diff --git a/mitogen/core.py b/mitogen/core.py
index b0fb8603..4c1e049b 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -1292,12 +1292,21 @@ class Router(object):
             return
 
         # Perform source verification.
-        if stream is not None:
-            expected_stream = self._stream_by_id.get(msg.auth_id,
-                self._stream_by_id.get(mitogen.parent_id))
-            if stream != expected_stream:
-                LOG.error('%r: bad source: got auth ID %r from %r, should be from %r',
-                          self, msg, stream, expected_stream)
+        if stream:
+            parent = self._stream_by_id.get(mitogen.parent_id)
+            expect = self._stream_by_id.get(msg.auth_id, parent)
+            if stream != expect:
+                LOG.error('%r: bad auth_id: got %r via %r, not %r: %r',
+                          self, msg.auth_id, stream, expect, msg)
+                return
+
+            if msg.src_id != msg.auth_id:
+                expect = self._stream_by_id.get(msg.src_id, parent)
+                if stream != expect:
+                    LOG.error('%r: bad src_id: got %r via %r, not %r: %r',
+                              self, msg.src_id, stream, expect, msg)
+                    return
+
             if stream.auth_id is not None:
                 msg.auth_id = stream.auth_id
 
diff --git a/tests/router_test.py b/tests/router_test.py
index 053037fb..c3d17b8b 100644
--- a/tests/router_test.py
+++ b/tests/router_test.py
@@ -11,6 +11,10 @@ import mitogen.master
 import mitogen.utils
 
 
+def ping():
+    return True
+
+
 @mitogen.core.takes_router
 def return_router_max_message_size(router):
     return router.max_message_size
@@ -21,6 +25,70 @@ def send_n_sized_reply(sender, n):
     return 123
 
 
+class SourceVerifyTest(testlib.RouterMixin, unittest2.TestCase):
+    def setUp(self):
+        super(SourceVerifyTest, self).setUp()
+        # Create some children, ping them, and store what their messages look
+        # like so we can mess with them later.
+        self.child1 = self.router.fork()
+        self.child1_msg = self.child1.call_async(ping).get()
+        self.child1_stream = self.router._stream_by_id[self.child1.context_id]
+
+        self.child2 = self.router.fork()
+        self.child2_msg = self.child2.call_async(ping).get()
+        self.child2_stream = self.router._stream_by_id[self.child2.context_id]
+
+    def test_bad_auth_id(self):
+        # Deliver a message locally from child2, but using child1's stream.
+        log = testlib.LogCapturer()
+        log.start()
+
+        # Used to ensure the message was dropped rather than routed after the
+        # error is logged.
+        recv = mitogen.core.Receiver(self.router)
+        self.child2_msg.handle = recv.handle
+
+        self.broker.defer(self.router._async_route,
+                          self.child2_msg,
+                          stream=self.child1_stream)
+
+        # Wait for IO loop to finish everything above.
+        self.sync_with_broker()
+
+        # Ensure message wasn't forwarded.
+        self.assertTrue(recv.empty())
+
+        # Ensure error was logged.
+        expect = 'bad auth_id: got %d via' % (self.child2_msg.auth_id,)
+        self.assertTrue(expect in log.stop())
+
+    def test_bad_src_id(self):
+        # Deliver a message locally from child2 with the correct auth_id, but
+        # the wrong src_id.
+        log = testlib.LogCapturer()
+        log.start()
+
+        # Used to ensure the message was dropped rather than routed after the
+        # error is logged.
+        recv = mitogen.core.Receiver(self.router)
+        self.child2_msg.handle = recv.handle
+        self.child2_msg.src_id = self.child1.context_id
+
+        self.broker.defer(self.router._async_route,
+                          self.child2_msg,
+                          self.child2_stream)
+
+        # Wait for IO loop to finish everything above.
+        self.sync_with_broker()
+
+        # Ensure message wasn't forwarded.
+        self.assertTrue(recv.empty())
+
+        # Ensure error was lgoged.
+        expect = 'bad src_id: got %d via' % (self.child1_msg.src_id,)
+        self.assertTrue(expect in log.stop())
+
+
 class CrashTest(testlib.BrokerMixin, unittest2.TestCase):
     # This is testing both Broker's ability to crash nicely, and Router's
     # ability to respond to the crash event.
diff --git a/tests/testlib.py b/tests/testlib.py
index 5d959b8d..cac5b1e9 100644
--- a/tests/testlib.py
+++ b/tests/testlib.py
@@ -11,6 +11,7 @@ import urlparse
 
 import unittest2
 
+import mitogen.core
 import mitogen.master
 import mitogen.utils
 
@@ -115,6 +116,20 @@ def wait_for_port(
                                  % (host, port))
 
 
+def sync_with_broker(broker, timeout=10.0):
+    """
+    Insert a synchronization barrier between the calling thread and the Broker
+    thread, ensuring it has completed at least one full IO loop before
+    returning.
+
+    Used to block while asynchronous stuff (like defer()) happens on the
+    broker.
+    """
+    sem = mitogen.core.Latch()
+    broker.defer(sem.put, None)
+    sem.get(timeout=10.0)
+
+
 class LogCapturer(object):
     def __init__(self, name=None):
         self.sio = StringIO.StringIO()
@@ -188,6 +203,9 @@ class BrokerMixin(object):
         self.broker.join()
         super(BrokerMixin, self).tearDown()
 
+    def sync_with_broker(self):
+        sync_with_broker(self.broker)
+
 
 class RouterMixin(BrokerMixin):
     router_class = mitogen.master.Router