This is no different from how it is already, though. If a user posts a CSAM link, your server immediately visits that link to scrape it, and you suddenly have that content on your server.
Of course, if you have a blocklist of forbidden sites, then your server will know not to scrape that site. And the same thing applies if we allow servers to send pre-scraped metadata. If the incoming message is for a blocked URL, just drop the message and don’t save any part of it.
The only new threat here is for a malicious server operator to modify outgoing messages to include CSAM metadata on a harmless link. However, that possibility already exists with any other photo posted on the fediverse.