SoK: A Literature and Engineering Review of Regular Expression Denial of Service

Regular expression denial of service (ReDoS) is an asymmetric cyberattack that has become prominent in recent years. Many research works examine ReDoS, measuring its impact or preventing its exploitation. However, there has been no systematic treatment of this topic in order to understand the limits of the state of the art and identify opportunities for further research. In this paper, we fill this gap by systematizing the existing knowledge on ReDoS. We review the algorithmic basis of ReDoS attacks and the pertinent history of regular expression engines. Next, we survey the literature, dividing works into two classes: measurement studies and defenses. We find no agreed-upon definition for ReDoS vulnerabilities, and observe weaknesses in the practical evaluations of many papers, making the impact of their findings hard to assess. The majority of academic work in this area limit themselves to showing the presence of an unexpected slow computation, without illustrating how this can be weaponized against real systems. Then, we survey the latest regex engines to examine whether and how the proposed defenses have been realized. In this way, we describe the new realities that should be considered in the next generation ReDoS research. We show that many academic threat models are out of date thanks to the adoption of defenses. Beyond this, we underscore the importance of simulating ReDoS attacks in realistic contexts, where factors like request size limiting or deployed mitigations are taken into account. We propose a tool, wrk-DoS, to facilitate these simulations.