CVE-2026-75899: Fast uri Remote Code Execution Vulnerability

HIGH
Advisory Identifier: GHSA-fph4-wmhf-6fwf
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CVSS 7.5
CWE-174 CWE-918
Published
Advisory Summary

CVE-2026-75899 is a HIGH (CVSS 7.5) remote code execution vulnerability (CWE-174: Double Decoding of the Same Data) affecting Fast uri libraries in the npm ecosystem. The issue involves fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding. Remediation is available by upgrading affected packages to version 2.4.5, 3.1.6, 4.1.3.

CVE-2026-75899 at a Glance

CVE Identifier CVE-2026-75899
GHSA Identifier GHSA-fph4-wmhf-6fwf
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-174: Double Decoding of the Same Data
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-2026-75899

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-2026-75899, 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-174: Double Decoding of the Same Data, where the product decodes the same input twice, which can limit the effectiveness of any protection mechanism that occurs in between the decoding operations. 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` 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()`.

CVE-2026-75899 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
≥ 2.4.1
< 2.4.5
2.4.5
fast-uri
npm/fast-uri
npm
≥ 3.1.2
< 3.1.6
3.1.6
fast-uri
npm/fast-uri
npm
≥ 4.0.0
< 4.1.3
4.1.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/fastify/fast-uri/security/advisories/GHSA-fph4-wmhf-6fwf
NVD — CVE-2026-75899 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-75899
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/2642290672e7211c990aaed3749c2ce667d0af4f
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/8c15dbe94ec50bb8e595791ebb42c35cef0fd304
Security Fix Commit FIX COMMIT
https://github.com/fastify/fast-uri/commit/ae92a4c5d8c4b6c9e447f048d5fcbde7eebd5514
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-2026-75899?
CVE-2026-75899 is a security vulnerability in Fast uri involving fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding under CWE-174: Double Decoding of the Same Data.
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-2026-75899?
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-2026-75899?
CVE-2026-75899 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-2026-75899?
CVE-2026-75899 is classified under CWE-174: Double Decoding of the Same Data (The product decodes the same input twice, which can limit the effectiveness of any protection mechanism that occurs in between the decoding operations.).
How can I check npm dependencies for CVE-2026-75899?
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-fph4-wmhf-6fwf",
  "modified" : "2026-09-02T15:42:39Z",
  "published" : "2026-09-02T15:42:39Z",
  "aliases" : [ "CVE-2026-75899" ],
  "summary" : "fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding",
  "details" : "### Impact\n\n`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.\n\n### Patches\n\nFixed in `fast-uri` 2.4.5, 3.1.6, and 4.1.3.\n\n### Workarounds\n\nReject untrusted URIs whose host component contains an encoded percent sign (`%25`) before passing them to `normalize()` or `resolve()`.",
  "severity" : [ {
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      }, {
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      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/security/advisories/GHSA-fph4-wmhf-6fwf"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-75899"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/2642290672e7211c990aaed3749c2ce667d0af4f"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/8c15dbe94ec50bb8e595791ebb42c35cef0fd304"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fastify/fast-uri/commit/ae92a4c5d8c4b6c9e447f048d5fcbde7eebd5514"
  }, {
    "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-174", "CWE-918" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T15:42:39Z",
    "nvd_published_at" : "2026-08-24T10:16:39Z"
  }
}