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Audit Level: Tactical

Records optimised: 05.08.2026 17:11 UTC

CVE-2026-71263 — Cwalter-At: FreeModbus

Published: 2026-08-05 | Updated: 2026-08-05

CRITICAL 9.1

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.

CVSS VECTOR: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

NVD Record ↗ | Vendor Advisory / Fix ↗

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The Marine OT Vulnerable Assets Library is a curated repository of security advisories identified through automated heuristic filtering. Users should verify hardware revisions with manufacturers before patching.