What is CVE-2026-71263?

CVE-2026-71263 is a critical-severity vulnerability in Cwalter-At Freemodbus, classified under Out-of-bounds Write. CVSS score: 9.1/10. Published 2026-08-05.

How severe is CVE-2026-71263?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Buffer overflow in Cwalter-At Freemodbus

CVE-2026-71263

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.

Vulnerability class: Buffer Overflow

EPSS: 0.003 (20.6th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.

Affected products

Cwalter-at Freemodbus — versions 0

Weakness classification (CWE)

CWE-787 — Out-of-bounds Write

References

309f9ea4-e3e9-4c6c-b79d-e8eb01244f2c (third-party-advisory)
309f9ea4-e3e9-4c6c-b79d-e8eb01244f2c (third-party-advisory)

Frequently asked questions

What is CVE-2026-71263?: CVE-2026-71263 is a critical-severity vulnerability in Cwalter-At Freemodbus, classified under Out-of-bounds Write. CVSS score: 9.1/10. Published 2026-08-05.
How severe is CVE-2026-71263?: Critical severity. CVSS v3 base score is 9.1 out of 10.