IT guy says I have malware but can't find any
I'm working on a project for my graduate program and they've contracted a web designer from outside the university. One of my jobs is updating the website, but every time I try to log into the website editor (Yootheme) in my browser, no matter what wifi I'm on, I get a 403 message--and I've tried it in more than one browser. The web designer says my computer (2015 MacBook Air running Monterey 12.6.7) is attempting to do a "direct file inclusion" attack and being blocked by his firewall. He says this means I have some kind of malware, especially since I wasn't actually trying to upload anything, just open the editor software. I've scanned the computer with Malwarebytes and Avira, but it didn't turn up anything except something it flagged as a "potentially unwanted program" and has quarantined. I'm still getting the 403 message. The web designer said this isn't surprising, that it's probably just undetectable by anti-virus, that Macs are quite vulnerable and people think they're safe but they're not, etc. etc., and urged me to take the computer to a specialist to do a deep cleaning. Do you guys think he could be right, and if so, is there anything I could do on my own to avoid having to bring my MacBook into the shop?
MacBook Air 13″, macOS 12.6