Counting in Regexes Considered Harmful: Exposing ReDoS Vulnerability of Nonbacktracking Matchers


Lenka Turoňová, Lukáš Holík, Ivan Homoliak, and Ondřej Lengál, Faculty of Information Technology, Brno University of Technology; Margus Veanes, Microsoft Research Redmond; Tomáš Vojnar, Faculty of Information Technology, Brno University of Technology


In this paper, we study the performance characteristics of nonbacktracking regex matchers and their vulnerability against ReDoS (regular expression denial of service) attacks. We focus on their known Achilles heel, which are extended regexes that use bounded quantifiers (e.g., '(ab){100}'). We propose a method for generating input texts that can cause ReDoS attacks on these matchers. The method exploits the bounded repetition and uses it to force expensive simulations of the deterministic automaton for the regex. We perform an extensive experimental evaluation of our and other state-of-the-art ReDoS generators on a large set of practical regexes with a comprehensive set of backtracking and nonbacktracking matchers, as well as experiments where we demonstrate ReDoS attacks on state-of-the-art real-world security applications containing SNORT with Hyperscan and the HW-accelerated regex matching engine on the NVIDIA BlueField-2 card. Our experiments show that bounded repetition is indeed a notable weakness of nonbacktracking matchers, with our generator being the only one capable of significantly increasing their running time.

Open Access Media

USENIX is committed to Open Access to the research presented at our events. Papers and proceedings are freely available to everyone once the event begins. Any video, audio, and/or slides that are posted after the event are also free and open to everyone. Support USENIX and our commitment to Open Access.

@inproceedings {279930,
author = {Lenka Turo{\v n}ov{\'a} and Luk{\'a}{\v s} Hol{\'\i}k and Ivan Homoliak and Ond{\v r}ej Leng{\'a}l and Margus Veanes and Tom{\'a}{\v s} Vojnar},
title = {Counting in Regexes Considered Harmful: Exposing {ReDoS} Vulnerability of Nonbacktracking Matchers},
booktitle = {31st USENIX Security Symposium (USENIX Security 22)},
year = {2022},
isbn = {978-1-939133-31-1},
address = {Boston, MA},
pages = {4165--4182},
url = {},
publisher = {USENIX Association},
month = aug

Presentation Video