What is ReDoS? Regular expression Denial of Service (ReDoS) is a security vulnerability that can occur in a regular expression (regex) when the regex is applied to a long string. This attack is designed to make a system or network unavailable to its intended users. An example occurrence of a ReDoS Imagine that a website has a form that accepts user input and uses a regex to validate the input. The regex is designed to only allow alphanumeric characters in the input, so it looks like this: /^[a-zA-Z0-9]+$/. An attacker could potentially craft a string of input that consists of