fix access key storage for different accounts (#6210)

moto/sts/models.py

@@ -99,7 +99,8 @@ class STSBackend(BaseBackend):
             duration,
             external_id,
         )
-        self.assumed_roles.append(role)
+        account_backend = sts_backends[account_id]["global"]
+        account_backend.assumed_roles.append(role)
         return role
 
     def get_assumed_role_from_access_key(self, access_key_id):

tests/test_sts/test_sts_integration.py

@@ -24,6 +24,17 @@ class TestStsAssumeRole(unittest.TestCase):
         )
 
         # Assume the new role
+        sts_account_b = boto3.client(
+            "sts",
+            aws_access_key_id=response["Credentials"]["AccessKeyId"],
+            aws_secret_access_key=response["Credentials"]["SecretAccessKey"],
+            aws_session_token=response["Credentials"]["SessionToken"],
+            region_name="us-east-1",
+        )
+        assumed_arn = sts_account_b.get_caller_identity()["Arn"]
+        assumed_arn.should.equal(
+            f"arn:aws:sts::{self.account_b}:assumed-role/my-role/test-session-name"
+        )
         iam_account_b = boto3.client(
             "iam",
             aws_access_key_id=response["Credentials"]["AccessKeyId"],