# CVE-2021-31755: Tenda AC11 Router Stack Buffer Overflow

> Attackers can execute arbitrary code on Tenda AC11 routers via a stack buffer overflow in the setmac endpoint.

- **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:** Yes, since 3 Nov 2021 (CISA KEV)
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 7 May 2021
- **Updated:** 21 Oct 2025
- **Also known as:** GHSA-539c-2w6v-9rwx

## What is CVE-2021-31755?

CVE-2021-31755 is a critical memory corruption vulnerability in n/a, published on 7 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered on Tenda AC11 devices with firmware through 02.03.01.104_CN. A stack buffer overflow vulnerability in /goform/setmac allows attackers to execute arbitrary code on the system via a crafted post request.

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

## Which versions are affected by CVE-2021-31755?

It affects 1 product.


Products: Tenda Ac11 Firmware.

## How do I fix CVE-2021-31755?

No fix has been published for CVE-2021-31755 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 17 Nov 2021.

## Is CVE-2021-31755 being exploited?

Yes. Attackers have exploited CVE-2021-31755 in the wild since at least 3 Nov 2021.

Public exploit code:

- Tenda Router AC11 - Remote Command Injection (nuclei): http/cves/2021/CVE-2021-31755.yaml

## Other vulnerabilities in n/a

- [CVE-2021-46262](https://insights.sbomify.com/CVE-2021-46262/): Memory corruption in n/a (critical 9.8)
- [CVE-2021-46263](https://insights.sbomify.com/CVE-2021-46263/): Memory corruption in n/a (critical 9.8)
- [CVE-2021-46264](https://insights.sbomify.com/CVE-2021-46264/): Memory corruption in n/a (critical 9.8)
- [CVE-2021-46265](https://insights.sbomify.com/CVE-2021-46265/): Memory corruption in n/a (critical 9.8)
- [CVE-2021-46321](https://insights.sbomify.com/CVE-2021-46321/): Memory corruption in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-31755 and fix it if it is affected.

What it is: CVE-2021-31755 is a critical memory corruption vulnerability in n/a, published on 7 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2021-31755 in the wild since at least 3 Nov 2021.

Affected product: n/a. 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-2021-31755/ (Markdown: https://insights.sbomify.com/CVE-2021-31755/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-31755
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-31755
- OSV: https://osv.dev/vulnerability/CVE-2021-31755

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-31755/
Summaries are generated and may contain mistakes; the official description above is the record.