Spam appears on websites

Hello there. I wanted to post in here to see if this is WordPress and plugin related issue or if it is entirely something else. I maintain two websites and the spam has been showing up on the top part of the header on both websites. I’ve kept WordPress and all plugins updated but couldn’t find where the offending source comes from.

For example, this spam with links shows up like this: “buy real phentermine diet pills” or “ou acheter du viagra a lyon sans ordonnance”. Both would lead to outside links not found on the websites.

Is there a way I can remove these and prevent them from showing up again in the future? Thanks.