Out-of-Bounds Write in nanoMODBUS Modbus Server

Executive Summary

This vulnerability is an out-of-bounds write in the nanoMODBUS library through version 1.23.0. It occurs in the Modbus server-side function handle_read_file_record() when processing FC 0x14 (Read File Record) requests. The function fails to validate the cumulative response size across multiple sub-requests, allowing an attacker to craft a request that overflows a buffer by writing up to 8490 bytes beyond its 260-byte limit. This can corrupt adjacent memory and potentially lead to denial of service or remote code execution.

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Detection Guidance

Detecting this vulnerability requires monitoring for out-of-bounds write attempts in the nanoMODBUS server. Check for crashes or memory corruption in the server process when handling Modbus FC 0x14 requests. Use network traffic analysis tools like Wireshark to inspect Modbus packets for unusually large or malformed FC 0x14 requests with excessive sub-requests or record lengths.

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Impact Analysis

If you use nanoMODBUS v1.23.0 or earlier in a Modbus server, an unauthenticated attacker on the network could exploit this to crash your server or execute arbitrary code. This is especially dangerous on embedded or bare-metal systems without memory protection, where out-of-bounds writes can directly corrupt critical data or control flow.

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Compliance Impact

This vulnerability could lead to unauthorized memory access, denial of service, or remote code execution, which may compromise data integrity and confidentiality. For systems handling sensitive data under GDPR or HIPAA, such breaches could result in non-compliance, legal penalties, or loss of protected health information (PHI) integrity.

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Mitigation Strategies

Immediately update nanoMODBUS to the latest version if available. If not, restrict network access to the Modbus server using firewalls or network segmentation. Disable FC 0x14 (Read File Record) functionality if not required. Monitor server logs for suspicious activity and consider deploying intrusion detection systems to block malformed Modbus requests.

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