2 files changed, 2 insertions, 1 deletions
diff --git a/changelog.d/3754.bugfix b/changelog.d/3754.bugfix
new file mode 100644
index 0000000000..6e3ec80194
--- /dev/null
+++ b/changelog.d/3754.bugfix
@@ -0,0 +1 @@
+Fix 'federation_domain_whitelist' such that an empty list correctly blocks all outbound federation traffic
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 44b61e70a4..b34bb8e31a 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -133,7 +133,7 @@ class MatrixFederationHttpClient(object):
                 failures, connection failures, SSL failures.)
         """
         if (
-            self.hs.config.federation_domain_whitelist and
+            self.hs.config.federation_domain_whitelist is not None and
             destination not in self.hs.config.federation_domain_whitelist
         ):
             raise FederationDeniedError(destination)