Update the WordPress to the latest available version (at least 5.7.2). All WordPress versions since 3.7 have also been updated to fix the following security issue.
WordPress Security Team discovered and reported this PHP Object Injection vulnerability in WordPress. This could allow a malicious actor to execute code injection, SQL injection, path traversal, denial of service, and more if a proper POP chain is present. This vulnerability has been fixed in version 5.7.2.