diff --git a/changelog.d/4284.bugfix b/changelog.d/4284.bugfix
new file mode 100644
index 000000000..4a9478fa2
--- /dev/null
+++ b/changelog.d/4284.bugfix
@@ -0,0 +1 @@
+Add 'sandbox' to CSP for media reprository
diff --git a/synapse/rest/media/v1/download_resource.py b/synapse/rest/media/v1/download_resource.py
index f911b120b..bdc5daecc 100644
--- a/synapse/rest/media/v1/download_resource.py
+++ b/synapse/rest/media/v1/download_resource.py
@@ -48,7 +48,8 @@ class DownloadResource(Resource):
         set_cors_headers(request)
         request.setHeader(
             b"Content-Security-Policy",
-            b"default-src 'none';"
+            b"sandbox;"
+            b" default-src 'none';"
             b" script-src 'none';"
             b" plugin-types application/pdf;"
             b" style-src 'unsafe-inline';"