SSHGuard protects hosts from brute-force attacks by monitoring system logs, detecting attacks, and blocking attackers using a firewall. It works by monitoring system log files and journal logs from various sources like cockpit, Common Log Format, macOS log, metalog, multilog, raw log files, syslog, syslog-ng, and systemd journal. SSHGuard's parser is fast, sandboxed, and secure, as it compiles attack signatures into a full lexical analyzer that does not slow down with more signatures, runs as a separate unprivileged process, and is not susceptible to regular expression denial of service attacks.