# CVE-2021-24040: Deserialization in ParlAI

> An attacker with local file write access can achieve remote code execution via unsafe YAML deserialization in the ParlAI library.

- **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:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Sep 2021
- **Updated:** 3 Aug 2024

## What is CVE-2021-24040?

CVE-2021-24040 is a critical unsafe deserialization vulnerability in Facebook ParlAI, published on 10 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Due to use of unsafe YAML deserialization logic, an attacker with the ability to modify local YAML configuration files could provide malicious input, resulting in remote code execution or similar risks. This issue affects ParlAI prior to v1.1.0.

Weakness: CWE-502 (Deserialization of Untrusted Data)

## Which versions are affected by CVE-2021-24040?

It affects 1 product.


Products: Facebook Parlai.

## How do I fix CVE-2021-24040?

Upgrade to a fixed version.

## Is CVE-2021-24040 being exploited?

There is no known exploitation of CVE-2021-24040 in the wild and no public exploit code.

Public exploit code:

- Facebook ParlAI 1.0.0 -  Deserialization of Untrusted Data in parlai (exploitdb)

## Other vulnerabilities in Facebook ParlAI

- [CVE-2021-39207](https://insights.sbomify.com/CVE-2021-39207/): Deserialization of Untrusted Data in parlai (high 8.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-24040 and fix it if it is affected.

What it is: CVE-2021-24040 is a critical unsafe deserialization vulnerability in Facebook ParlAI, published on 10 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-2021-24040 in the wild and no public exploit code.

Affected product: Facebook ParlAI. 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-2021-24040/ (Markdown: https://insights.sbomify.com/CVE-2021-24040/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-24040
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-24040
- OSV: https://osv.dev/vulnerability/CVE-2021-24040

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-24040/
Summaries are generated and may contain mistakes; the official description above is the record.