fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding

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-2026-75899: 0.22% estimated exploitation activity in the next 30 days; percentile 12.61 (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 decodes a hostname's percent escapes twice in a single normalize() or resolve() call: once during parsing and again during authority recomposition. A nested percent-encoded host therefore survives the first decode and is turned into a live destination by the second, so normalize('http://%256c%256f%2563%2561%256c%2568%256f%2573%2574/') returns http://localhost/. Applications that normalize or resolve an untrusted URI before an SSRF check, redirect validation, or host allowlist can be steered to a different destination, including internal addresses such as loopback or a cloud metadata endpoint, than the encoded input appeared to contain. This is an incomplete-fix variant of CVE-2026-6322, whose encoded-authority-delimiter fix introduced the second decode. Patches Fixed in fast-uri 2.4.5, 3.1.6, and 4.1.3. Workarounds Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().

Published by the source

2 Sep 2026, 15:42 UTC

Last modified by the source

2 Sep 2026, 15:42 UTC

Source last checked

26 Oct 2026, 13:02 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-fph4-wmhf-6fwf

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# Patch Next advisory brief: CVE-2026-75899

Advisory: fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding
IDs: CVE-2026-75899, GHSA-fph4-wmhf-6fwf
Record: https://patchnext.com/advisories/cve-2026-75899 (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 >= 2.4.1, < 2.4.5; >= 3.1.2, < 3.1.6; >= 4.0.0, < 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-fph4-wmhf-6fwf (GitHub-reviewed advisory GHSA-fph4-wmhf-6fwf lists affected ranges: npm fast-uri >=2.4.1 =3.1.2 =4.0.0 <4.1.3)

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