Reject FIC leg-2 apps on silent flows too

The _reject_if_mtls_binding_cert guard already covered the six
non-client-credential acquisition flows, but not acquire_token_silent /
acquire_token_silent_with_error. With a shared token cache holding user
refresh tokens, a FIC leg-2 app (client_assertion + mtls_binding_certificate)
could silently redeem one of those RTs, putting the cert-bound leg-1
assertion on a non-mTLS jwt-bearer request -- which ESTS rejects with a
confusing invalid_client rather than a clear message.

Add the guard to both public silent entry points. Both call the private
_acquire_token_silent_with_error directly (not each other), and
acquire_token_for_client also uses that private helper, so the guard lives
on the two public wrappers -- never on the private method -- to keep the
legitimate mTLS client-credential flow working. No-op for normal apps.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: Robbie-Microsoft

msal/application.py

@@ -1796,6 +1796,7 @@ def acquire_token_silent(
               if cache lookup succeeded.
             - None when cache lookup does not yield a token.
         """
+        self._reject_if_mtls_binding_cert("acquire_token_silent")
         if not account:
             return None  # A backward-compatible NO-OP to drop the account=None usage
         result = _clean_up(self._acquire_token_silent_with_error(
@@ -1850,6 +1851,7 @@ def acquire_token_silent_with_error(
             - None when there is simply no token in the cache.
             - A dict containing an "error" key, when token refresh failed.
         """
+        self._reject_if_mtls_binding_cert("acquire_token_silent_with_error")
         if not account:
             return None  # A backward-compatible NO-OP to drop the account=None usage
         return _clean_up(self._acquire_token_silent_with_error(

tests/test_application.py

@@ -1769,6 +1769,16 @@ def test_fic_leg2_app_rejects_non_client_credential_flows(self):
         with self.assertRaises(ValueError):
             app.acquire_token_by_user_federated_identity_credential(
                 ["s1"], "assertion_token", username="user@contoso.com")
+        # Silent flows are also unsupported: a shared cache could hold user RTs
+        # whose redemption would put the cert-bound assertion on a non-mTLS
+        # jwt-bearer request. Guard both silent entry points (they call the
+        # private _acquire_token_silent_with_error directly, not each other).
+        account = {"home_account_id": "uid.utid",
+                   "environment": "login.microsoftonline.com", "username": "u"}
+        with self.assertRaises(ValueError):
+            app.acquire_token_silent(["s1"], account)
+        with self.assertRaises(ValueError):
+            app.acquire_token_silent_with_error(["s1"], account)
 
     def test_secret_credential_with_flag_raises(self):
         app = ConfidentialClientApplication(