# CVE-2021-40556: Memory corruption in n/a

> Authenticated attackers can trigger a stack overflow in the httpd service to execute arbitrary commands remotely on ASUS RT-AX56U routers.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Oct 2022
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-9v8q-97m7-4x6q

## What is CVE-2021-40556?

CVE-2021-40556 is a high-severity memory corruption vulnerability in n/a, published on 6 Oct 2022. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A stack overflow vulnerability exists in the httpd service in ASUS RT-AX56U Router Version 3.0.0.4.386.44266. This vulnerability is caused by the strcat function called by "caupload" input handle function allowing the user to enter 0xFFFF bytes into the stack. This vulnerability allows an attacker to execute commands remotely. The vulnerability requires authentication.

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

## Which versions are affected by CVE-2021-40556?

It affects 1 product.


Products: Asus Rt-Ax56u Firmware.

## How do I fix CVE-2021-40556?

No fix has been published for CVE-2021-40556 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-40556 being exploited?

There is no known exploitation of CVE-2021-40556 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2022-23972](https://insights.sbomify.com/CVE-2022-23972/): ASUS RT-AX56U - SQL Injection (high 8.8)
- [CVE-2022-23973](https://insights.sbomify.com/CVE-2022-23973/): ASUS RT-AX56U - Stack overflew (high 8.8)
- [CVE-2022-23970](https://insights.sbomify.com/CVE-2022-23970/): ASUS RT-AX56U - Path Traversal (high 8.1)
- [CVE-2022-23971](https://insights.sbomify.com/CVE-2022-23971/): ASUS RT-AX56U - Path Traversal (high 8.1)
- [CVE-2021-44158](https://insights.sbomify.com/CVE-2021-44158/): ASUS RT-AX56U Router - Stack-based buffer overflow (high 8.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40556 and fix it if it is affected.

What it is: CVE-2021-40556 is a high-severity memory corruption vulnerability in n/a, published on 6 Oct 2022. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-40556 in the wild and no public exploit code.

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-2021-40556/ (Markdown: https://insights.sbomify.com/CVE-2021-40556/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40556
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40556
- OSV: https://osv.dev/vulnerability/CVE-2021-40556

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40556/
Summaries are generated and may contain mistakes; the official description above is the record.