Synapse has URL deny list bypass via oEmbed and image URLs when generating previews

Impact

A discovered oEmbed or image URL can bypass the url_preview_url_blacklist setting potentially allowing server side request forgery or bypassing network policies. Impact is limited to IP addresses allowed by the url_preview_ip_range_blacklist setting (by default this only allows public IPs) and by the limited information returned to the client:

• For discovered oEmbed URLs, any non-JSON response or a JSON response which includes non-oEmbed information is discarded.
• For discovered image URLs, any non-image response is discarded.

Systems which have URL preview disabled (via the url_preview_enabled setting) or have not configured a url_preview_url_blacklist are not affected.

Because of the uncommon configuration required, the limited information a malicious user, and the amount of guesses/time the attack would need; the severity is rated as low.

Patches

The issue is fixed by #15601.

Workarounds

The default configuration of the url_preview_ip_range_blacklist should protect against requests being made to internal infrastructure, URL previews of public URLs is expected.

Alternately URL previews could be disabled using the url_preview_enabled setting.