Impact

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

Proof of concept

python3 -c 'print("![[]()" * 10000)' | cmark-gfm

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

Patches

This vulnerability has been patched in 0.29.0.gfm.7.

Note on cmark and cmark-gfm

cmark-gfm is a fork of cmark that adds the GitHub Flavored Markdown extensions. The two codebases have diverged over time, but share a common core. This bug affects both codebases. We would like to thank @jgm for his help with fixing and disclosing this bug.

Credit

We would like to thank @jgm for implementing the fix for this bug.

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