fast-uri vulnerable to host confusion via backslash authority introducer

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-2026-18446: 0.38% estimated exploitation activity in the next 30 days; percentile 32.04 (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 v4.1.1 and earlier require a literal // to recognize a URI authority, so a reference that uses \\, /\, or \/ as the authority introducer (in place of //, after an optional scheme) is parsed with no authority: the sequence and everything after it fold into the path. Node's native WHATWG URL (used by fetch(), undici, and Node's http/https clients) instead treats \ as interchangeable with / for special schemes (http, https, ws, wss, ftp, file), so the two parsers extract different hosts from the same input. For example, fast-uri resolves \\evil.com/path against base https://allowed.com/ to https://allowed.com/%5C%5Cevil.com/path (confined to the trusted host), while Node's WHATWG URL resolves the same reference to https://evil.com/path. Applications that use fast-uri to enforce host-based policy (allowlists, denylists, loopback/SSRF filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or fetch() consumers see a policy/use…

Published by the source

3 Aug 2026, 19:16 UTC

Last modified by the source

3 Aug 2026, 19:16 UTC

Source last checked

26 Oct 2026, 13:00 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-7p8r-x3mc-p8w7

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# Patch Next advisory brief: CVE-2026-18446

Advisory: fast-uri vulnerable to host confusion via backslash authority introducer
IDs: CVE-2026-18446, GHSA-7p8r-x3mc-p8w7
Record: https://patchnext.com/advisories/cve-2026-18446 (revision 1, source checked 26 Oct 2026, 13:00 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 4.1.2, 3.1.5, 2.4.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-uri: affected < 2.4.4; >= 3.0.0, < 3.1.5; >= 4.0.0, < 4.1.2; fixed in 3.1.5, 4.1.2, 2.4.4

## 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-7p8r-x3mc-p8w7 (GitHub-reviewed advisory GHSA-7p8r-x3mc-p8w7 lists affected ranges: npm fast-uri >=0 =3.0.0 =4.0.0 <4.1.2)

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