diff --git a/changelog.d/7979.misc b/changelog.d/7979.misc
new file mode 100644
index 000000000..4304bbdd2
--- /dev/null
+++ b/changelog.d/7979.misc
@@ -0,0 +1 @@
+Switch to the JSON implementation from the standard library and bump the minimum version of the canonicaljson library to 1.2.0.
diff --git a/synapse/http/client.py b/synapse/http/client.py
index 6bc51202c..155b7460d 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -395,7 +395,9 @@ class SimpleHttpClient(object):
         if 200 <= response.code < 300:
             return json.loads(body.decode("utf-8"))
         else:
-            raise HttpResponseException(response.code, response.phrase, body)
+            raise HttpResponseException(
+                response.code, response.phrase.decode("ascii", errors="replace"), body
+            )
 
     @defer.inlineCallbacks
     def post_json_get_json(self, uri, post_json, headers=None):
@@ -436,7 +438,9 @@ class SimpleHttpClient(object):
         if 200 <= response.code < 300:
             return json.loads(body.decode("utf-8"))
         else:
-            raise HttpResponseException(response.code, response.phrase, body)
+            raise HttpResponseException(
+                response.code, response.phrase.decode("ascii", errors="replace"), body
+            )
 
     @defer.inlineCallbacks
     def get_json(self, uri, args={}, headers=None):
@@ -509,7 +513,9 @@ class SimpleHttpClient(object):
         if 200 <= response.code < 300:
             return json.loads(body.decode("utf-8"))
         else:
-            raise HttpResponseException(response.code, response.phrase, body)
+            raise HttpResponseException(
+                response.code, response.phrase.decode("ascii", errors="replace"), body
+            )
 
     @defer.inlineCallbacks
     def get_raw(self, uri, args={}, headers=None):
@@ -544,7 +550,9 @@ class SimpleHttpClient(object):
         if 200 <= response.code < 300:
             return body
         else:
-            raise HttpResponseException(response.code, response.phrase, body)
+            raise HttpResponseException(
+                response.code, response.phrase.decode("ascii", errors="replace"), body
+            )
 
     # XXX: FIXME: This is horribly copy-pasted from matrixfederationclient.
     # The two should be factored out.
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 148eeb19d..ea026ed9f 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -447,6 +447,7 @@ class MatrixFederationHttpClient(object):
                     ).inc()
 
                     set_tag(tags.HTTP_STATUS_CODE, response.code)
+                    response_phrase = response.phrase.decode("ascii", errors="replace")
 
                     if 200 <= response.code < 300:
                         logger.debug(
@@ -454,7 +455,7 @@ class MatrixFederationHttpClient(object):
                             request.txn_id,
                             request.destination,
                             response.code,
-                            response.phrase.decode("ascii", errors="replace"),
+                            response_phrase,
                         )
                         pass
                     else:
@@ -463,7 +464,7 @@ class MatrixFederationHttpClient(object):
                             request.txn_id,
                             request.destination,
                             response.code,
-                            response.phrase.decode("ascii", errors="replace"),
+                            response_phrase,
                         )
                         # :'(
                         # Update transactions table?
@@ -487,7 +488,7 @@ class MatrixFederationHttpClient(object):
                             )
                             body = None
 
-                        e = HttpResponseException(response.code, response.phrase, body)
+                        e = HttpResponseException(response.code, response_phrase, body)
 
                         # Retry if the error is a 429 (Too Many Requests),
                         # otherwise just raise a standard HttpResponseException