CVE-2025-51986
📋 TL;DR
This vulnerability in the FreeMODBUS demo/LINUXTCP implementation allows attackers to trigger an infinite loop by sending a specially crafted packet with a malicious length value. This affects systems running the vulnerable FreeMODBUS library version, potentially causing denial of service. The issue specifically impacts the TCP implementation in the demo code.
💻 Affected Systems
- FreeMODBUS
⚠️ Manual Verification Required
This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.
Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).
🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.
- Review the CVE details at NVD
- Check vendor security advisories for your specific version
- Test if the vulnerability is exploitable in your environment
- Consider updating to the latest version as a precaution
⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service through resource exhaustion, potentially crashing the service or consuming all CPU resources indefinitely.
Likely Case
Service disruption or unresponsiveness of the MODBUS TCP service, requiring manual intervention to restart.
If Mitigated
Minimal impact if proper network segmentation and monitoring are in place to detect anomalous traffic patterns.
🎯 Exploit Status
The GitHub gist provides technical details that could be used to create an exploit. Crafting the malicious packet requires minimal technical skill.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Not available
Vendor Advisory: No official vendor advisory found
Restart Required: No
Instructions:
1. Review the FreeMODBUS source code for the demo/LINUXTCP implementation. 2. Implement proper input validation for packet length values. 3. Recompile and redeploy the modified library.
🔧 Temporary Workarounds
Network Filtering
linuxBlock or filter MODBUS TCP traffic from untrusted sources using firewall rules.
iptables -A INPUT -p tcp --dport 502 -s untrusted_network -j DROP
Rate Limiting
linuxImplement rate limiting on MODBUS TCP port to prevent rapid exploitation attempts.
iptables -A INPUT -p tcp --dport 502 -m limit --limit 10/min --limit-burst 20 -j ACCEPT
🧯 If You Can't Patch
- Isolate MODBUS TCP services in a separate network segment with strict access controls.
- Implement monitoring and alerting for abnormal traffic patterns or service unavailability.
🔍 How to Verify
Check if Vulnerable:
Check if FreeMODBUS v.2018-09-12 demo/LINUXTCP implementation is in use. Review source code for lack of length validation in TCP packet handling.Check Version:
Check build configuration or source code headers for FreeMODBUS version information.Verify Fix Applied:
Test with crafted packets containing malicious length values to ensure the service remains responsive.📡 Detection & Monitoring
Log Indicators:
- Repeated connection attempts on port 502
- Service crash or restart logs
- High CPU usage by MODBUS process
Network Indicators:
- Abnormal packet sizes in MODBUS TCP traffic
- Rapid connection attempts to port 502
SIEM Query:
source_port=502 AND (packet_size>normal_threshold OR connection_rate>threshold)