path-to-regexp vulnerable to Regular Expression Denial of Service via multiple route parameters

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 [email protected] 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 [email protected]

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
• Detailed blog post: ReDoS the web