CVE-2026-16221: Fast uri Fast-uri Vulnerable To Host Confusion Vulnerability

HIGH
Advisory Identifier: GHSA-v2hh-gcrm-f6hx
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CVSS 7.5
CWE-436
Published
Advisory Summary

CVE-2026-16221 is a HIGH (CVSS 7.5) fast-uri vulnerable to host confusion vulnerability (CWE-436: Interpretation Conflict) affecting Fast uri libraries in the npm ecosystem. The issue involves fast-uri vulnerable to host confusion via literal backslash authority delimiter. Remediation is available by upgrading affected packages to version 2.4.3, 3.1.4, 4.1.1.

CVE-2026-16221 at a Glance

CVE Identifier CVE-2026-16221
GHSA Identifier GHSA-v2hh-gcrm-f6hx
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-436: Interpretation Conflict
Affected Product Fast uri
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability None (A:N)

How to Fix CVE-2026-16221

To remediate this vulnerability, upgrade all affected Fast uri dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

fast-uri fixed: 2.4.3 
npm install fast-uri@2.4.3
fast-uri fixed: 3.1.4 
npm install fast-uri@3.1.4
fast-uri fixed: 4.1.1 
npm install fast-uri@4.1.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls fast-uri

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-16221, inspect your project dependencies using npm:

npm ls fast-uri

Analyze the terminal output to locate any version of Fast uri in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.4.3), your application is affected.

Note: The npm ls fast-uri command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-436: Interpretation Conflict, where product a handles inputs or steps differently than product b, which causes a to perform incorrect actions based on its perception of b's state. In Fast uri, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact `fast-uri` v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG `URL` (used by `fetch()`, `undici`, and Node's `http`/`https` clients) normalizes `\` to `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input string. For example, `http://evil.com\@allowed.com` is treated by `fast-uri` as host `allowed.com` with userinfo `evil.com\`, while Node's WHATWG URL parser and `fetch()` see host `evil.com` with path `/@allowed.com`. 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 desync and can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts. ### Patches Upgrade to `fast-uri` v4.1.1, v3.1.4, or v2.4.3. ### Workarounds None. Upgrade to the patched version.

CVE-2026-16221 Affected Fast uri Versions

3 component(s)
Fixed Versions:
2.4.33.1.44.1.1
Product / Package Ecosystem Affected Versions Fixed Version
fast-uri
npm/fast-uri
npm
≥ 2.3.1
< 2.4.3
2.4.3
fast-uri
npm/fast-uri
npm
≥ 3.0.0
< 3.1.4
3.1.4
fast-uri
npm/fast-uri
npm
≥ 4.0.0
< 4.1.1
4.1.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fast-uri/security/advisories/GHSA-v2hh-gcrm-f6hx
NVD — CVE-2026-16221 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-16221
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/0542a216860fd70c062a4730e620576f62ded057
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/2d50fbabc80e4d0884fe0f6a98fe118ce6faa353
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/9438266d6a7ded688c8bc7c4ba506da17f44a17b
Fast uri Security Information SECURITY
https://cna.openjsf.org/security-advisories.html
Fast uri Source Repository REPOSITORY
https://github.com/fastify/fast-uri
Fast uri Release RELEASE
https://github.com/fastify/fast-uri/releases/tag/v2.4.3
Fast uri Release RELEASE
https://github.com/fastify/fast-uri/releases/tag/v3.1.4
Fast uri Release RELEASE
https://github.com/fastify/fast-uri/releases/tag/v4.1.1

Frequently Asked Questions

What is CVE-2026-16221?
CVE-2026-16221 is a security vulnerability in Fast uri involving fast-uri vulnerable to host confusion via literal backslash authority delimiter under CWE-436: Interpretation Conflict.
Which Fast uri versions are affected?
Affected versions include releases prior to 2.4.3, 3.1.4, 4.1.1 across affected packages: fast-uri.
What version fixes CVE-2026-16221?
Upgrading to 2.4.3, 3.1.4, 4.1.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is fast-uri affected?
Yes, fast-uri is affected in releases prior to 2.4.3, 3.1.4, 4.1.1. Upgrade your dependency to 2.4.3, 3.1.4, 4.1.1 or later.
What is the CVSS score of CVE-2026-16221?
CVE-2026-16221 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
What CWE is associated with CVE-2026-16221?
CVE-2026-16221 is classified under CWE-436: Interpretation Conflict (Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.).
How can I check npm dependencies for CVE-2026-16221?
Run the dependency tree command: 'npm ls fast-uri'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-v2hh-gcrm-f6hx",
  "modified" : "2026-07-21T22:08:29Z",
  "published" : "2026-07-21T22:08:28Z",
  "aliases" : [ "CVE-2026-16221" ],
  "summary" : "fast-uri vulnerable to host confusion via literal backslash authority delimiter",
  "details" : "### Impact\n\n`fast-uri` v4.1.0 and earlier do not treat a literal backslash (U+005C) as an authority delimiter. Node's native WHATWG `URL` (used by `fetch()`, `undici`, and Node's `http`/`https` clients) normalizes `\\` to `/` for special schemes (`http`, `https`, `ws`, `wss`, `ftp`, `file`), so the two parsers extract different hosts from the same input string.\n\nFor example, `http://evil.com\\@allowed.com` is treated by `fast-uri` as host `allowed.com` with userinfo `evil.com\\`, while Node's WHATWG URL parser and `fetch()` see host `evil.com` with path `/@allowed.com`.\n\nApplications 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 desync and can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts.\n\n### Patches\n\nUpgrade to `fast-uri` v4.1.1, v3.1.4, or v2.4.3.\n\n### Workarounds\n\nNone. Upgrade to the patched version.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
  } ],
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    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.3.1"
      }, {
        "fixed" : "2.4.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.4.2"
    }
  }, {
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      "ecosystem" : "npm",
      "name" : "fast-uri"
    },
    "ranges" : [ {
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      }, {
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      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.1.3"
    }
  }, {
    "package" : {
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    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.1.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.1.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/security/advisories/GHSA-v2hh-gcrm-f6hx"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-16221"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/0542a216860fd70c062a4730e620576f62ded057"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/2d50fbabc80e4d0884fe0f6a98fe118ce6faa353"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/9438266d6a7ded688c8bc7c4ba506da17f44a17b"
  }, {
    "type" : "WEB",
    "url" : "https://cna.openjsf.org/security-advisories.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fastify/fast-uri"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/releases/tag/v2.4.3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/releases/tag/v3.1.4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/releases/tag/v4.1.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-436" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-21T22:08:28Z",
    "nvd_published_at" : "2026-07-19T15:16:49Z"
  }
}