Stack Out-of-Bounds Write in nanoMODBUS

Executive Summary

This vulnerability is an out-of-bounds stack read in nanoMODBUS through v1.23.0 that leads to a wild-pointer write. The issue occurs in the function nmbs_read_device_identification_basic() / recv_read_device_identification_res() where a 3-element stack array maps object IDs to buffer indices. A malicious Modbus server can send a Read Device Identification response with an object_id value greater than or equal to 3, causing an out-of-bounds read from the stack. This garbage value is then used as an index into a 3-element buffers array, resulting in an arbitrary-address write with server-controlled data.

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Detection Guidance

Detecting this vulnerability requires monitoring Modbus traffic for Read Device Identification responses with object_id values greater than or equal to 3. Use network sniffing tools like tcpdump or Wireshark to capture Modbus TCP/RTU traffic and filter for function code 0x2B (Read Device Identification) with invalid object IDs.

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Impact Analysis

This vulnerability can allow an attacker to execute arbitrary code or cause a denial of service by exploiting the wild-pointer write. Since the attack is triggered by a malicious Modbus server, any system using the vulnerable nanoMODBUS library to communicate with Modbus devices could be affected. The high CVSS score (9.8) indicates severe impact on confidentiality, integrity, and availability.

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Compliance Impact

This vulnerability could lead to unauthorized data access, modification, or system disruption, which may violate compliance requirements under GDPR (data protection), HIPAA (healthcare data security), and other regulations. The high impact on confidentiality and integrity means organizations could face legal penalties or data breach notifications if exploited.

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Mitigation Strategies

Immediately update nanoMODBUS to the latest patched version (v1.23.1 or later). If updating is not possible, implement strict input validation to reject Read Device Identification responses with object_id >= 3. Disable Modbus Read Device Identification functionality if unused. Monitor network traffic for suspicious Modbus responses.

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