diff --git a/mitogen/core.py b/mitogen/core.py
index 334166c1..65432062 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -638,7 +638,7 @@ class Stream(BasicStream):
 
         msg.data = self._input_buf[self.HEADER_LEN:self.HEADER_LEN+msg_len]
         self._input_buf = self._input_buf[self.HEADER_LEN+msg_len:]
-        self._router._async_route(msg)
+        self._router._async_route(msg, self)
         return True
 
     def on_transmit(self, broker):
@@ -929,8 +929,16 @@ class Router(object):
         except Exception:
             LOG.exception('%r._invoke(%r): %r crashed', self, msg, fn)
 
-    def _async_route(self, msg):
-        IOLOG.debug('%r._async_route(%r)', self, msg)
+    def _async_route(self, msg, stream=None):
+        IOLOG.debug('%r._async_route(%r, %r)', self, msg, stream)
+        # Perform source verification.
+        if stream is not None:
+            expected_stream = self._stream_by_id.get(msg.src_id,
+                self._stream_by_id.get(mitogen.parent_id))
+            if stream != expected_stream:
+                LOG.error('%r: bad source: got %r from %r, should be from %r',
+                          self, msg, stream, expected_stream)
+
         if msg.dst_id == mitogen.context_id:
             return self._invoke(msg)