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
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.
Weakness
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Path-to-regexp | Pillarjs | * | 0.1.13 (excluding) |
| Node-path-to-regexp | Ubuntu | esm-apps/xenial | * |
| Node-path-to-regexp | Ubuntu | upstream | * |