Path-to-regexp, a utility for converting Express-style path strings into regular expressions, saw significant changes moving from version 3.3.0 to 4.0.0. While both versions share the same MIT license and repository on GitHub, targeting developers who need flexible and performant route matching, the internal tooling and package size underwent a transformation. Version 4.0.0 embraces a more modern development workflow, evident in its devDependencies. Jest replaced Mocha and Chai for testing, indicating a preference for a more comprehensive and integrated testing experience. Linting also evolved, with TSLint, prettier, and lint-staged superseding standard. Typescript is present in both versions of the package. The introduction of Husky suggests a commitment to Git hooks for code quality enforcement.
However, this modernization came at a cost. The unpacked size of the package ballooned from ~26KB in version 3.3.0 to ~470KB in version 4.0.0, a substantial increase potentially impacting installation times and bundle sizes. This size increase is likely attributed to the newer version's tooling dependencies and potentially more comprehensive type definitions or a more complex implementation. Version 3.3.0, although older, offers a leaner footprint. The release date of the newer version is significantly older. While version 4.0.0 boasts the latest development practices, developers should carefully consider the trade-off between modern tooling and package size when choosing between the two versions. Ultimately, the selection hinges on the project's specific needs and priorities.
All the vulnerabilities related to the version 4.0.0 of the package
path-to-regexp outputs backtracking regular expressions
A bad regular expression is generated any time you have two parameters within a single segment, separated by something that is not a period (.). For example, /:a-:b.
For users of 0.1, upgrade to 0.1.10. All other users should upgrade to 8.0.0.
These versions add backtrack protection when a custom regex pattern is not provided:
They do not protect against vulnerable user supplied capture groups. Protecting against explicit user patterns is out of scope for old versions and not considered a vulnerability.
Version 7.1.0 can enable strict: true and get an error when the regular expression might be bad.
Version 8.0.0 removes the features that can cause a ReDoS.
All versions can be patched by providing a custom regular expression for parameters after the first in a single segment. As long as the custom regular expression does not match the text before the parameter, you will be safe. For example, change /:a-:b to /:a-:b([^-/]+).
If paths cannot be rewritten and versions cannot be upgraded, another alternative is to limit the URL length. For example, halving the attack string improves performance by 4x faster.
Using /:a-:b will produce the regular expression /^\/([^\/]+?)-([^\/]+?)\/?$/. This can be exploited by a path such as /a${'-a'.repeat(8_000)}/a. OWASP has a good example of why this occurs, but the TL;DR is the /a at the end ensures this route would never match but due to naive backtracking it will still attempt every combination of the :a-:b on the repeated 8,000 -a.
Because JavaScript is single threaded and regex matching runs on the main thread, poor performance will block the event loop and can lead to a DoS. In local benchmarks, exploiting the unsafe regex will result in performance that is over 1000x worse than the safe regex. In a more realistic environment using Express v4 and 10 concurrent connections, this translated to average latency of ~600ms vs 1ms.
