CVE-2022-41952

Synapse before 1.52.0 with URL preview functionality enabled will attempt
to generate URL previews for media stream URLs without properly limiting
connection time. Connections will only be terminated after
max_spider_size (default: 10M) bytes have been downloaded, which can in
some cases lead to long-lived connections towards the streaming media
server (for instance, Icecast). This can cause excessive traffic and
connections toward such servers if their stream URL is, for example, posted
to a large room with many Synapse instances with URL preview enabled.
Version 1.52.0 implements a timeout mechanism which will terminate URL
preview connections after 30 seconds. Since generating URL previews for
media streams is not supported and always fails, 1.53.0 additionally
implements an allow list for content types for which Synapse will even
attempt to generate a URL preview. Upgrade to 1.53.0 to fully resolve the
issue. As a workaround, turn off URL preview functionality by setting
url_preview_enabled: false in the Synapse configuration file.