# CVE-2021-41168: Hash-Collision Denial-of-Service Vulnerability in snudown

> An attacker can cause denial of service in snudown by exploiting weak hash functions in its reference table implementation.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 21 Oct 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-6gvv-9q92-w5f6

## What is CVE-2021-41168?

CVE-2021-41168 is a medium-severity denial of service vulnerability in reddit snudown, published on 21 Oct 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Snudown is a reddit-specific fork of the Sundown Markdown parser used by GitHub, with Python integration added. In affected versions snudown was found to be vulnerable to denial of service attacks to its reference table implementation. References written in markdown ` [reference_name]: https://www.example.com` are inserted into a hash table which was found to have a weak hash function, meaning that an attacker can reliably generate a large number of collisions for it. This makes the hash table vulnerable to a hash-collision DoS attack, a type of algorithmic complexity attack. Further the hash table allowed for duplicate entries resulting in long retrieval times. Proofs of concept and further discussion of the hash collision issue are discussed on the snudown GHSA(https://github.com/reddit/snudown/security/advisories/GHSA-6gvv-9q92-w5f6). Users are advised to update to version 1.7.0.

Weakness: CWE-400 (Uncontrolled Resource Consumption), CWE-407 (Inefficient Algorithmic Complexity), CWE-327 (Use of a Broken or Risky Cryptographic Algorithm), CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2021-41168?

It affects 1 product.


Products: Reddit Snudown.

## How do I fix CVE-2021-41168?

No fix has been published for CVE-2021-41168 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-41168 being exploited?

There is no known exploitation of CVE-2021-41168 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-41168 and fix it if it is affected.

What it is: CVE-2021-41168 is a medium-severity denial of service vulnerability in reddit snudown, published on 21 Oct 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-41168 in the wild and no public exploit code.

Affected product: reddit snudown. 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-2021-41168/ (Markdown: https://insights.sbomify.com/CVE-2021-41168/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-41168
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-41168
- OSV: https://osv.dev/vulnerability/CVE-2021-41168

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-41168/
Summaries are generated and may contain mistakes; the official description above is the record.