path-to-regexp turns path strings into a regular expressions. In certain cases, path-to-regexp will output a regular expression that can be exploited to cause poor performance. Because JavaScript is single threaded and regex matching runs on the main thread, poor performance will block the event loop and lead to a DoS. The bad regular expression is generated any time you have two parameters within a single segment, separated by something that is not a period (.). For users of 0.1, upgrade to 0.1.10. All other users should upgrade to 8.0.0.
The product uses a regular expression with an inefficient, possibly exponential worst-case computational complexity that consumes excessive CPU cycles.