CVE-2026-4867

CVE-2026-4867 · path-to-regexp vulnerable to Regular Expression Denial of Service via multiple route parameters

This page contains facts from public vulnerability sources. Check the original advisory before updating.

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.

CVSS 7.5 · High · EPSS 0.5% · Fix available

Published

2026-03-26T16:16:25.501Z

Updated

2026-03-26T16:52:14.893Z

Stack: system

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