From d2e3d5b9db346c88b31ff5eef2793c5cf82f698e Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Thu, 1 Aug 2019 13:23:00 +0100
Subject: [PATCH 1/2] Handle incorrectly encoded query params correctly

---
 synapse/http/servlet.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/synapse/http/servlet.py b/synapse/http/servlet.py
index f0ca7d9ab..fd07bf7b8 100644
--- a/synapse/http/servlet.py
+++ b/synapse/http/servlet.py
@@ -166,7 +166,12 @@ def parse_string_from_args(
         value = args[name][0]
 
         if encoding:
-            value = value.decode(encoding)
+            try:
+                value = value.decode(encoding)
+            except ValueError:
+                raise SynapseError(
+                    400, "Query parameter %r must be %s" % (name, encoding)
+                )
 
         if allowed_values is not None and value not in allowed_values:
             message = "Query parameter %r must be one of [%s]" % (

From da378af44517d96d461431c21400e44b00edb285 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Thu, 1 Aug 2019 13:24:00 +0100
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/5808.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/5808.misc

diff --git a/changelog.d/5808.misc b/changelog.d/5808.misc
new file mode 100644
index 000000000..cac3fd34d
--- /dev/null
+++ b/changelog.d/5808.misc
@@ -0,0 +1 @@
+Handle incorrectly encoded query params correctly by returning a 400.