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

Medium 5.3 Published 31 Mar 2023 Updated 11 Feb 2025

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

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

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, CWE-400

Which versions are affected by CVE-2023-26485?

It affects 1 product.

Products include Github Cmark-Gfm.

How do I fix CVE-2023-26485?

Upgrade to a fixed version.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2023-26485 being exploited?

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

Sources

Also known as DEBIAN-CVE-2023-26485, GHSA-r8vr-c48j-fcc5, UBUNTU-CVE-2023-26485.