# CVE-2026-71263: FreeModbus LINUXTCP Port Off-by-One Global Buffer Overflow in xMBPortTCPPool()

> An off-by-one error in FreeModbus's LINUXTCP port allows attackers to trigger a global buffer overflow, potentially leading to code execution or crashes.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Aug 2026
- **Updated:** 10 Aug 2026
- **Also known as:** GHSA-cjvm-2pqq-6v6h

## What is CVE-2026-71263?

CVE-2026-71263 is a critical memory corruption vulnerability in cwalter-at FreeModbus, published on 5 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2026-71263?


## How do I fix CVE-2026-71263?

No fix has been published for CVE-2026-71263 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-71263 being exploited?

There is no known exploitation of CVE-2026-71263 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-71263 and fix it if it is affected.

What it is: CVE-2026-71263 is a critical memory corruption vulnerability in cwalter-at FreeModbus, published on 5 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-71263 in the wild and no public exploit code.

Affected product: cwalter-at FreeModbus. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-71263/ (Markdown: https://insights.sbomify.com/CVE-2026-71263/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-71263
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-71263
- OSV: https://osv.dev/vulnerability/CVE-2026-71263

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-71263/
Summaries are generated and may contain mistakes; the official description above is the record.