CVE-2023-26485: 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 ...

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)

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

1.03%

59.3th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Github	Cmark-Gfm	< 0.29.0.gfm.10

References

https://github.com/github/cmark-gfm/commit/07a66c9bc341f902878e37d7da8647d6ef150987Patch
https://github.com/github/cmark-gfm/security/advisories/GHSA-r8vr-c48j-fcc5Exploit, Vendor Advisory
https://github.com/github/cmark-gfm/commit/07a66c9bc341f902878e37d7da8647d6ef150987Patch
https://github.com/github/cmark-gfm/security/advisories/GHSA-r8vr-c48j-fcc5Exploit, Vendor Advisory

Timeline

Published: Mar 31, 2023
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2023-26485?

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)

How severe is CVE-2023-26485?

CVE-2023-26485 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.03% probability of exploitation in the next 30 days.

How do I fix CVE-2023-26485?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.