Path-to-regexp, a popular utility for converting Express-style path strings into regular expressions, has two recent versions of interest: 6.0.0 and 5.0.0. Both versions share the same core functionality, allowing developers to easily match routes in their applications. They also utilize an identical suite of development dependencies, including testing frameworks like Jest, linting tools such as TSLint, and build tools like TypeScript. This suggests a consistent development environment and code quality practices across both versions.
However, the key difference lies in the unpacked size of the package; version 6.0.0 is slightly smaller at 468051 bytes compared to version 5.0.0 with 468899 bytes. This potentially indicates minor optimizations within the v6.0.0 release, which may lead to slight improvements in performance and reduced disk space usage. The release date also separates the two, with v6.0.0 published on November 20, 2019, shortly after v5.0.0 which was released on November 12, 2019. This quick succession of releases might indicate a relatively small set of focused changes. Additionally, both are MIT licensed and hosted on GitHub, ensuring open-source accessibility and collaborative potential for developers. For developers, the library provides a straightforward way to define routes using familiar syntax and benefit from the generated regular expressions, making route matching simple and maintainable. The small difference in size likely doesn't represent a lot of significant code changes, so upgrading should be relatively easy.
All the vulnerabilities related to the version 6.0.0 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.
