CVE-2026-4926: path-to-regexp vulnerable to Denial of Service via sequential optional groups

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-26 · updated 2026-09-09
Weakness: CWE-400

Description

Impact: A bad regular expression is generated any time you have multiple sequential optional groups (curly brace syntax), such as `{a}{b}{c}:z`. The generated regex grows exponentially with the number of groups, causing denial of service. Patches: Fixed in version 8.4.0. Workarounds: Limit the number of sequential optional groups in route patterns. Avoid passing user-controlled input as route patterns.

Affected products

path-to-regexp: from 8.0.0 before 8.4.0 — fixed in 8.4.0

How to fix

Upgrade to a fixed version: path-to-regexp 8.4.0. Check the vendor advisory for mitigations.

References

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