How are ads being inserted into iMessage chats and how to stop?

I was texting with a friend this morning, just the two of us in an iMessage chat on our iPhones, and suddenly an ad appeared from a company called Replit in the form of a YouTube link. Neither he nor I inserted it! It shocked and surprised us both. How could such a thing even happen? Anyone know anything about this and how to stop it from recurring? Very disturbing. Thanks.