CVE-2026-71263: Cwalter-At Freemodbus

Critical severity, CVSS 9.1. EPSS: 0.5% chance of exploitation in the next 30 days.

The LINUXTCP port of FreeModbus contains an off-by-one bounds check in xMBPortTCPPool (demo/LINUXTCP/port/porttcp.c). The check uses a strict greater-than comparison instead of greater-than-or-equal against the 263-byte MB_TCP_BUF_SIZE limit.

Affected products

• Cwalter-At Freemodbus: any version

Published 2026-08-05. Last modified 2026-08-26.