CVE-2026-71256 in nanoMODBUS
Summary
by MITRE • 08/05/2026
nanoMODBUS through v1.23.0 contains an out-of-bounds stack read leading to a wild-pointer write in nmbs_read_device_identification_basic() / recv_read_device_identification_res() in nanomodbus.c. A fixed 3-element stack array order[3] = {0,1,2} maps object IDs to buffer indices. The server-supplied object_id field (0-255, read directly from the wire) is used without any bounds check as buf_index = order[object_id]. When a malicious Modbus server sends a Read Device Identification response with object_id >= 3, this reads an out-of-bounds/garbage byte from the stack adjacent to order[], which is then used as an index into a 3-element buffers[] array of char* pointers. The resulting wild pointer is passed to strncpy() as the destination, causing an arbitrary-address write with server-controlled data.
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Analysis
by VulDB Data Team • 08/05/2026
The nanoMODBUS library version 1.23.0 contains a critical out-of-bounds stack read vulnerability that can lead to arbitrary memory writes through a wild pointer dereference in the nmbs_read_device_identification_basic() function. This vulnerability exists within the nanomodbus.c file where a fixed 3-element stack array order[3] = {0,1,2} is used to map object IDs to buffer indices during Modbus device identification response processing. The flaw stems from insufficient input validation as the server-supplied object_id field ranging from 0-255 is read directly from the network wire without any bounds checking mechanism. When a malicious Modbus server sends a Read Device Identification response with an object_id value of 3 or greater, the code performs an out-of-bounds stack read that retrieves adjacent garbage bytes from memory locations immediately following the order array. These garbage bytes are then interpreted as buf_index values and used to access a 3-element buffers[] array containing char* pointers, creating a wild pointer that points to arbitrary memory locations. The vulnerability follows the CWE-129 weakness pattern for improper input validation and demonstrates characteristics consistent with CWE-787 Out-of-bounds Write in the context of buffer management errors. This issue directly maps to ATT&CK technique T1059.007 for command and scripting interpreter and can be leveraged by attackers to execute arbitrary code through memory corruption, as the wild pointer is subsequently passed to strncpy() as a destination parameter, enabling an arbitrary-address write with data controlled by the malicious Modbus server. The vulnerability represents a classic stack-based buffer overflow scenario where improper bounds checking allows attackers to manipulate program execution flow and potentially achieve remote code execution. Security practitioners should note this issue aligns with common attack patterns described in the MITRE ATT&CK framework for network-based exploitation and memory corruption vulnerabilities.
The technical implementation of this vulnerability exploits the fundamental weakness in array indexing without proper validation checks. The order array serves as a lookup table mapping object IDs to buffer indices, but when the object_id exceeds the valid range of 0-2, the code performs an out-of-bounds read operation that retrieves unpredictable memory values from adjacent stack locations. These retrieved values become the buf_index used to access the buffers[] array, which contains only three elements indexed from 0-2. This misalignment between expected and actual array access creates a wild pointer situation where the destination address for strncpy() points to arbitrary memory locations rather than intended buffer boundaries. The vulnerability's impact is amplified by the fact that Modbus protocol responses are typically unauthenticated and can be sent by any device on the network, making this attack vector particularly dangerous in industrial control systems and SCADA environments. The issue fundamentally violates proper bounds checking practices and demonstrates how simple input validation failures can lead to complex memory corruption vulnerabilities. This type of vulnerability is classified under CWE-129 as improper validation of array index and also relates to CWE-787 for out-of-bounds write conditions, both of which are well-documented weaknesses in software security engineering practices.
The operational impact of this vulnerability extends beyond simple data corruption to potentially enable complete system compromise when exploited by malicious actors. Attackers can leverage this vulnerability to perform arbitrary memory writes with controlled data, which can be used to overwrite function pointers, return addresses, or other critical program structures within the application's memory space. The wild pointer dereference creates opportunities for attackers to manipulate program execution flow, potentially leading to privilege escalation or complete system takeover in vulnerable environments. This vulnerability is particularly concerning in industrial automation and control systems where nanoMODBUS libraries are commonly used for communication with programmable logic controllers and other embedded devices. The attack surface includes any application that uses the affected nanoMODBUS library version to process Modbus device identification responses from potentially untrusted network sources. Organizations should be aware that this vulnerability can be exploited without requiring special privileges or authentication, as it operates at the protocol level where network traffic is typically accepted and processed without additional security checks. The memory corruption nature of this vulnerability makes it particularly challenging to detect through traditional network monitoring systems, as the malicious behavior may appear as normal Modbus protocol communication until exploitation occurs.
Mitigation strategies for this vulnerability require immediate remediation efforts including upgrading to a patched version of the nanoMODBUS library where bounds checking has been properly implemented for object_id validation. Organizations should ensure that all instances of the vulnerable library are updated to versions that include proper input validation mechanisms preventing out-of-bounds array access. Network segmentation and firewall rules can provide additional defense-in-depth by limiting access to Modbus communication ports and implementing network access controls to prevent unauthorized devices from sending malicious responses. The implementation of proper input validation should include bounds checking for all user-supplied data, particularly when mapping external values to internal array indices. Security engineers should also consider implementing intrusion detection systems capable of monitoring for suspicious Modbus traffic patterns that may indicate exploitation attempts. Regular vulnerability assessments and penetration testing should be conducted to identify other potential instances of similar vulnerabilities in industrial control system components. Additionally, network monitoring solutions should be configured to detect anomalous behavior in Modbus communication that could indicate exploitation attempts. The fix should specifically address the root cause by validating object_id values against the expected range before using them as array indices, ensuring that any value outside the valid 0-2 range triggers appropriate error handling rather than proceeding with potentially dangerous memory operations. Organizations implementing industrial security measures should also consider implementing secure coding practices and regular code reviews to prevent similar vulnerabilities in custom Modbus implementations and other network communication libraries.