CVE-2026-4867

### Impact A bad regular expression is generated any time you have three or more parameters within a single segment, separated by something that is not a period (`.`). For example, `/:a-:b-:c` or `/:a-:b-:c-:d`. The backtrack protection added in `path-to-regexp@0.1.12` only prevents ambiguity for two parameters. With three or more, the generated lookahead does not block single separator characters, so capture groups overlap and cause catastrophic backtracking. ### Patches Upgrade to [path-to-regexp@0.1.13](https://github.com/pillarjs/path-to-regexp/releases/tag/v.0.1.13) Custom regex patterns in route definitions (e.g., `/:a-:b([^-/]+)-:c([^-/]+)`) are not affected because they override the default capture group. ### Workarounds 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-:c` to `/:a-:b([^-/]+)-:c([^-/]+)`. If paths cannot be rewritten and versions cannot be upgraded, another alternative is to limit the URL length. ### References - [GHSA-9wv6-86v2-598j](https://github.com/advisories/GHSA-9wv6-86v2-598j) - [Detailed blog post: ReDoS the web](https://blakeembrey.com/posts/2024-09-web-redos/)

### Impact 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`. ### Patches 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: - [0.1.10](https://github.com/pillarjs/path-to-regexp/releases/tag/v0.1.10) - [1.9.0](https://github.com/pillarjs/path-to-regexp/releases/tag/v1.9.0) - [3.3.0](https://github.com/pillarjs/path-to-regexp/releases/tag/v3.3.0) - [6.3.0](https://github.com/pillarjs/path-to-regexp/releases/tag/v6.3.0) 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](https://github.com/pillarjs/path-to-regexp/releases/tag/v7.1.0) can enable `strict: true` and get an error when the regular expression might be bad. Version [8.0.0](https://github.com/pillarjs/path-to-regexp/releases/tag/v8.0.0) removes the features that can cause a ReDoS. ### Workarounds 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. ### Details Using `/:a-:b` will produce the regular expression `/^\/([^\/]+?)-([^\/]+?)\/?$/`. This can be exploited by a path such as `/a${'-a'.repeat(8_000)}/a`. [OWASP](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS) 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. ### References * [OWASP](https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS) * [Detailed blog post](https://blakeembrey.com/posts/2024-09-web-redos/)

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.