CVE-2026-71263 in FreeModbus
Summary
by MITRE • 08/05/2026
The LINUXTCP port of FreeModbus contains an off-by-one bounds check in xMBPortTCPPool() (demo/LINUXTCP/port/porttcp.c). The check `if (usTCPFrameBytesLeft > MB_TCP_BUF_SIZE)` uses a strict greater-than comparison instead of greater-than-or-equal against the 263-byte MB_TCP_BUF_SIZE limit. An MBAP frame with a Length field of 264 makes usTCPFrameBytesLeft equal to 263, which passes the flawed check, and the subsequent recv() call writes up to 263 bytes starting at buffer offset 7 into the 263-byte static buffer aucTCPBuf, overflowing it by 7 bytes into the adjacent static variable usTCPBufPos. A single crafted, unauthenticated Modbus TCP packet triggers the overflow, since Modbus has no built-in authentication.
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Analysis
by VulDB Data Team • 08/05/2026
The vulnerability identified in the Linux TCP port of FreeModbus represents a critical buffer overflow flaw within the network communication handling mechanism of this widely-used Modbus protocol implementation. This issue resides in the xMBPortTCPPool() function located in demo/LINUXTCP/port/porttcp.c, where an improper bounds checking mechanism creates a dangerous condition that can be exploited through a simple network packet. The flaw manifests as an off-by-one error in the validation logic, specifically involving the comparison operation that determines whether incoming Modbus TCP frames exceed acceptable buffer limits.
The technical implementation of this vulnerability stems from the use of a strict greater-than comparison rather than a greater-than-or-equal check against the MB_TCP_BUF_SIZE limit of 263 bytes. This subtle but critical difference allows packets with Length fields exactly equal to 264 bytes to bypass the validation entirely, as the calculation usTCPFrameBytesLeft equals 263 when the Length field is set to 264. The subsequent recv() system call proceeds to write data into the static buffer aucTCPBuf starting at offset 7, which results in an overflow that extends 7 bytes beyond the intended 263-byte buffer boundary into adjacent memory space occupied by the usTCPBufPos variable.
This memory corruption vulnerability operates entirely within the Modbus TCP protocol stack without requiring any authentication or authorization, making it particularly dangerous as any network-connected system running this FreeModbus implementation becomes immediately vulnerable to exploitation. The single packet nature of the attack means that an attacker needs only to send one specially crafted Modbus TCP packet with a Length field of 264 bytes to trigger the buffer overflow condition and potentially corrupt adjacent memory locations. This scenario aligns with CWE-121 Stack-based Buffer Overflow, though the implementation involves static buffers rather than stack allocations, and represents a classic example of improper input validation leading to memory corruption.
The operational impact of this vulnerability extends beyond simple buffer overflows to potentially enable arbitrary code execution or system instability depending on how the corrupted memory affects program flow. Since Modbus protocol lacks built-in authentication mechanisms, attackers can exploit this vulnerability remotely without requiring credentials, making it particularly concerning for industrial control systems and networked devices that rely on FreeModbus implementations. The attack surface includes any device or system that utilizes this specific version of FreeModbus in its Linux TCP deployment, potentially affecting critical infrastructure applications where Modbus TCP communication is prevalent.
Mitigation strategies should focus on immediate code fixes including correcting the bounds check to use greater-than-or-equal comparison instead of strict greater-than, implementing proper input validation for all incoming packet lengths, and adding additional bounds checking before any buffer operations. Security patches should also consider implementing more robust error handling for malformed packets, ensuring that all received data undergoes comprehensive validation before being processed. The vulnerability demonstrates the critical importance of thorough input validation in network protocol implementations and highlights how seemingly minor logical errors can create severe security implications. Organizations should review their FreeModbus implementations to ensure they are running patched versions and consider implementing network segmentation or intrusion detection systems to monitor for suspicious Modbus TCP traffic patterns that might indicate exploitation attempts.