# CVE-2026-21888: MQTT v5 Variable Byte Integer parsing out-of-bounds: get_var_integer()

> An out-of-bounds read in NanoMQ's MQTT v5 parser allows attackers to crash the broker via malformed variable byte integers.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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:** 11 Mar 2026
- **Updated:** 11 Mar 2026
- **Also known as:** GHSA-cggc-6m7w-j7x5

## What is CVE-2026-21888?

CVE-2026-21888 is a high-severity memory corruption vulnerability in nanomq, published on 11 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

NanoMQ MQTT Broker (NanoMQ) is an all-around Edge Messaging Platform. MQTT v5 Variable Byte Integer parsing out-of-bounds: get_var_integer() accepts 5-byte varints without bounds checks; reliably triggers OOB read / crash when built with ASan. This affects 0.24.6 and earlier.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-21888?

It affects 1 product.


Products: Emqx Nanomq.

## How do I fix CVE-2026-21888?

No fix has been published for CVE-2026-21888 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21888 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21888 exists.

## Other vulnerabilities in nanomq

- [CVE-2024-42655](https://insights.sbomify.com/CVE-2024-42655/): Missing authorization in n/a (high 8.8)
- [CVE-2025-59947](https://insights.sbomify.com/CVE-2025-59947/): NanoMQ has Buffer Overflow (high 8.5)
- [CVE-2023-34488](https://insights.sbomify.com/CVE-2023-34488/): Memory corruption in n/a (high 8.0)
- [CVE-2026-32135](https://insights.sbomify.com/CVE-2026-32135/): NanoMQ has Heap Buffer Overflow in URI Parameter Parsing (high 7.7)
- [CVE-2026-36590](https://insights.sbomify.com/CVE-2026-36590/): Denial of service in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21888 and fix it if it is affected.

What it is: CVE-2026-21888 is a high-severity memory corruption vulnerability in nanomq, published on 11 Mar 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-2026-21888 exists.

Affected product: nanomq. 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-2026-21888/ (Markdown: https://insights.sbomify.com/CVE-2026-21888/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21888
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21888
- OSV: https://osv.dev/vulnerability/CVE-2026-21888

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21888/
Summaries are generated and may contain mistakes; the official description above is the record.