--- name: hardening-linux-endpoint-with-cis-benchmark description: 'Hardens Linux endpoints using CIS Benchmark recommendations for Ubuntu, RHEL, and CentOS to reduce attack surface, enforce security baselines, and meet compliance requirements. Use when deploying new Linux servers, remediating audit findings, or establishing security baselines for Linux infrastructure. Activates for requests involving Linux hardening, CIS benchmarks for Linux, server security baselines, or Linux configuration compliance. ' domain: cybersecurity subdomain: endpoint-security tags: - endpoint - hardening - linux-security - CIS-benchmark - Ubuntu - RHEL version: 1.0.0 author: mahipal license: Apache-2.0 nist_csf: - PR.PS-01 - PR.PS-02 - DE.CM-01 - PR.IR-01 --- # Hardening Linux Endpoint with CIS Benchmark ## When to Use Use this skill when: - Hardening Linux servers (Ubuntu, RHEL, CentOS, Debian) against CIS benchmarks - Automating Linux security baselines using Ansible, OpenSCAP, or shell scripts - Meeting compliance requirements (PCI DSS, HIPAA, SOC 2) for Linux endpoints - Remediating findings from vulnerability scans or security audits **Do not use** for Windows hardening (use hardening-windows-endpoint-with-cis-benchmark). ## Prerequisites - Root or sudo access on target Linux endpoints - CIS Benchmark PDF for target distribution (from cisecurity.org) - OpenSCAP or CIS-CAT for automated assessment - Ansible for enterprise-scale remediation (optional) ## Workflow ### Step 1: Filesystem Configuration (Section 1) ```bash # 1.1.1 Disable unused filesystems cat >> /etc/modprobe.d/CIS.conf << 'EOF' install cramfs /bin/true install freevxfs /bin/true install jffs2 /bin/true install hfs /bin/true install hfsplus /bin/true install squashfs /bin/true install udf /bin/true EOF # 1.1.2 Ensure /tmp is a separate partition with nodev,nosuid,noexec # /etc/fstab entry: # tmpfs /tmp tmpfs defaults,rw,nosuid,nodev,noexec,relatime 0 0 systemctl unmask tmp.mount systemctl enable tmp.mount # 1.1.8 Ensure nodev option on /dev/shm mount -o remount,nodev,nosuid,noexec /dev/shm echo "tmpfs /dev/shm tmpfs defaults,nodev,nosuid,noexec 0 0" >> /etc/fstab # 1.4 Secure boot settings chown root:root /boot/grub/grub.cfg chmod 600 /boot/grub/grub.cfg # Set GRUB password grub-mkpasswd-pbkdf2 # Generate hash, add to /etc/grub.d/40_custom ``` ### Step 2: Services and Network (Sections 2-3) ```bash # 2.1 Disable unnecessary services systemctl disable --now avahi-daemon systemctl disable --now cups systemctl disable --now rpcbind systemctl disable --now xinetd # 2.2 Ensure NTP is configured apt install chrony -y # or systemd-timesyncd systemctl enable --now chrony # 3.1 Network parameters (host only, not router) cat >> /etc/sysctl.d/99-cis.conf << 'EOF' net.ipv4.ip_forward = 0 net.ipv4.conf.all.send_redirects = 0 net.ipv4.conf.default.send_redirects = 0 net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.default.accept_redirects = 0 net.ipv4.conf.all.secure_redirects = 0 net.ipv4.conf.default.secure_redirects = 0 net.ipv4.conf.all.log_martians = 1 net.ipv4.conf.default.log_martians = 1 net.ipv4.icmp_echo_ignore_broadcasts = 1 net.ipv4.icmp_ignore_bogus_error_responses = 1 net.ipv4.conf.all.rp_filter = 1 net.ipv4.conf.default.rp_filter = 1 net.ipv4.tcp_syncookies = 1 net.ipv6.conf.all.accept_ra = 0 net.ipv6.conf.default.accept_ra = 0 EOF sysctl --system # 3.4 Configure firewall (UFW or firewalld) ufw enable ufw default deny incoming ufw default allow outgoing ufw allow ssh ``` ### Step 3: Access Control (Sections 4-5) ```bash # 5.2 SSH Server Configuration (/etc/ssh/sshd_config) sed -i 's/#Protocol 2/Protocol 2/' /etc/ssh/sshd_config cat >> /etc/ssh/sshd_config << 'EOF' LogLevel VERBOSE MaxAuthTries 4 PermitRootLogin no PermitEmptyPasswords no PasswordAuthentication no X11Forwarding no MaxStartups 10:30:60 LoginGraceTime 60 AllowTcpForwarding no ClientAliveInterval 300 ClientAliveCountMax 3 EOF systemctl restart sshd # 5.3 Password policy (PAM) # /etc/security/pwquality.conf minlen = 14 dcredit = -1 ucredit = -1 ocredit = -1 lcredit = -1 # 5.4 User account settings # /etc/login.defs PASS_MAX_DAYS 365 PASS_MIN_DAYS 1 PASS_WARN_AGE 7 # Lock inactive accounts useradd -D -f 30 ``` ### Step 4: Audit and Logging (Section 4) ```bash # Install and configure auditd apt install auditd audispd-plugins -y systemctl enable --now auditd # /etc/audit/rules.d/cis.rules cat > /etc/audit/rules.d/cis.rules << 'EOF' -w /etc/sudoers -p wa -k scope -w /etc/sudoers.d/ -p wa -k scope -w /var/log/sudo.log -p wa -k actions -a always,exit -F arch=b64 -S adjtimex -S settimeofday -k time-change -a always,exit -F arch=b64 -S sethostname -S setdomainname -k system-locale -w /etc/group -p wa -k identity -w /etc/passwd -p wa -k identity -w /etc/shadow -p wa -k identity -w /var/log/faillog -p wa -k logins -w /var/log/lastlog -p wa -k logins -a always,exit -F arch=b64 -S chmod -S fchmod -S fchmodat -k perm_mod -a always,exit -F arch=b64 -S unlink -S rmdir -S rename -k delete -w /sbin/insmod -p x -k modules -w /sbin/modprobe -p x -k modules -e 2 EOF augenrules --load # Configure rsyslog for remote logging echo "*.* @@syslog-server.corp.com:514" >> /etc/rsyslog.d/50-remote.conf systemctl restart rsyslog ``` ### Step 5: Assess with OpenSCAP ```bash # Install OpenSCAP apt install openscap-scanner scap-security-guide -y # Run CIS benchmark assessment oscap xccdf eval \ --profile xccdf_org.ssgproject.content_profile_cis_level1_server \ --results /tmp/cis_results.xml \ --report /tmp/cis_report.html \ /usr/share/xml/scap/ssg/content/ssg-ubuntu2204-ds.xml # View HTML report in browser for detailed results ``` ## Key Concepts | Term | Definition | |------|-----------| | **OpenSCAP** | Open-source SCAP (Security Content Automation Protocol) scanner for automated compliance | | **auditd** | Linux audit framework for monitoring system calls and file access | | **PAM** | Pluggable Authentication Modules; configurable authentication framework for Linux | | **sysctl** | Linux kernel parameter configuration for network and system security tuning | | **AIDE** | Advanced Intrusion Detection Environment; file integrity checker for Linux | ## Tools & Systems - **OpenSCAP**: Automated CIS benchmark assessment for Linux - **Ansible Lockdown**: Ansible roles for automated CIS benchmark remediation - **Lynis**: Open-source security auditing tool for Linux/Unix systems - **AIDE**: File integrity monitoring for Linux endpoints - **auditd**: Linux audit framework for system call monitoring ## Common Pitfalls - **Applying server benchmarks to workstations**: CIS provides separate benchmarks for server and workstation profiles. Server benchmarks disable desktop services. - **Breaking SSH access**: Misconfiguring sshd_config (especially PermitRootLogin, PasswordAuthentication) can lock out administrators. Always test SSH configuration changes from a second session. - **Not testing firewall rules**: Enabling UFW without allowing SSH first will disconnect remote sessions permanently. - **Kernel parameter changes without testing**: Some sysctl settings can break application networking. Test in staging first.