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Medium, CVSS 4.8amended

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

CVE-2026-86472GHSA-hrr3-gc8f-f4qj

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (fast-uri 4.1.5, 3.1.8, 2.4.7), 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
    < 2.4.7>= 3.0.0, < 3.1.8>= 4.0.0, < 4.1.5
    Fixed in
    4.1.5, 2.4.7, 3.1.8

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
4.8 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
EPSS
  • CVE-2026-86472: 0.25% estimated exploitation activity in the next 30 days; percentile 16.83 (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 folds the host to lowercase before it percent-decodes the host, so a percent-encoded uppercase unreserved octet such as %41 decodes to a literal A that is never folded. For a scheme-relative reference (//host) there is no scheme, so the host canonicalization that repairs this on a scheme-bearing URL does not run. As a result parse, normalize, and equal disagree on the same host: parse("//%41.com").host returns "A.com" while parse("//a.com").host and parse("//A.com").host return "a.com", and equal("//%41.com", "//a.com") is false even though equal("//A.com", "//a.com") is true. An application that makes a case-sensitive host decision on fast-uri output for a scheme-relative reference, for example a host allowlist or denylist that compares parse(url).host or uses fast-uri.equal, can be steered past the check with a percent-encoded uppercase octet. Because a hostname is case-insensitive in DNS and HTTP routing, the evading spelling reaches the same host the check meant…

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

    Amended from ghsa record GHSA-hrr3-gc8f-f4qj

  2. Revision 2 · amended ·

    Amended from osv record GHSA-hrr3-gc8f-f4qj

  3. Revision 1 · created ·

    Published from ghsa record GHSA-hrr3-gc8f-f4qj

Agent brief

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# Patch Next advisory brief: CVE-2026-86472

Advisory: fast-uri vulnerable to inconsistent host case normalization via percent-encoded octets
IDs: CVE-2026-86472, GHSA-hrr3-gc8f-f4qj
Record: https://patchnext.com/advisories/cve-2026-86472 (revision 3, source checked 26 Oct 2026, 13:06 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-uri 4.1.5, 3.1.8, 2.4.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-uri: affected < 2.4.7; >= 3.0.0, < 3.1.8; >= 4.0.0, < 4.1.5; fixed in 4.1.5, 2.4.7, 3.1.8

## 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-hrr3-gc8f-f4qj (GitHub-reviewed advisory GHSA-hrr3-gc8f-f4qj lists affected ranges: npm fast-uri >=0 =3.0.0 =4.0.0 <4.1.5)
- Verified: https://osv.dev/vulnerability/GHSA-hrr3-gc8f-f4qj (OSV record GHSA-hrr3-gc8f-f4qj lists affected ranges: npm fast-uri >=0 =3.0.0 =4.0.0 <4.1.5 (fixed))

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