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 (.).
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.50%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
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.
- CVSS 3.1
- 7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- EPSS
- 0.50% probability · 41th percentile
- CISA KEV
- Not listed
- Weakness
- CWE-1333
- Affected
- pillarjs/path-to-regexp
- Source
- ce714d77-add3-4f53-aff5-83d477b104bb
References
- https://blakeembrey.com/posts/2024-09-web-redosTechnical Description
- https://cna.openjsf.org/security-advisories.htmlThird Party Advisory
- https://github.com/advisories/GHSA-9wv6-86v2-598jNot Applicable
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.