Description
fast-uri is a dependency-free RFC 3986 URI parser for Node.js, used by Fastify and ajv. In versions before 2.4.7, from 3.0.0 through 3.1.7, and from 4.0.0 through 4.1.4, fast-uri folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference such as //host there is no scheme, so the host canonicalization that would normally repair this does not run, and parse, normalize, and equal then disagree on the same host. An application that makes a case-sensitive host decision on fast-uri output, for example a host allowlist or denylist that compares the parsed host or uses equal, can be steered past the check with a percent-encoded uppercase octet, and because hostnames are case-insensitive in DNS and HTTP the evading spelling still reaches the host the check meant to gate. The issue is fixed in fast-uri 2.4.7, 3.1.8, and 4.1.5, and users should upgrade to one of those versions or later. As a workaround, compare hosts case-insensitively by lowercasing the parsed host before any allowlist or denylist decision.
Published: 2026-09-15
Score: 4.8 Medium
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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History

Tue, 15 Sep 2026 13:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Tue, 15 Sep 2026 11:45:00 +0000

Type Values Removed Values Added
First Time appeared Fast-uri
Fast-uri fast-uri
Vendors & Products Fast-uri
Fast-uri fast-uri

Tue, 15 Sep 2026 10:45:00 +0000

Type Values Removed Values Added
Description fast-uri is a dependency-free RFC 3986 URI parser for Node.js, used by Fastify and ajv. In versions before 2.4.7, from 3.0.0 through 3.1.7, and from 4.0.0 through 4.1.4, fast-uri folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference such as //host there is no scheme, so the host canonicalization that would normally repair this does not run, and parse, normalize, and equal then disagree on the same host. An application that makes a case-sensitive host decision on fast-uri output, for example a host allowlist or denylist that compares the parsed host or uses equal, can be steered past the check with a percent-encoded uppercase octet, and because hostnames are case-insensitive in DNS and HTTP the evading spelling still reaches the host the check meant to gate. The issue is fixed in fast-uri 2.4.7, 3.1.8, and 4.1.5, and users should upgrade to one of those versions or later. As a workaround, compare hosts case-insensitively by lowercasing the parsed host before any allowlist or denylist decision.
Title fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets
Weaknesses CWE-178
References
Metrics cvssV3_1

{'score': 4.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N'}


Subscriptions

Fast-uri Fast-uri
cve-icon MITRE

Status: PUBLISHED

Assigner: openjs

Published:

Updated: 2026-09-15T12:35:44.789Z

Reserved: 2026-09-07T15:02:14.920Z

Link: CVE-2026-86472

cve-icon Vulnrichment

Updated: 2026-09-15T12:35:33.623Z

cve-icon NVD

Status : Received

Published: 2026-09-15T11:17:12.327

Modified: 2026-09-15T13:16:43.810

Link: CVE-2026-86472

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-09-15T11:30:11Z

Weaknesses