---
id: CVE-2026-110306
aliases:
  - GHSA-w74x-v6f9-j3xg
title: fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding
summary: fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding
severity: medium
cvss: 6.1
cwe:
  - CWE-172
  - CWE-180
  - CWE-1286
vendor: fast-uri
product: fast-uri
ecosystem: npm
affected:
  - fast-uri < 2.4.8
  - 'fast-uri >= 3.0.0, < 3.1.9'
  - 'fast-uri >= 4.0.0, < 4.2.3'
patched:
  - fast-uri 2.4.8
  - fast-uri 3.1.9
  - fast-uri 4.2.3
published: '2026-10-23'
updated: '2026-10-23'
sourceUpdated: '2026-10-23T21:41:38Z'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-w74x-v6f9-j3xg'
references:
  - url: >-
      https://github.com/fastify/fast-uri/security/advisories/GHSA-w74x-v6f9-j3xg
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-110306'
  - url: >-
      https://github.com/fastify/fast-uri/commit/39933a1f1422986f14e2d844f552238feb40f5d4
  - url: >-
      https://github.com/fastify/fast-uri/commit/597f254c86d151568511eeb9a1df761fd12520b3
  - url: >-
      https://github.com/fastify/fast-uri/commit/e60e716f41d1d73717e2d803d20b68d2c6d20a0f
  - url: 'https://cna.openjsf.org/security-advisories.html'
  - url: 'https://github.com/fastify/fast-uri/releases/tag/v2.4.8'
  - url: 'https://github.com/fastify/fast-uri/releases/tag/v3.1.9'
  - url: 'https://github.com/fastify/fast-uri/releases/tag/v4.2.3'
  - url: 'https://github.com/advisories/GHSA-w74x-v6f9-j3xg'
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-110306.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-110306'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2556005'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-110306'
tags:
  - ghsa
  - npm
  - cve.org
  - csaf
  - vex
  - red-hat
epss: 0.0025
epssPercentile: 0.16213
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-10-20T19:14:03.847725Z'
ingestedAt: '2026-10-20T07:55:43.638Z'
---

## Overview

### Impact

`fast-uri` percent-decodes the scheme component with the legacy global `unescape()` before validating it, so an input whose decoded scheme is a valid but dangerous token is emitted as a live scheme. RFC 3986 section 3.1 does not allow percent-encoding in a scheme, so no RFC or WHATWG parser treats such input as having that scheme, but `fast-uri` does.

For example, `normalize('%6Aavascript:alert(1)')` returns `javascript:alert(1)`, `resolve('https://a.example/', '%6Aavascript:alert(1)')` returns `javascript:alert(1)`, and `parse('%6Aavascript:alert(1)').scheme` returns `javascript`. The `%uXXXX` form (`%u006Aavascript:`) produces the same result, and `equal('%68ttp://a/', 'http://a/')` returns `true`.

An application that screens dangerous schemes on the raw input and then stores or renders `normalize(input)` or `resolve(base, input)` turns an inert string into an executable `javascript:`, `data:`, or `vbscript:` URL, a stored or reflected cross-site scripting vector in the consuming application. `equal()` also treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists.

### Patches

Upgrade to `fast-uri` `2.4.8`, `3.1.9`, or `4.2.3`.

### Workarounds

Reject input whose scheme part (before the first `:`, and before any `/`, `?`, or `#`) contains `%`. Alternatively, check `parse(x).scheme` against an allowlist after normalization and use that same parsed result rather than the raw string.

## Affected packages

- `fast-uri < 2.4.8`
- `fast-uri >= 3.0.0, < 3.1.9`
- `fast-uri >= 4.0.0, < 4.2.3`

## Remediation

Upgrade to a patched release:

- `fast-uri 2.4.8`
- `fast-uri 3.1.9`
- `fast-uri 4.2.3`

## Vendor advisories

- **Red Hat VEX** · Moderate · affected: Cost Management On Premise, Cryostat 4, Migration Toolkit for Applications 8, Multicluster Engine for Kubernetes, Network Observability Operator, OpenShift Lightspeed, … · no fix planned: Red Hat Ansible Automation Platform 2, Red Hat Directory Server 11, Red Hat Directory Server 12, Red Hat Openshift Data Foundation 4, … · updated 2026-10-20 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-110306.json)