# CVE-2026-56111: Marlin Firmware 2.1.2.7 Out-of-Bounds Write via M421 G-code Handler

> Attackers can corrupt firmware memory and cause denial of service by sending crafted G-code commands to Marlin firmware with mesh bed leveling enabled.

- **Severity:** High 8.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 24 Jun 2026
- **Updated:** 28 Jul 2026
- **Also known as:** GHSA-x5qg-qgv8-q7cr

## What is CVE-2026-56111?

CVE-2026-56111 is a high-severity memory corruption vulnerability in MarlinFirmware Marlin, published on 24 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Marlin Firmware through 2.1.2.7, fixed in commit 1f255d1, when built with MESH_BED_LEVELING enabled, contains an out-of-bounds write vulnerability in the M421 G-code handler that allows attackers to corrupt firmware memory by supplying out-of-range X and Y grid indices. Attackers can send a single crafted G-code command via USB serial, network interface, or malicious gcode file to write an attacker-controlled 32-bit float value past the z_values array bounds, corrupting adjacent firmware variables and causing denial of service or firmware state corruption.

Weakness: CWE-129 (Improper Validation of Array Index)

## Which versions are affected by CVE-2026-56111?


## How do I fix CVE-2026-56111?

No fix has been published for CVE-2026-56111 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-56111 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-56111 exists.

Public exploit code:

- Proof of concept for CVE-2026-56111, an out-of-bounds write in the M421 G-code handler of Marlin Firmware (github_poc): https://github.com/Christbowel/CVE-2026-56111

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-56111 and fix it if it is affected.

What it is: CVE-2026-56111 is a high-severity memory corruption vulnerability in MarlinFirmware Marlin, published on 24 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-56111 exists.

Affected product: MarlinFirmware Marlin. 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-2026-56111/ (Markdown: https://insights.sbomify.com/CVE-2026-56111/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-56111
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-56111
- OSV: https://osv.dev/vulnerability/CVE-2026-56111

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-56111/
Summaries are generated and may contain mistakes; the official description above is the record.