CVE-2020-27213
📋 TL;DR
This vulnerability in Ethernut Nut/OS allows attackers to predict TCP Initial Sequence Numbers (ISNs) due to insufficient randomness in generation. This enables TCP connection hijacking or spoofing attacks. Affected systems are those running vulnerable versions of Ethernut Nut/OS, particularly in embedded/IoT devices.
💻 Affected Systems
- Ethernut Nut/OS
📦 What is this software?
Nut\/os by Ethernut
⚠️ Risk & Real-World Impact
Worst Case
Complete network compromise allowing man-in-the-middle attacks, session hijacking, data interception/modification, and unauthorized access to connected systems.
Likely Case
TCP session hijacking leading to data theft or manipulation in vulnerable embedded systems, particularly in industrial/ICS environments.
If Mitigated
Limited impact with proper network segmentation, monitoring, and updated systems; attackers would need network access and specific targeting.
🎯 Exploit Status
Exploitation requires network access and ability to sniff/analyze TCP traffic. The Forescout research provides technical details and proof-of-concept.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor updates post-5.1
Vendor Advisory: http://lists.egnite.de/mailman/listinfo/en-nut-announce
Restart Required: Yes
Instructions:
1. Check current Nut/OS version. 2. Update to patched version from Ethernut website. 3. Recompile and redeploy firmware. 4. Restart affected devices.
🔧 Temporary Workarounds
Network Segmentation
allIsolate vulnerable devices in separate network segments with strict access controls
Traffic Monitoring
allImplement network monitoring for TCP sequence anomalies and connection hijacking attempts
🧯 If You Can't Patch
- Implement strict network segmentation and firewall rules to limit exposure
- Deploy intrusion detection systems monitoring for TCP sequence prediction attacks
🔍 How to Verify
Check if Vulnerable:
Check Nut/OS version; if version is 5.1 or earlier, system is vulnerable. Review TCP ISN generation in source code if available.Check Version:
Check device firmware version or consult device documentationVerify Fix Applied:
Verify updated Nut/OS version implements RFC 6528 compliant ISN generation with proper randomness📡 Detection & Monitoring
Log Indicators:
- Unexpected TCP connection resets
- Multiple TCP SYN packets from same source with predictable sequence numbers
- Connection attempts with out-of-window sequence numbers
Network Indicators:
- TCP sequence number predictability patterns
- Man-in-the-middle attack signatures
- Abnormal TCP handshake patterns
SIEM Query:
tcp.flags.syn==1 AND tcp.seq < threshold OR tcp.analysis.retransmission🔗 References
- http://lists.egnite.de/mailman/listinfo/en-nut-announce
- http://www.ethernut.de/en/download/index.html
- https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
- https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
- http://lists.egnite.de/mailman/listinfo/en-nut-announce
- http://www.ethernut.de/en/download/index.html
- https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
- https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
