# CVE-2023-26485: Quadratic complexity may lead to a denial of service in cmark-gfm

> Attackers can cause denial of service in cmark-gfm by submitting input with many underscores, triggering quadratic time complexity and resource exhaustion.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 31 Mar 2023
- **Updated:** 11 Feb 2025
- **Also known as:** DEBIAN-CVE-2023-26485, GHSA-r8vr-c48j-fcc5, UBUNTU-CVE-2023-26485

## What is CVE-2023-26485?

CVE-2023-26485 is a medium-severity denial of service vulnerability in github cmark-gfm, published on 31 Mar 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

cmark-gfm is GitHub's fork of cmark, a CommonMark parsing and rendering library and program in C. A polynomial time complexity issue in cmark-gfm may lead to unbounded resource exhaustion and subsequent denial of service. This CVE covers quadratic complexity issues when parsing text which leads with either large numbers of `_` characters. This issue has been addressed in version 0.29.0.gfm.10. Users are advised to upgrade. Users unable to upgrade should validate that their input comes from trusted sources.

### Impact

A polynomial time complexity issue in cmark-gfm may lead to unbounded resource exhaustion and subsequent denial of service.

### Proof of concept

```
$ ~/cmark-gfm$ python3 -c 'pad = "_" * 100000; print(pad + "." + pad, end="")' | time ./build/src/cmark-gfm --to plaintext
```

Increasing the number 10000 in the above commands causes the running time to increase quadratically.

### Patches

This vulnerability have been patched in 0.29.0.gfm.10.

### Note on cmark and cmark-gfm

XXX: TBD

[cmark-gfm](https://github.com/github/cmark-gfm) is a fork of [cmark](https://github.com/commonmark/cmark) that adds the GitHub Flavored Markdown extensions. The two codebases have diverged over time, but share a common core. These bugs affect both `cmark` and `cmark-gfm`. 

### Credit

We would like to thank @gravypod for reporting this vulnerability.

### References

https://en.wikipedia.org/wiki/Time_complexity

### For more information

If you have any questions or comments about this advisory:

* Open an issue in [github/cmark-gfm](https://github.com/github/cmark-gfm)

Weakness: CWE-407 (Inefficient Algorithmic Complexity), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2023-26485?

It affects 1 product.


Products: Github Cmark-Gfm.

## How do I fix CVE-2023-26485?

Upgrade to a fixed version.

## Is CVE-2023-26485 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2023-26485 exists.

## Other vulnerabilities in github cmark-gfm

- [CVE-2024-22051](https://insights.sbomify.com/CVE-2024-22051/): CommonMarker Integer Overflow Vulnerability (critical 9.8)
- [CVE-2023-37463](https://insights.sbomify.com/CVE-2023-37463/): Quadratic complexity bugs may lead to a denial of service (medium 6.4)
- [CVE-2023-24824](https://insights.sbomify.com/CVE-2023-24824/): Quadratic complexity may lead to a denial of service in cmark-gfm (medium 5.3)
- [CVE-2023-22485](https://insights.sbomify.com/CVE-2023-22485/): cmark-gfm out-of-bounds read in validate_protocol (medium 5.3)
- [CVE-2023-22486](https://insights.sbomify.com/CVE-2023-22486/): cmark-gfm Quadratic complexity bug in handle_close_bracket may lead to a denial of service (low 3.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-26485 and fix it if it is affected.

What it is: CVE-2023-26485 is a medium-severity denial of service vulnerability in github cmark-gfm, published on 31 Mar 2023. 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-2023-26485 exists.

Affected product: github cmark-gfm. 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-2023-26485/ (Markdown: https://insights.sbomify.com/CVE-2023-26485/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-26485
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-26485
- OSV: https://osv.dev/vulnerability/CVE-2023-26485

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-26485/
Summaries are generated and may contain mistakes; the official description above is the record.