diff --git a/ios/chrome/browser/credential_provider/model/credential_provider_migrator_unittest.mm b/ios/chrome/browser/credential_provider/model/credential_provider_migrator_unittest.mm
index 0ced9ed9a6690..8e6e648371367 100644
--- a/ios/chrome/browser/credential_provider/model/credential_provider_migrator_unittest.mm
+++ b/ios/chrome/browser/credential_provider/model/credential_provider_migrator_unittest.mm
@@ -39,6 +39,22 @@ NSData* StringToData(std::string str) {
   return [NSData dataWithBytes:str.data() length:str.length()];
 }
 
+// Creates a password credential tagged with a specific gaia ID — simulating
+// what the CPE writes into the shared App-Group defaults.
+ArchivableCredential* TestPasswordCredentialForGaia(NSString* gaia) {
+  return [[ArchivableCredential alloc]
+            initWithFavicon:nil
+                       gaia:gaia
+                   password:@"qwerty123"
+                       rank:1
+           recordIdentifier:@"recordIdentifier"
+          serviceIdentifier:@"http://www.alpha.example.com/path/and?args=8"
+                serviceName:nil
+   registryControlledDomain:nil
+                   username:@"username_value"
+                       note:@"note"];
+}
+
 ArchivableCredential* TestPasswordCredential() {
   NSString* username = @"username_value";
   NSString* password = @"qwerty123";
@@ -141,6 +157,89 @@ TEST_F(CredentialProviderMigratorTest, Migration) {
   EXPECT_EQ(store.credentials.count, 0u);
 }
 
+// POC: cross-profile credential leak.
+//
+// CredentialProviderMigratorAppAgent::-migrateCredentialForAllPasskeyModels
+// loops GetLoadedProfiles() synchronously on the main thread and creates one
+// CredentialProviderMigrator per profile, all reading the SAME profile-agnostic
+// App-Group key. Each migrator instantiates its own UserDefaultsCredentialStore
+// with its own dispatch_queue; -startMigrationWithCompletion: synchronously
+// loads the blob and calls AddLogin() / CreatePasskey() into THAT profile's
+// store; the clearing write (-saveDataWithCompletion:) is
+// dispatch_barrier_async on the per-instance queue, so the next iteration of
+// the loop re-reads the still-populated NSUserDefaults entry.
+//
+// Additionally, neither PasswordFormFromCredential nor PasskeyFromCredential
+// consult `credential.gaia`, so even the FIRST iteration consumes credentials
+// gaia-tagged for a different profile.
+//
+// This test mirrors the app agent's loop with two "profiles" (two distinct
+// password stores) and a credential explicitly gaia-tagged for the personal
+// account. It asserts the SECURE behaviour (managed store NEVER receives the
+// credential). The test FAILS — which is the proof of the leak.
+TEST_F(CredentialProviderMigratorTest, POC_CrossProfileLeak) {
+  // 1) The CPE writes a NEW password into the shared App-Group key, tagged
+  //    with the personal account's gaia ID.
+  UserDefaultsCredentialStore* cpeStore =
+      [[UserDefaultsCredentialStore alloc] initWithUserDefaults:user_defaults_
+                                                            key:store_key_];
+  id<Credential> credential =
+      TestPasswordCredentialForGaia(@"personal-gaia-id");
+  [cpeStore addCredential:credential];
+  __block BOOL saved = NO;
+  [cpeStore saveDataWithCompletion:^(NSError* error) {
+    saved = YES;
+  }];
+  ASSERT_TRUE(WaitUntilConditionOrTimeout(kWaitForFileOperationTimeout, ^bool {
+    return saved;
+  }));
+  ASSERT_EQ(cpeStore.credentials.count, 1u);
+
+  // 2) Two loaded profiles → two destination password stores.
+  scoped_refptr<MockPasswordStoreInterface> personal_store =
+      base::MakeRefCounted<testing::StrictMock<MockPasswordStoreInterface>>();
+  scoped_refptr<MockPasswordStoreInterface> managed_store =
+      base::MakeRefCounted<testing::StrictMock<MockPasswordStoreInterface>>();
+
+  PasswordForm expected = PasswordFormFromCredential(credential);
+
+  // Personal store SHOULD receive it (gaia matches).
+  EXPECT_CALL(*personal_store, AddLogin(expected, _)).Times(1);
+
+  // Managed store should NEVER receive a credential gaia-tagged for the
+  // personal account.
+  //
+  // *** THIS EXPECTATION FAILS ***
+  // The migrator ignores `credential.gaia` and the per-profile loop re-reads
+  // the unflushed App-Group blob, so the personal cleartext password is
+  // imported into the managed profile's password store as well.
+  EXPECT_CALL(*managed_store, AddLogin(_, _)).Times(0);
+
+  // 3) Mirror -migrateCredentialForAllPasskeyModels: synchronous main-thread
+  //    loop, one fresh migrator per profile, same key/userDefaults, no wait
+  //    for the previous migrator's async flush.
+  scoped_refptr<MockPasswordStoreInterface> stores[] = {personal_store,
+                                                        managed_store};
+  __block int completions = 0;
+  for (auto& store : stores) {
+    CredentialProviderMigrator* migrator =
+        [[CredentialProviderMigrator alloc] initWithUserDefaults:user_defaults_
+                                                             key:store_key_
+                                                   passwordStore:store
+                                                    passkeyStore:nil];
+    // -startMigrationWithCompletion: synchronously loads NSUserDefaults
+    // (dispatch_sync on a fresh per-instance queue) and calls AddLogin()
+    // before returning. The clearing save is dispatch_barrier_async.
+    [migrator startMigrationWithCompletion:^(BOOL success, NSError* error) {
+      completions++;
+    }];
+  }
+
+  EXPECT_TRUE(WaitUntilConditionOrTimeout(kWaitForFileOperationTimeout, ^bool {
+    return completions == 2;
+  }));
+}
+
 // Tests basic migration for 1 passkey credential.
 TEST_F(CredentialProviderMigratorTest, PasskeyMigration) {
   // Create temp store and add 1 credential.