CVE-2026-86472

fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets

Published Sep 15, 2026 CVSS 3.1 · 4.8 Medium EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-hrr3-gc8f-f4qj fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets
History
Type Values Removed Values Added
Metrics ssvc

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

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

Weaknesses

Sources

Status
PUBLISHED
Assigner
openjs
Published
Updated
Sep 15, 2026, 12:35 PM
Reserved
Sep 7, 2026, 3:02 PM
Status
Awaiting Analysis
Published
Sep 15, 2026, 11:17 AM
Modified
Sep 16, 2026, 7:40 PM
Severity
Moderate
Public date
Sep 15, 2026, 10:29 AM