Description
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.
Published: 2026-08-05
Score: 9.1 Critical
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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Remediation

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History

Wed, 05 Aug 2026 13:15:00 +0000

Type Values Removed Values Added
Description 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.
Title FreeModbus LINUXTCP Port Off-by-One Global Buffer Overflow in xMBPortTCPPool()
Weaknesses CWE-787
References
Metrics cvssV3_1

{'score': 9.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H'}

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cve-icon MITRE

Status: PUBLISHED

Assigner: TuranSec

Published:

Updated: 2026-08-05T12:52:30.063Z

Reserved: 2026-08-05T12:23:34.966Z

Link: CVE-2026-71263

cve-icon Vulnrichment

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cve-icon NVD

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cve-icon Redhat

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cve-icon OpenCVE Enrichment

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Weaknesses