FreeModbus LINUXTCP Port Off-by-One Global Buffer Overflow in xMBPortTCPPool() (CVE-2026-71263) | HOL Guard CVE

Critical · CVSS 9.1CVE-2026-71263

FreeModbus LINUXTCP Port Off-by-One Global Buffer Overflow in xMBPortTCPPool()

Answer in brief

CVE-2026-71263 records a Critical severity (CVSS 9.1) vulnerability in FreeModbus LINUXTCP Port Off-by-One Global Buffer Overflow in xMBPortTCPPool(). The current sources do not mark it as known exploited. The current feed maps cwalter-at/FreeModbus (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Aug 5, 2026Updated Aug 26, 2026Source checked Sep 21, 2026First seen by HOL Aug 5, 2026Material review Aug 26, 2026

Upstream Advisory

Key facts

Risk: Critical · CVSS 9.1
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Not reported

Why this deserves its current priority

CVSS is 9.1. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps cwalter-at/FreeModbus (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
cwalter-at/FreeModbusgeneric	0	Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for cwalter-at/FreeModbus.
Review the reported fix. Monitor this advisory for an available fix and review any installs of the affected package.

Evidence timeline and material changes

Published upstream
Aug 5, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Aug 26, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Aug 5, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.