CVE-2026-4867 path-to-regexp vulnerable to Regular Expression Denial of Service via multiple route parameters
An attacker can cause a denial of service by triggering catastrophic backtracking in path-to-regexp when using three or more route parameters in a single segment.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-4867?
CVE-2026-4867 is a high-severity denial of service vulnerability in path-to-regexp, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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 [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.
Weakness: CWE-1333
Which versions are affected by CVE-2026-4867?
It affects 1 product.
Products include Pillarjs Path-to-Regexp.
How do I fix CVE-2026-4867?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-4867 being exploited?
There is no known exploitation of CVE-2026-4867 in the wild and no public exploit code.
Other vulnerabilities in path-to-regexp
Sources
Also known as CLEANSTART-2026-QC14914, CLEANSTART-2026-TC19011, CLSA-2026-1777250311, CLSA-2026-1779238048, CLSA-2026-1792663862, CLSA-2026-1792789806 and 17 more.