The vulnerabilities

• Log4Shell: an unauthenticated flaw in the Log4j logging library that let attackers get code executed just by getting it logged. Its attack surface was enormous, hitting companies like Apple, Amazon, Microsoft, and Steam.
• ProxyShell: three Exchange Server vulnerabilities chained together: get in, escalate privileges, then execute code remotely on unpatched servers.
• ProxyLogon: another Exchange chain, starting with a server-side request forgery over port 443, used to steal mailbox content.
• CVE-2021-26084: an OGNL injection in Confluence Server and Data Center that let unauthenticated users run code. Crypto-miners used it on unpatched servers.

Port review

I also reviewed risky open ports on my own machine: FTP (20/21) is outdated and open to password brute-forcing, SMB (445) is exposed to EternalBlue, and Telnet (23) has been replaced by SSH. If a service isn't in use, turn it off.