# CVE-2021-40524: Denial of service in n/a

> Attackers can upload unbounded files due to a quota mechanism flaw, causing server hangs or denial of service in Pure-FTPd.

- **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:** Yes
- **Published:** 5 Sep 2021
- **Updated:** 4 Nov 2025
- **Also known as:** DEBIAN-CVE-2021-40524, DLA-4360-1, GHSA-p53v-96v3-p5jg, UBUNTU-CVE-2021-40524

## What is CVE-2021-40524?

CVE-2021-40524 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 Pure-FTPd before 1.0.50, an incorrect max_filesize quota mechanism in the server allows attackers to upload files of unbounded size, which may lead to denial of service or a server hang. This occurs because a certain greater-than-zero test does not anticipate an initial -1 value. (Versions 1.0.23 through 1.0.49 are affected.)

Weakness: CWE-434 (Unrestricted Upload of File with Dangerous Type)

## Which versions are affected by CVE-2021-40524?

It affects 1 product.


Products: Pureftpd Pure-Ftpd.

## How do I fix CVE-2021-40524?

Upgrade to a fixed version.

## Is CVE-2021-40524 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-40524 exists.

Public exploit code:

- Pure-FTPd 1.0.23 < 1.0.50 - Arbitrary File Upload (nuclei)

## Other vulnerabilities in n/a

- [CVE-2024-48208](https://insights.sbomify.com/CVE-2024-48208/): Memory corruption in n/a (high 8.6)
- [CVE-2020-35359](https://insights.sbomify.com/CVE-2020-35359/): Denial of service in n/a (high 7.5)
- [CVE-2011-1575](https://insights.sbomify.com/CVE-2011-1575/): Other in n/a (medium 5.8)
- [CVE-2011-3171](https://insights.sbomify.com/CVE-2011-3171/): Path traversal in n/a (low 3.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40524 and fix it if it is affected.

What it is: CVE-2021-40524 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: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-40524 exists.

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-2021-40524/ (Markdown: https://insights.sbomify.com/CVE-2021-40524/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40524
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40524
- OSV: https://osv.dev/vulnerability/CVE-2021-40524

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40524/
Summaries are generated and may contain mistakes; the official description above is the record.