diff --git a/src/theme/__tests__/provider.test.ts b/src/theme/__tests__/provider.test.ts
index acac88b491..b1f421be36 100644
--- a/src/theme/__tests__/provider.test.ts
+++ b/src/theme/__tests__/provider.test.ts
@@ -124,6 +124,49 @@ describe('safeMerge', () => {
     expect(result.colors.secondary).toBe('#999');
     expect(result.colors.tertiary).toBe('#222');
   });
+
+  it('ignores a top-level __proto__ key', () => {
+    const overrides = JSON.parse('{"__proto__": {"polluted": "yes"}, "a": 2}');
+
+    const result = safeMerge<Record<string, unknown>>({ a: 1 }, overrides);
+    expect(Object.getPrototypeOf(result)).toBe(Object.prototype);
+    expect(result).toEqual({ a: 2 });
+    expect(result.polluted).toBeUndefined();
+    const fresh: Record<string, unknown> = {};
+    expect(fresh.polluted).toBeUndefined();
+  });
+
+  it('ignores a nested __proto__ key', () => {
+    const base = { colors: { primary: 'blue', secondary: 'green' } };
+    const overrides = JSON.parse(
+      '{"colors": {"__proto__": {"polluted": "yes"}, "primary": "red"}}'
+    );
+
+    const result = safeMerge<{ colors: Record<string, unknown> }>(
+      base,
+      overrides
+    );
+    expect(Object.getPrototypeOf(result.colors)).toBe(Object.prototype);
+    expect(result.colors.primary).toBe('red');
+    expect(result.colors.secondary).toBe('green');
+    expect(result.colors.polluted).toBeUndefined();
+  });
+
+  it('skips constructor and prototype keys', () => {
+    const overrides = JSON.parse(
+      '{"constructor": {"x": 1}, "prototype": {"y": 2}, "a": 2}'
+    );
+
+    const result = safeMerge<Record<string, unknown>>({ a: 1 }, overrides);
+    expect(Object.prototype.hasOwnProperty.call(result, 'constructor')).toBe(
+      false
+    );
+    expect(Object.prototype.hasOwnProperty.call(result, 'prototype')).toBe(
+      false
+    );
+    expect(result.constructor).toBe(Object);
+    expect(result).toEqual({ a: 2 });
+  });
 });
 
 describe('useInternalTheme', () => {
diff --git a/src/theme/provider.tsx b/src/theme/provider.tsx
index 2c1f8ca9f9..11d697bd19 100644
--- a/src/theme/provider.tsx
+++ b/src/theme/provider.tsx
@@ -29,6 +29,8 @@ export const isPlatformColorSentinel = (v: unknown): boolean =>
   typeof v === 'object' &&
   ('resource_paths' in v || 'semantic' in v || 'dynamic' in v);
 
+const UNSAFE_KEYS = new Set(['__proto__', 'constructor', 'prototype']);
+
 /* eslint-disable @typescript-eslint/no-unsafe-type-assertion */
 export const safeMerge = <T,>(base: T, overrides: unknown): T => {
   if (
@@ -46,6 +48,9 @@ export const safeMerge = <T,>(base: T, overrides: unknown): T => {
   }
   const out: Record<string, unknown> = { ...(base as Record<string, unknown>) };
   for (const key of Object.keys(overrides)) {
+    if (UNSAFE_KEYS.has(key)) {
+      continue;
+    }
     out[key] = safeMerge(
       (base as Record<string, unknown>)[key],
       (overrides as Record<string, unknown>)[key]