CVE-2026-35209: defu Prototype Pollution Vulnerability

HIGH
Advisory Identifier: GHSA-737v-mqg7-c878
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CVSS 7.5
CWE-1321
Published
Advisory Summary

CVE-2026-35209 is a HIGH (CVSS 7.5) prototype pollution vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting defu libraries in the npm ecosystem. The issue involves prototype pollution via `__proto__` key in defaults argument. Remediation is available by upgrading affected packages to version 6.1.5.

CVE-2026-35209 at a Glance

CVE Identifier CVE-2026-35209
GHSA Identifier GHSA-737v-mqg7-c878
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product defu
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-2026-35209

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

defu fixed: 6.1.5 
npm install defu@6.1.5
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 defu

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-35209, inspect your project dependencies using npm:

npm ls defu

Analyze the terminal output to locate any version of defu in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.1.5), your application is affected.

Note: The npm ls defu 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-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), where the product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. In defu, 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 Applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution. A crafted payload containing a `__proto__` key can override intended default values in the merged result: ```js import { defu } from 'defu' const userInput = JSON.parse('{"__proto__":{"isAdmin":true}}') const config = defu(userInput, { isAdmin: false }) config.isAdmin // true — attacker overrides the server default ``` ### Root Cause The internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object's `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result. ### Fix Replace `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter. ### Affected Versions <= 6.1.4 ### Credits Reported by [@BlackHatExploitation](https://github.com/BlackHatExploitation)

CVE-2026-35209 Affected defu Versions

1 component(s)
Fixed Versions:
6.1.5
Product / Package Ecosystem Affected Versions Fixed Version
defu
npm/defu
npm
< 6.1.5
6.1.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878
NVD — CVE-2026-35209 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-35209
defu Source Repository REPOSITORY
https://github.com/unjs/defu/pull/156
Security Fix Commit FIX COMMIT
https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29
defu Source Repository REPOSITORY
https://github.com/unjs/defu
defu Release RELEASE
https://github.com/unjs/defu/releases/tag/v6.1.5

Frequently Asked Questions

What is CVE-2026-35209?
CVE-2026-35209 is a security vulnerability in defu involving Prototype pollution via `__proto__` key in defaults argument under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which defu versions are affected?
Affected versions include releases prior to 6.1.5 across affected packages: defu.
What version fixes CVE-2026-35209?
Upgrading to 6.1.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is defu affected?
Yes, defu is affected in releases prior to 6.1.5. Upgrade your dependency to 6.1.5 or later.
What is the CVSS score of CVE-2026-35209?
CVE-2026-35209 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-2026-35209?
CVE-2026-35209 is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') (The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.).
How can I check npm dependencies for CVE-2026-35209?
Run the dependency tree command: 'npm ls defu'. 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-737v-mqg7-c878",
  "modified" : "2026-04-06T23:42:28Z",
  "published" : "2026-04-04T06:17:53Z",
  "aliases" : [ "CVE-2026-35209" ],
  "summary" : "defu: Prototype pollution via `__proto__` key in defaults argument",
  "details" : "### Impact\n\nApplications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution.\n\nA crafted payload containing a `__proto__` key can override intended default values in the merged result:\n\n```js\nimport { defu } from 'defu'\n\nconst userInput = JSON.parse('{\"__proto__\":{\"isAdmin\":true}}')\nconst config = defu(userInput, { isAdmin: false })\n\nconfig.isAdmin // true — attacker overrides the server default\n```\n\n### Root Cause\n\nThe internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object's `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result.\n\n### Fix\n\nReplace `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter.\n\n### Affected Versions\n\n<= 6.1.4\n\n### Credits\n\nReported by [@BlackHatExploitation](https://github.com/BlackHatExploitation)",
  "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" : "defu"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.1.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.1.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/unjs/defu/security/advisories/GHSA-737v-mqg7-c878"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-35209"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/unjs/defu/pull/156"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/unjs/defu/commit/3942bfbbcaa72084bd4284846c83bd61ed7c8b29"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/unjs/defu"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/unjs/defu/releases/tag/v6.1.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-04T06:17:53Z",
    "nvd_published_at" : "2026-04-06T18:16:44Z"
  }
}