Index: /trunk/src/VBox/Main/src-client/GuestCtrlImpl.cpp
===================================================================
--- /trunk/src/VBox/Main/src-client/GuestCtrlImpl.cpp	(revision 39668)
+++ /trunk/src/VBox/Main/src-client/GuestCtrlImpl.cpp	(revision 39669)
@@ -2246,7 +2246,9 @@
     CheckComArgStrNotEmptyOrNull(aSource);
     CheckComArgStrNotEmptyOrNull(aDest);
-    CheckComArgStrNotEmptyOrNull(aUsername);
-    CheckComArgStrNotEmptyOrNull(aPassword);
     CheckComArgOutPointerValid(aProgress);
+
+    /* Do not allow anonymous executions (with system rights). */
+    if (RT_UNLIKELY((aUsername) == NULL || *(aUsername) == '\0'))
+        return setError(E_INVALIDARG, tr("No user name specified"));
 
     AutoCaller autoCaller(this);
@@ -2321,7 +2323,9 @@
     CheckComArgStrNotEmptyOrNull(aSource);
     CheckComArgStrNotEmptyOrNull(aDest);
-    CheckComArgStrNotEmptyOrNull(aUsername);
-    CheckComArgStrNotEmptyOrNull(aPassword);
     CheckComArgOutPointerValid(aProgress);
+
+    /* Do not allow anonymous executions (with system rights). */
+    if (RT_UNLIKELY((aUsername) == NULL || *(aUsername) == '\0'))
+        return setError(E_INVALIDARG, tr("No user name specified"));
 
     AutoCaller autoCaller(this);