Mitigate media repo XSSs on IE11. (#10468)

IE11 doesn't support Content-Security-Policy but it has support for
a non-standard X-Content-Security-Policy header, which only supports the
sandbox directive. This prevents script execution, so it at least offers
some protection against media repo-based attacks.

Signed-off-by: Denis Kasak <dkasak@termina.org.uk>

changelog.d/10468.misc

@@ -0,0 +1 @@
+Mitigate media repo XSS attacks on IE11 via the non-standard X-Content-Security-Policy header.

synapse/rest/media/v1/download_resource.py

@@ -49,6 +49,8 @@ class DownloadResource(DirectServeJsonResource):
             b" media-src 'self';"
             b" object-src 'self';",
         )
+        # Limited non-standard form of CSP for IE11
+        request.setHeader(b"X-Content-Security-Policy", b"sandbox;")
         request.setHeader(
             b"Referrer-Policy",
             b"no-referrer",