CVE-2020-27633
📋 TL;DR
CVE-2020-27633 is a vulnerability in FNET 4.6.3 where TCP Initial Sequence Numbers (ISNs) are generated with insufficient randomness, allowing attackers to predict sequence numbers and hijack TCP sessions. This affects systems using FNET TCP/IP stack, particularly in embedded and industrial control systems. Attackers can intercept, inject, or manipulate network traffic between vulnerable devices.
💻 Affected Systems
- FNET TCP/IP stack
📦 What is this software?
Fnet by Butok
⚠️ Risk & Real-World Impact
Worst Case
Complete network session hijacking allowing man-in-the-middle attacks, data interception, injection of malicious commands, and potential system compromise in critical infrastructure environments.
Likely Case
Session hijacking leading to data interception, unauthorized command injection, and disruption of industrial control system operations.
If Mitigated
Limited impact with proper network segmentation, monitoring, and compensating controls that detect anomalous TCP behavior.
🎯 Exploit Status
Exploitation requires network access and ability to sniff traffic. The Forescout research provides technical details and demonstrates the attack methodology.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Update to FNET version with proper ISN randomization
Vendor Advisory: https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
Restart Required: Yes
Instructions:
1. Contact FNET vendor for patched version. 2. Apply update to affected embedded systems. 3. Restart systems to load new TCP/IP stack implementation. 4. Verify ISN randomness post-update.
🔧 Temporary Workarounds
Network Segmentation
allIsolate vulnerable systems in separate network segments to limit attack surface
Encryption Enforcement
allRequire TLS/SSL for all TCP communications to prevent session hijacking even if ISNs are predictable
🧯 If You Can't Patch
- Implement strict network access controls and firewall rules to limit connections to vulnerable systems
- Deploy network monitoring and intrusion detection systems to identify TCP session hijacking attempts
🔍 How to Verify
Check if Vulnerable:
Check FNET version on embedded systems. If running 4.6.3, system is vulnerable. Network testing tools can analyze ISN randomness patterns.Check Version:
System-specific command to check FNET version (varies by implementation)Verify Fix Applied:
Test ISN generation randomness using network analysis tools after patch application. Verify FNET version is updated beyond 4.6.3.📡 Detection & Monitoring
Log Indicators:
- Unexpected TCP resets
- Sequence number anomalies
- Multiple failed connection attempts with predictable patterns
Network Indicators:
- TCP session hijacking attempts
- Predictable ISN patterns in packet captures
- Man-in-the-middle attack signatures
SIEM Query:
Search for TCP packets with anomalous sequence number patterns or repeated connection attempts from same source with predictable ISNs🔗 References
- https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
- https://www.forescout.com
- https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
- https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
- https://www.forescout.com
- https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
