CVE-2026-4867

Impact:<br /> <br /> 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.<br /> <br /> Patches:<br /> <br /> Upgrade to path-to-regexp@0.1.13<br /> <br /> Custom regex patterns in route definitions (e.g., /:a-:b([^-/]+)-:c([^-/]+)) are not affected because they override the default capture group.<br /> <br /> Workarounds:<br /> <br /> 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([^-/]+).<br /> <br /> If paths cannot be rewritten and versions cannot be upgraded, another alternative is to limit the URL length.