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Medium, CVSS 6.1amended

fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding

CVE-2026-110306GHSA-w74x-v6f9-j3xg

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (fast-uri 4.2.3, 3.1.9, 2.4.8), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm fast-uri

    Affected
    < 2.4.8>= 3.0.0, < 3.1.9>= 4.0.0, < 4.2.3
    Fixed in
    2.4.8, 4.2.3, 3.1.9

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
EPSS
  • CVE-2026-110306: 0.25% estimated exploitation activity in the next 30 days; percentile 16.38 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-w74x-v6f9-j3xg

  2. Revision 2 · amended ·

    Amended from osv record GHSA-w74x-v6f9-j3xg

  3. Revision 1 · created ·

    Published from ghsa record GHSA-w74x-v6f9-j3xg

Agent brief

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# Patch Next advisory brief: CVE-2026-110306

Advisory: fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding
IDs: CVE-2026-110306, GHSA-w74x-v6f9-j3xg
Record: https://patchnext.com/advisories/cve-2026-110306 (revision 3, source checked 26 Oct 2026, 13:07 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-uri 4.2.3, 3.1.9, 2.4.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-uri: affected < 2.4.8; >= 3.0.0, < 3.1.9; >= 4.0.0, < 4.2.3; fixed in 2.4.8, 4.2.3, 3.1.9

## Suggested inspection (read-only)
- `npm ls fast-uri --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fast-uri.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-w74x-v6f9-j3xg (GitHub-reviewed advisory GHSA-w74x-v6f9-j3xg lists affected ranges: npm fast-uri >=0 =3.0.0 =4.0.0 <4.2.3)
- Verified: https://osv.dev/vulnerability/GHSA-w74x-v6f9-j3xg (OSV record GHSA-w74x-v6f9-j3xg lists affected ranges: npm fast-uri >=0 =3.0.0 =4.0.0 <4.2.3 (fixed))

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