Hallo Guys,
I had the same problem today and i sorted the issue by using the plugin Anti-Malware Security and Brute-Force Firewall
It confirmed the infected file was the functions.php file in the current theme of the website. Some malicious code had been planted there. The plugin removes it and you’re good to go.
Kindly try it and let others know if worked or not for you.
All the best.
M.A
