CVE-2026-71263

Received Received - Intake

Buffer Overflow in FreeModbus LINUXTCP Port

Publication date: 2026-08-05

Last updated on: 2026-08-10

Assigner: TuranSec

Description

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.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-08-05

Last Modified

2026-08-10

Generated

2026-08-26

AI Q&A

2026-08-05

EPSS Evaluated

2026-08-24

NVD

CVE-2026-71263

EUVD

EUVD-2026-53348

Affected Vendors & Products

Vendor	Product	Version / Range
freemodbus	freemodbus	*
christian_walter	freemodbus	1.6.0

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-787	The product writes data past the end, or before the beginning, of the intended buffer.

Attack-Flow Graph

Executive Summary

This vulnerability is an off-by-one buffer overflow in the FreeModbus library's LINUXTCP port. The flaw occurs in the xMBPortTCPPool() function where a strict greater-than check against a 263-byte buffer size allows a crafted Modbus TCP packet with a Length field of 264 to overflow the buffer by 7 bytes. This happens because the check fails to account for the exact buffer size limit, causing data to overwrite adjacent memory.

Detection Guidance

Detecting this vulnerability requires monitoring for malformed Modbus TCP packets with a Length field of 264 bytes. Use network traffic analyzers like Wireshark to filter Modbus TCP traffic and inspect packet lengths. Check for crashes or memory corruption in FreeModbus applications after receiving such packets.

Impact Analysis

This vulnerability allows a remote attacker to send a single unauthenticated Modbus TCP packet that triggers a buffer overflow. This could lead to arbitrary code execution, denial of service, or data corruption on systems running vulnerable FreeModbus versions. The impact depends on the system's use of FreeModbus, particularly in embedded devices managing Modbus TCP communications.

Compliance Impact

This vulnerability could lead to unauthorized code execution or data corruption in systems handling sensitive data, potentially violating GDPR's integrity and confidentiality requirements or HIPAA's security rules for protected health information. The buffer overflow may allow attackers to manipulate system behavior or access restricted memory, compromising compliance with data protection standards.

Mitigation Strategies

Immediately update FreeModbus to the latest patched version. If unavailable, apply input validation to reject Modbus TCP packets with Length fields exceeding 263 bytes. Use network firewalls to block unauthorized Modbus TCP traffic. Monitor systems for signs of exploitation or crashes.

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