diff --git a/src/datastore/local-store/repos/api-key.repo.spec.ts b/src/datastore/local-store/repos/api-key.repo.spec.ts
index 48c5dcb9..d664a5a7 100644
--- a/src/datastore/local-store/repos/api-key.repo.spec.ts
+++ b/src/datastore/local-store/repos/api-key.repo.spec.ts
@@ -2,6 +2,7 @@ import { createMock } from '@golevelup/ts-jest';
 import { ConfigService } from '@nestjs/config';
 
 import { ApiKeyRepo } from '~/auth/repos/api-key.repo';
+import { AppNotFoundError } from '~/common/errors';
 import { LocalApiKeysRepo } from '~/datastore/local-store/repos/api-key.repo';
 import { defaultUser } from '~/users/user.consts';
 
@@ -23,4 +24,16 @@ describe('LocalApiKeyRepo', () => {
 
     return expect(repo.getUserByApiKey(config)).resolves.toEqual(defaultUser);
   });
+
+  it.each(['constructor', '__proto__', 'toString', 'valueOf', 'hasOwnProperty'])(
+    'should not treat the Object.prototype property "%s" as a configured key',
+    key => {
+      const mockConfig = createMock<ConfigService>();
+      mockConfig.getOrThrow.mockReturnValue(config);
+
+      const repo = new LocalApiKeysRepo(mockConfig);
+
+      return expect(repo.getUserByApiKey(key)).rejects.toThrow(AppNotFoundError);
+    }
+  );
 });
diff --git a/src/datastore/local-store/repos/api-key.repo.ts b/src/datastore/local-store/repos/api-key.repo.ts
index 5f26cfcf..2df33069 100644
--- a/src/datastore/local-store/repos/api-key.repo.ts
+++ b/src/datastore/local-store/repos/api-key.repo.ts
@@ -11,7 +11,7 @@ import { defaultUser } from '~/users/user.consts';
 
 @Injectable()
 export class LocalApiKeysRepo implements ApiKeyRepo {
-  private apiKeys: Record<string, UserModel> = {};
+  private apiKeys: Record<string, UserModel> = Object.create(null);
 
   constructor(readonly config: ConfigService) {
     const givenApiKeys = config.getOrThrow<string>('API_KEYS');
diff --git a/src/signing/services/local-signing.service.spec.ts b/src/signing/services/local-signing.service.spec.ts
index 6b1873b3..02b5be97 100644
--- a/src/signing/services/local-signing.service.spec.ts
+++ b/src/signing/services/local-signing.service.spec.ts
@@ -75,6 +75,13 @@ describe('LocalSigningService', () => {
     it('should throw if an Account is not loaded', () => {
       expect(() => service.getAddressByHandle('badId')).toThrow(AppNotFoundError);
     });
+
+    it.each(['constructor', '__proto__', 'toString'])(
+      'should throw for the Object.prototype property "%s" when it is not loaded',
+      handle => {
+        expect(() => service.getAddressByHandle(handle)).toThrow(AppNotFoundError);
+      }
+    );
   });
 
   describe('isAddress', () => {
@@ -122,5 +129,13 @@ describe('LocalSigningService', () => {
 
       await expect(service.addSigner(handle, mnemonic)).rejects.toThrow(AppConflictError);
     });
+
+    it('should add a signer whose handle is an Object.prototype property name', async () => {
+      const handle = 'constructor';
+      const expectedAddress = '15oF4uVJwmo4TdGW7VfQxNLavjCXviqxT9S1MgbjMNHr6Sp5';
+
+      await expect(service.addSigner(handle, '//Alice')).resolves.toBe(expectedAddress);
+      await expect(service.getAddressByHandle(handle)).resolves.toBe(expectedAddress);
+    });
   });
 });
diff --git a/src/signing/services/local-signing.service.ts b/src/signing/services/local-signing.service.ts
index 3ee3c403..cf667089 100644
--- a/src/signing/services/local-signing.service.ts
+++ b/src/signing/services/local-signing.service.ts
@@ -7,7 +7,7 @@ import { PolymeshService } from '~/polymesh/polymesh.service';
 import { SigningService } from '~/signing/services/signing.service';
 
 export class LocalSigningService extends SigningService {
-  private addressBook: Record<string, string> = {};
+  private addressBook: Record<string, string> = Object.create(null);
 
   constructor(
     protected readonly signingManager: LocalSigningManager,