Off-by-One Vulnerability in FreeModbus Affecting LINUXTCP Port

CVE-2026-71263

What is CVE-2026-71263?

The FreeModbus LINUXTCP port is susceptible to an off-by-one buffer overflow due to a flawed bounds check in the xMBPortTCPPool() function. This vulnerability allows an attacker to send a specially crafted Modbus TCP packet with a length field that exceeds the expected buffer size. The comparison improperly uses a 'greater-than' rather than a 'greater-than-or-equal' condition, resulting in a write operation exceeding the allocated buffer by 7 bytes, potentially corrupting adjacent memory. This flaw underscores the importance of rigorous validation in network protocols, particularly since Modbus lacks built-in authentication mechanisms.

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