fix: Revert "prevent prototype pollution via constructor.prototype access (CVE-2026-XXXX) (#1259)"

This reverts commit 48fc378.
This fixes #1266 and #1268, #1265 while the security reported could not find a reasonable way to show the designated fixes was closing a real vulnerability, nor could I think of anything.

Author: mweststrate

.vscode/settings.json

@@ -5,8 +5,5 @@
   "javascript.validate.enable": false,
   "typescript.tsdk": "node_modules/typescript/lib",
   "jest.enableInlineErrorMessages": true,
-  "cSpell.enabled": true,
-  "chat.tools.terminal.autoApprove": {
-    "yarn vitest": true
-  }
+  "cSpell.enabled": true
 }

__tests__/base.js

@@ -2953,98 +2953,6 @@ function runBaseTest(
 			expect(result.objConstructed).toEqual(new Object().constructor(1))
 		})
 
-		it("does not allow prototype pollution via reserved constructor access", () => {
-			const pollutedKey = "__immer_test_polluted__"
-			const original = Object.prototype[pollutedKey]
-
-			delete Object.prototype[pollutedKey]
-
-			try {
-				// The attack should either throw an error or silently fail
-				// but NOT pollute Object.prototype
-				let hadError = false
-				try {
-					produce({}, draft => {
-						draft.constructor.prototype[pollutedKey] = true
-						draft["__proto__"][pollutedKey] = true
-					})
-				} catch (e) {
-					// Expected: error when trying to set on undefined/frozen objects
-					hadError = true
-				}
-
-				// The critical check: Object.prototype must not be polluted
-				// either because we threw an error OR because the assignment was silently blocked
-				expect(Object.prototype[pollutedKey]).toBeUndefined()
-			} finally {
-				if (original === undefined) {
-					delete Object.prototype[pollutedKey]
-				} else {
-					Object.prototype[pollutedKey] = original
-				}
-			}
-		})
-
-		it("blocks prototype pollution via stored constructor reference (CVE bypass)", () => {
-			const pollutedKey = "__immer_test_ref__"
-			const original = Object.prototype[pollutedKey]
-
-			delete Object.prototype[pollutedKey]
-
-			try {
-				// Attack 3 from CVE: store constructor reference and mutate
-				let hadError = false
-				try {
-					produce({data: {}}, draft => {
-						const ctor = draft.data.constructor
-						ctor.prototype[pollutedKey] = true
-					})
-				} catch (e) {
-					hadError = true
-				}
-
-				expect(Object.prototype[pollutedKey]).toBeUndefined()
-			} finally {
-				if (original === undefined) {
-					delete Object.prototype[pollutedKey]
-				} else {
-					Object.prototype[pollutedKey] = original
-				}
-			}
-		})
-
-		it("blocks prototype pollution via Object.assign with malicious payload", () => {
-			const pollutedKey = "__immer_test_assign__"
-			const original = Object.prototype[pollutedKey]
-
-			delete Object.prototype[pollutedKey]
-
-			try {
-				// Simulates the real-world attack scenario where user input is Object.assign'd to draft
-				const userInput = {
-					constructor: {prototype: {[pollutedKey]: true}}
-				}
-
-				let hadError = false
-				try {
-					produce({}, draft => {
-						Object.assign(draft, userInput)
-					})
-				} catch (e) {
-					hadError = true
-				}
-
-				// Must NOT pollute via Object.assign path
-				expect(Object.prototype[pollutedKey]).toBeUndefined()
-			} finally {
-				if (original === undefined) {
-					delete Object.prototype[pollutedKey]
-				} else {
-					Object.prototype[pollutedKey] = original
-				}
-			}
-		})
-
 		it("should handle equality correctly - 1", () => {
 			const baseState = {
 				y: 3 / 0,

src/core/proxy.ts

@@ -110,32 +110,6 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 	get(state, prop) {
 		if (prop === DRAFT_STATE) return state
 
-		// Guard against prototype pollution via constructor and __proto__
-		// We allow access but wrap in a proxy that blocks prototype chain traversal
-		if (prop === "constructor" || prop === "__proto__") {
-			const source = latest(state)
-			const value = source[prop]
-			// Return a proxy that allows calling the constructor but blocks access to prototype
-			return new Proxy(value || {}, {
-				get: (target, key) => {
-					// Block __proto__ and prototype access chains
-					if (key === "__proto__" || key === "prototype") {
-						return Object.freeze(Object.create(null))
-					}
-					// Allow normal property access for legitimate use
-					return Reflect.get(target, key)
-				},
-				set: () => {
-					// Silently ignore writes to prevent pollution
-					return true
-				},
-				apply: (target, thisArg, args) => {
-					// Allow constructor to be called as a function (e.g., draft.arr.constructor(1))
-					return Reflect.apply(target as Function, thisArg, args)
-				}
-			})
-		}
-
 		let arrayPlugin = state.scope_.arrayMethodsPlugin_
 		const isArrayWithStringProp =
 			state.type_ === ArchType.Array && typeof prop === "string"
@@ -183,14 +157,6 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 		return value
 	},
 	has(state, prop) {
-		// Block reserved properties from being detected
-		if (
-			prop === "constructor" ||
-			prop === "__proto__" ||
-			prop === "prototype"
-		) {
-			return false
-		}
 		return prop in latest(state)
 	},
 	ownKeys(state) {
@@ -201,16 +167,6 @@ export const objectTraps: ProxyHandler<ProxyState> = {
 		prop: string /* strictly not, but helps TS */,
 		value
 	) {
-		// Guard against prototype pollution - prevent assignment to reserved properties
-		// that could lead to Object.prototype pollution
-		if (
-			prop === "constructor" ||
-			prop === "__proto__" ||
-			prop === "prototype"
-		) {
-			return true
-		}
-
 		const desc = getDescriptorFromProto(latest(state), prop)
 		if (desc?.set) {
 			// special case: if this write is captured by a setter, we have