title: Quadratic complexity may lead to a denial of service in cmark-gfm
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)
problem types: CWE-400: Uncontrolled Resource Consumption
affected:
{
vendor: github
product: cmark-gfm
versions:
version: < 0.29.0.gfm.10, status: affected
}
metrics:
cvssV3_1: [
attackComplexity: LOW
attackVector: NETWORK
availabilityImpact: LOW
baseScore: 5.3
baseSeverity: MEDIUM
confidentialityImpact: NONE
integrityImpact: NONE
privilegesRequired: NONE
scope: UNCHANGED
userInteraction: NONE
vectorString: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
version: 3.1
]