diff --git a/src/canonicalize.ts b/src/canonicalize.ts
index b45e4a7..b7da5f9 100644
--- a/src/canonicalize.ts
+++ b/src/canonicalize.ts
@@ -43,23 +43,31 @@ const SIGNED_FIELDS = [
 
 /**
  * Recursively sort object keys for deterministic JSON.
+ * Uses Object.create(null) for the result object to avoid
+ * prototype-pollution style assignments being silently
+ * discarded (or hijacked) by Object.prototype setters.
  */
 function sortKeys(obj: unknown): unknown {
   if (obj === null || typeof obj !== 'object') {
     return obj;
   }
-  
+
   if (Array.isArray(obj)) {
     return obj.map(sortKeys);
   }
-  
-  const sorted: Record<string, unknown> = {};
+
+  const sorted = Object.create(null) as Record<string, unknown>;
   const keys = Object.keys(obj as Record<string, unknown>).sort();
-  
+
   for (const key of keys) {
-    sorted[key] = sortKeys((obj as Record<string, unknown>)[key]);
+    Object.defineProperty(sorted, key, {
+      value: sortKeys((obj as Record<string, unknown>)[key]),
+      writable: true,
+      enumerable: true,
+      configurable: true,
+    });
   }
-  
+
   return sorted;
 }
 
@@ -77,15 +85,24 @@ function serializeValue(value: unknown): unknown {
  * Extract and canonicalize receipt fields for signing.
  */
 export function canonicalizeReceipt(receipt: Record<string, unknown>): string {
-  const canonical: Record<string, unknown> = {};
-  
+  // Use a null-prototype object so that a malicious receipt cannot
+  // route `__proto__` (or any other inherited key) through
+  // Object.prototype setters and silently drop its signed value from
+  // the canonical bytes.
+  const canonical = Object.create(null) as Record<string, unknown>;
+
   for (const field of SIGNED_FIELDS) {
     const value = receipt[field];
     if (value !== undefined && value !== null) {
-      canonical[field] = serializeValue(value);
+      Object.defineProperty(canonical, field, {
+        value: serializeValue(value),
+        writable: true,
+        enumerable: true,
+        configurable: true,
+      });
     }
   }
-  
+
   // Sort keys and stringify deterministically
   const sorted = sortKeys(canonical);
   return JSON.stringify(sorted);
diff --git a/tests/verify.test.ts b/tests/verify.test.ts
index 0fd26db..1341616 100644
--- a/tests/verify.test.ts
+++ b/tests/verify.test.ts
@@ -2,6 +2,7 @@ import { readFile } from 'node:fs/promises';
 import { resolve } from 'node:path';
 import { describe, expect, it } from 'vitest';
 import { verifyReceipt } from '../src/verify.js';
+import { canonicalizeReceipt, canonicalizeReceiptBytes } from '../src/canonicalize.js';
 
 async function loadJson(name: string): Promise<unknown> {
   const path = resolve(process.cwd(), 'tests/fixtures', name);
@@ -40,6 +41,35 @@ describe('verifyReceipt', () => {
     }
   });
 
+  it('canonicalize preserves nested __proto__ keys inside signed fields', () => {
+    // Forge a requestJson whose value contains a nested __proto__ own
+    // key. After the receipt is signed, mutate that value and confirm
+    // the canonical signing bytes change. If the canonicalizer routed
+    // __proto__ through Object.prototype.__proto__ the value change
+    // would be silently dropped and the bytes would not change.
+    const inner: Record<string, unknown> = {};
+    Object.defineProperty(inner, '__proto__', {
+      value: { evil: 'first' },
+      writable: true,
+      enumerable: true,
+      configurable: true,
+    });
+    const base = {
+      id: 'rcpt_proto_001',
+      requestJson: inner,
+    } as unknown as Record<string, unknown>;
+    const originalBytes = canonicalizeReceiptBytes(base);
+
+    // Mutate the nested __proto__ value. With the old sortKeys
+    // implementation, this would either silently no-op or rewrite the
+    // prototype chain instead of changing the own key. With the fix
+    // the canonical bytes must change.
+    (base.requestJson as unknown as { __proto__: { evil: string } }).__proto__.evil = 'second';
+    const mutatedBytes = canonicalizeReceiptBytes(base);
+
+    expect(mutatedBytes.equals(originalBytes)).toBe(false);
+  });
+
   it('returns signature invalid for wrong key id pin', async () => {
     const receipt = await loadJson('wrong-key.json');
     const result = await verifyReceipt(receipt, { keyFile, keyId: 'pp-test-2026-q2', noNetwork: true });