diff --git a/jest.config.js b/jest.config.js
index c109c65382..6a7cab682b 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -30,4 +30,12 @@ module.exports = {
   transformIgnorePatterns: [
     'node_modules/(?!((jest-)?react-native|@firebase|@react-native(-community)?))',
   ],
+  // Packages cross-import each other's built output (e.g. database imports
+  // `@react-native-firebase/app/dist/module/common/deeps`), which resolves
+  // through the workspace symlink to `packages/*/dist/**`. Without this,
+  // Jest instruments both that build artifact and the original `lib/**.ts`
+  // source it was compiled from, so any shared file with branches gets a
+  // second, all-zero coverage entry alongside the real one, and codecov's
+  // patch coverage misreports lines that are actually fully tested.
+  coveragePathIgnorePatterns: ['/node_modules/', '/dist/'],
 };
diff --git a/packages/app/__tests__/common.test.ts b/packages/app/__tests__/common.test.ts
index 511f657b01..2ddd03ad61 100644
--- a/packages/app/__tests__/common.test.ts
+++ b/packages/app/__tests__/common.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it } from '@jest/globals';
 
 import { Base64, getDataUrlParts } from '../lib/common';
+import { deepSet } from '../lib/common/deeps';
 
 describe('common utilities', () => {
   describe('getDataUrlParts', () => {
@@ -42,4 +43,25 @@ describe('common utilities', () => {
       },
     );
   });
+
+  describe('deepSet', () => {
+    it('rejects paths that mutate the target prototype', () => {
+      const target = {};
+
+      expect(deepSet(target, '__proto__.polluted', true)).toBe(false);
+      expect((target as Record<string, unknown>).polluted).toBeUndefined();
+    });
+
+    it('rejects paths that mutate Object.prototype', () => {
+      expect(deepSet({}, 'constructor.prototype.polluted', true, false)).toBe(false);
+      expect(({} as Record<string, unknown>).polluted).toBeUndefined();
+    });
+
+    it('still assigns values along an ordinary nested path', () => {
+      const target: Record<string, unknown> = {};
+
+      expect(deepSet(target, 'a.b', 5)).toBe(true);
+      expect(target).toEqual({ a: { b: 5 } });
+    });
+  });
 });
diff --git a/packages/app/lib/common/deeps.ts b/packages/app/lib/common/deeps.ts
index 8ca859ac99..fda8723ad6 100644
--- a/packages/app/lib/common/deeps.ts
+++ b/packages/app/lib/common/deeps.ts
@@ -69,6 +69,9 @@ export function deepSet(
     return false;
   }
   const keys = path.split(joiner);
+  if (keys.some(key => ['__proto__', 'prototype', 'constructor'].includes(key))) {
+    return false;
+  }
 
   let i = 0;
   let _object: unknown = object;