CVE-2026-75931: Fast uri Fast-uri Vulnerable To Host Confusion Vulnerability

HIGH
Advisory Identifier: GHSA-5jgf-p345-68v8
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CVSS 7.5
CWE-436
Published
Advisory Summary

CVE-2026-75931 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 skipped idn canonicalization on scheme-relative references. Remediation is available by upgrading affected packages to version 2.4.5, 3.1.6, 4.1.3.

CVE-2026-75931 at a Glance

CVE Identifier CVE-2026-75931
GHSA Identifier GHSA-5jgf-p345-68v8
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-2026-75931

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: 2.4.5 
npm install fast-uri@2.4.5
fast-uri fixed: 3.1.6 
npm install fast-uri@3.1.6
fast-uri fixed: 4.1.3 
npm install fast-uri@4.1.3
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-2026-75931, 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 2.4.5), 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` 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.

CVE-2026-75931 Affected Fast uri Versions

3 component(s)
Fixed Versions:
2.4.53.1.64.1.3
Product / Package Ecosystem Affected Versions Fixed Version
fast-uri
npm/fast-uri
npm
≥ 2.4.2
< 2.4.5
2.4.5
fast-uri
npm/fast-uri
npm
≥ 3.1.3
< 3.1.6
3.1.6
fast-uri
npm/fast-uri
npm
≥ 4.0.1
< 4.1.3
4.1.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8
NVD — CVE-2026-75931 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-75931
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/0256bc8d1f28b5d0ac657faf67e2411a189dfcb5
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/444ecdad447db2cc23c4d422acc6f0daa6fa8eef
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/4e4ebd8b245f6ca16e448203559eea3e545453a3
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/v2.4.5
Fast uri Release RELEASE
https://github.com/fastify/fast-uri/releases/tag/v3.1.6
Fast uri Release RELEASE
https://github.com/fastify/fast-uri/releases/tag/v4.1.3

Frequently Asked Questions

What is CVE-2026-75931?
CVE-2026-75931 is a security vulnerability in Fast uri involving fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references under CWE-436: Interpretation Conflict.
Which Fast uri versions are affected?
Affected versions include releases prior to 2.4.5, 3.1.6, 4.1.3 across affected packages: fast-uri.
What version fixes CVE-2026-75931?
Upgrading to 2.4.5, 3.1.6, 4.1.3 (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 2.4.5, 3.1.6, 4.1.3. Upgrade your dependency to 2.4.5, 3.1.6, 4.1.3 or later.
What is the CVSS score of CVE-2026-75931?
CVE-2026-75931 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-2026-75931?
CVE-2026-75931 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-2026-75931?
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-5jgf-p345-68v8",
  "modified" : "2026-09-02T15:44:30Z",
  "published" : "2026-09-02T15:44:30Z",
  "aliases" : [ "CVE-2026-75931" ],
  "summary" : "fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references",
  "details" : "### Impact\n\n`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.\n\n### Patches\n\nUpgrade 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.\n\n### Workarounds\n\nResolve scheme-relative references against a base that carries a scheme before performing any host-policy or origin check.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fast-uri"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.4.2"
      }, {
        "fixed" : "2.4.5"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fast-uri"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.1.3"
      }, {
        "fixed" : "3.1.6"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fast-uri"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.1"
      }, {
        "fixed" : "4.1.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-75931"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/0256bc8d1f28b5d0ac657faf67e2411a189dfcb5"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/444ecdad447db2cc23c4d422acc6f0daa6fa8eef"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/4e4ebd8b245f6ca16e448203559eea3e545453a3"
  }, {
    "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/v2.4.5"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/releases/tag/v3.1.6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/releases/tag/v4.1.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-436" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:44:30Z",
    "nvd_published_at" : "2026-08-24T10:16:40Z"
  }
}