CVE-2021-34549
📋 TL;DR
This vulnerability in Tor allows an attacker to manipulate circuit ID hashing, potentially causing algorithm inefficiency that could degrade performance or enable denial-of-service attacks. It affects all Tor clients and relays running vulnerable versions before 0.4.6.5.
💻 Affected Systems
- Tor
📦 What is this software?
Tor by Torproject
Tor by Torproject
Tor by Torproject
Tor by Torproject
⚠️ Risk & Real-World Impact
Worst Case
An attacker could cause significant performance degradation or denial-of-service on Tor relays, potentially disrupting the Tor network's functionality for users.
Likely Case
Performance degradation in Tor circuits, potentially slowing down user connections and increasing latency for affected nodes.
If Mitigated
Minimal impact with proper monitoring and quick patching, though some performance degradation might still occur during exploitation attempts.
🎯 Exploit Status
Exploitation requires network access to Tor nodes but no authentication. The technical details suggest specialized knowledge of Tor's circuit handling is needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 0.4.6.5 and later
Vendor Advisory: https://blog.torproject.org/node/2041
Restart Required: Yes
Instructions:
1. Download Tor 0.4.6.5 or later from torproject.org
2. Stop Tor service
3. Install/upgrade to patched version
4. Restart Tor service
🔧 Temporary Workarounds
Rate limiting
allImplement rate limiting on Tor relays to reduce impact of potential DoS attacks
# Configure in torrc: MaxCircuitDirtiness 600
# Configure in torrc: CircuitBuildTimeout 60
🧯 If You Can't Patch
- Monitor Tor relay performance metrics for unusual degradation
- Implement network segmentation to isolate Tor nodes from untrusted networks
🔍 How to Verify
Check if Vulnerable:
Check Tor version with 'tor --version' or check running process versionCheck Version:
tor --versionVerify Fix Applied:
Verify version is 0.4.6.5 or higher and monitor for abnormal circuit behavior📡 Detection & Monitoring
Log Indicators:
- Unusual circuit failure rates
- Abnormal circuit build times
- Excessive circuit ID collisions
Network Indicators:
- Increased latency on Tor circuits
- Unusual patterns in circuit establishment
SIEM Query:
source="tor.log" AND ("circuit failure" OR "circuit timeout" OR "unusual circuit")