# CVE-2026-71254: nanoMODBUS Server-Side Out-of-Bounds Write in handle_read_file_record()

> An out-of-bounds write in nanoMODBUS allows attackers to execute arbitrary code or cause denial of service via crafted Modbus requests.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/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-29rm-wvww-5gjv

## What is CVE-2026-71254?

CVE-2026-71254 is a critical memory corruption vulnerability in debevv nanoMODBUS, published on 5 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

nanoMODBUS through v1.23.0 contains an out-of-bounds write in the Modbus server-side handle_read_file_record function (FC 0x14, Read File Record) in nanomodbus.c. The function validates that the total request size does not exceed 245 bytes and that each sub-request's record_length is at most 124, but it never validates the CUMULATIVE response size across all sub-requests before processing them.

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

## Which versions are affected by CVE-2026-71254?


## How do I fix CVE-2026-71254?

No fix has been published for CVE-2026-71254 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-71254 being exploited?

There is no known exploitation of CVE-2026-71254 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-71254 and fix it if it is affected.

What it is: CVE-2026-71254 is a critical memory corruption vulnerability in debevv nanoMODBUS, 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-2026-71254 in the wild and no public exploit code.

Affected product: debevv nanoMODBUS. 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-2026-71254/ (Markdown: https://insights.sbomify.com/CVE-2026-71254/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-71254
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-71254
- OSV: https://osv.dev/vulnerability/CVE-2026-71254

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-71254/
Summaries are generated and may contain mistakes; the official description above is the record.