Add Linux security hardening guide

Author: bad-antics

SECURITY.md

@@ -1,5 +1,75 @@
-# Security Policy
+# Linux Security Hardening Guide
 
-## Reporting a Vulnerability
+## Overview
+Comprehensive Linux security hardening procedures.
 
-Report security vulnerabilities via GitHub Security Advisories.
+## Kernel Hardening
+
+### Sysctl Settings
+- ASLR configuration
+- Core dump restrictions
+- Exec shield
+- Panic settings
+
+### Module Security
+- Module loading restrictions
+- Signing requirements
+- Blacklisting
+
+### Syscall Filtering
+- seccomp profiles
+- Audit subsystem
+- SELinux/AppArmor
+
+## Filesystem Security
+
+### Permissions
+- umask settings
+- Sticky bits
+- SUID/SGID auditing
+- ACL implementation
+
+### Mount Options
+- noexec
+- nosuid
+- nodev
+- read-only mounts
+
+### Encryption
+- LUKS configuration
+- dm-crypt
+- ecryptfs
+- fscrypt
+
+## Network Security
+
+### Firewall
+- iptables/nftables
+- Default deny
+- Logging
+- Rate limiting
+
+### Service Hardening
+- SSH configuration
+- Network services
+- Listening ports
+
+## Authentication
+
+### PAM Configuration
+- Password policies
+- Account lockout
+- Two-factor auth
+
+### SSH Hardening
+- Key-based auth
+- Allowed users
+- Protocol settings
+
+## Monitoring
+- auditd rules
+- Log management
+- AIDE/Tripwire
+
+## Legal Notice
+For authorized system administration.