

CVE-2022-33903 is a great example of why subtle bugs in logic—especially involving time and network reliability—can have huge implications for security and privacy software. Even something as simple as a stuck RTT estimation can grind an anonymity system like Tor to a halt.

If you run or care about Tor, always keep it up to date, and watch for maintenance releases and security advisories. Vulnerabilities like this are a reminder that security is a marathon, not a sprint!


*Have questions or want a more detailed technical breakdown? Drop them below!*

Timeline

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