# CVE-2021-40523: Denial of service in n/a

> A flaw in Contiki OS Telnet negotiation handling can cause a denial of service by exhausting response buffers.

- **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:** 5 Sep 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-9rqj-h33r-qgm8

## What is CVE-2021-40523?

CVE-2021-40523 is a high-severity denial of service vulnerability in n/a, published on 5 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In Contiki 3.0, Telnet option negotiation is mishandled. During negotiation between a server and a client, the server may fail to give the WILL/WONT or DO/DONT response for DO and WILL commands because of improper handling of exception condition, which leads to property violations and denial of service. Specifically, a server sometimes sends no response, because a fixed buffer space is available for all responses and that space may have been exhausted.

Weakness: CWE-754 (Improper Check for Unusual or Exceptional Conditions)

## Which versions are affected by CVE-2021-40523?

It affects 1 product.


Products: Contiki-Os Contiki.

## How do I fix CVE-2021-40523?

No fix has been published for CVE-2021-40523 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-40523 being exploited?

There is no known exploitation of CVE-2021-40523 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2021-38386](https://insights.sbomify.com/CVE-2021-38386/): Memory corruption in n/a (high 7.5)
- [CVE-2021-38387](https://insights.sbomify.com/CVE-2021-38387/): Denial of service in n/a (high 7.5)
- [CVE-2021-38311](https://insights.sbomify.com/CVE-2021-38311/): Denial of service in n/a (high 7.5)
- [CVE-2021-28362](https://insights.sbomify.com/CVE-2021-28362/): Integer overflow in n/a (high 7.5)
- [CVE-2020-13984](https://insights.sbomify.com/CVE-2020-13984/): Denial of service in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40523 and fix it if it is affected.

What it is: CVE-2021-40523 is a high-severity denial of service vulnerability in n/a, published on 5 Sep 2021. 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-2021-40523 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-2021-40523/ (Markdown: https://insights.sbomify.com/CVE-2021-40523/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40523
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40523
- OSV: https://osv.dev/vulnerability/CVE-2021-40523

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40523/
Summaries are generated and may contain mistakes; the official description above is the record.