fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS

• CVE-2026-75931: 0.40% estimated exploitation activity in the next 30 days; percentile 34.08 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact fast-uri canonicalizes a host to its ASCII form only when the input carries an explicit scheme. When resolve() resolves a scheme-relative reference (//host/) against a scheme-bearing base, it still emits the host verbatim even though the effective scheme is known, so re-parsing the resolved URI yields a different host than the one resolve() returned. An application that resolves an untrusted reference with fast-uri and then checks or routes on the resulting host can make a policy decision on one host and reach another. This is an incomplete-fix variant of CVE-2026-13676, whose IDN canonicalization was applied only to the scheme-bearing form. Patches Upgrade to fast-uri 2.4.5, 3.1.6, or 4.1.3. resolve() now canonicalizes the host once the effective scheme is known, and fails closed if a raw non-ASCII host cannot be converted. Workarounds Resolve scheme-relative references against a base that carries a scheme before performing any host-policy or origin check.

Published by the source

2 Sep 2026, 15:44 UTC

Last modified by the source

2 Sep 2026, 15:44 UTC

Source last checked

26 Oct 2026, 13:02 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-5jgf-p345-68v8

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# Patch Next advisory brief: CVE-2026-75931

Advisory: fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references
IDs: CVE-2026-75931, GHSA-5jgf-p345-68v8
Record: https://patchnext.com/advisories/cve-2026-75931 (revision 1, source checked 26 Oct 2026, 13:02 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-uri 4.1.3, 3.1.6, 2.4.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-uri: affected >= 2.4.2, < 2.4.5; >= 3.1.3, < 3.1.6; >= 4.0.1, < 4.1.3; fixed in 2.4.5, 4.1.3, 3.1.6

## 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-5jgf-p345-68v8 (GitHub-reviewed advisory GHSA-5jgf-p345-68v8 lists affected ranges: npm fast-uri >=2.4.2 =3.1.3 =4.0.1 <4.1.3)

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