# CVE-2021-40839: Infinite Loop in rencode

> A remote attacker can cause a denial of service by triggering an infinite loop in the rencode Python library's typecode decoding.

- **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:** 10 Sep 2021
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-40839, GHSA-gh8j-2pgf-x458, MGASA-2022-0167, PYSEC-2021-345, UBUNTU-CVE-2021-40839

## What is CVE-2021-40839?

CVE-2021-40839 is a high-severity denial of service vulnerability in Fedoraproject Fedora, published on 10 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The rencode package through 1.0.6 for Python allows an infinite loop in typecode decoding (such as via ;\x2f\x7f), enabling a remote attack that consumes CPU and memory.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2021-40839?

3 open source packages ship an affected version, including rencode. It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `rencode` | PyPI | All versions | None |
| `rencode` | PyPI | All versions to 1.0.6 | None |
| `rencode` | PyPI | All versions to 1.0.6 | None |

Products: Fedoraproject Fedora, Rencode Project Rencode.

## How do I fix CVE-2021-40839?

Upgrade to a fixed version.

## Is CVE-2021-40839 being exploited?

There is no known exploitation of CVE-2021-40839 in the wild and no public exploit code.

Public exploit code:

- Dirty Cow kernel exploit without libcrypt dependency (github_poc): https://github.com/itlabbet/CVE-2021-40839

## Other vulnerabilities in Fedoraproject Fedora

- [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild)
- [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild)
- [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40839 and fix it if it is affected.

What it is: CVE-2021-40839 is a high-severity denial of service vulnerability in Fedoraproject Fedora, 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-2021-40839 in the wild and no public exploit code.

Affected packages and fixed versions:
- rencode (PyPI): affected all versions; no fix listed
- rencode (PyPI): affected all versions up to 1.0.6; no fix listed
- rencode (PyPI): affected all versions up to 1.0.6; no fix listed

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-2021-40839/ (Markdown: https://insights.sbomify.com/CVE-2021-40839/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40839
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40839
- OSV: https://osv.dev/vulnerability/CVE-2021-40839

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40839/
Summaries are generated and may contain mistakes; the official description above is the record.