Linux Security Hardening

Use When

• Use when hardening a Debian/Ubuntu server — user/group/sudo hardening, file permission audits, PAM password policy + MFA, AppArmor mandatory access control, auditd system call logging, kernel sysctl hardening, file integrity monitoring (AIDE), rootkit detection (rkhunter/chkrootkit), unattended security patching, GRUB + UEFI + LUKS boot security, and CIS benchmark compliance.
• The task needs reusable judgment, domain constraints, or a proven workflow rather than ad hoc advice.

Do Not Use When

• The task is unrelated to linux-security-hardening or would be better handled by a more specific companion skill.
• The request only needs a trivial answer and none of this skill's constraints or references materially help.

Required Inputs

• Gather relevant project context, constraints, and the concrete problem to solve; load references only as needed.
• Confirm the desired deliverable: design, code, review, migration plan, audit, or documentation.

Workflow

• Read this SKILL.md first, then load only the referenced deep-dive files that are necessary for the task.
• Apply the ordered guidance, checklists, and decision rules in this skill instead of cherry-picking isolated snippets.
• Produce the deliverable with assumptions, risks, and follow-up work made explicit when they matter.

Quality Standards

• Keep outputs execution-oriented, concise, and aligned with the repository's baseline engineering standards.
• Preserve compatibility with existing project conventions unless the skill explicitly requires a stronger standard.
• Prefer deterministic, reviewable steps over vague advice or tool-specific magic.

Anti-Patterns

• Treating examples as copy-paste truth without checking fit, constraints, or failure modes.
• Loading every reference file by default instead of using progressive disclosure.

Outputs

• A concrete result that fits the task: implementation guidance, review findings, architecture decisions, templates, or generated artifacts.
• Clear assumptions, tradeoffs, or unresolved gaps when the task cannot be completed from available context alone.
• References used, companion skills, or follow-up actions when they materially improve execution.

Evidence Produced

Category	Artifact	Format	Example
Security	Server hardening checklist	Markdown doc covering user/sudo, file permissions, PAM/MFA, and auditd findings	docs/server/hardening-checklist.md

References

• Use the references/ directory for deep detail after reading the core workflow below.

Defensive hardening for Debian 12 / Ubuntu 24.04 servers running multi-tenant SaaS. Covers the OS layer — users, permissions, PAM, mandatory access control, kernel, integrity, patching, and compliance.

Core principle: A fresh Debian install is not production-ready. Every server that accepts traffic needs hardening. Do it once with automation, audit it forever.

Scope: Linux OS security on your own servers. For network-layer defence use network-security. For secrets/CI hardening use cicd-devsecops. For app-code vulnerabilities use web-app-security-audit.

Cross-references: network-security, cicd-devsecops, cicd-jenkins-debian, database-reliability, web-app-security-audit

See references/ for: users-groups-sudo.md, file-permissions-acls.md, pam-authentication.md, selinux-apparmor.md, auditd-logging.md, kernel-sysctl-hardening.md, file-integrity.md, rootkit-detection.md, patch-management.md, boot-security.md, cis-benchmark-checklist.md

When to Use

• Before putting a fresh VPS into production
• During quarterly security audit cadence
• After a CVE affecting a library on your server
• When onboarding a new server into the fleet
• When a compliance requirement (SOC 2, ISO 27001, PCI-DSS) demands baseline hardening
• After an incident — to verify baseline still holds
• When migrating from one cloud/VPS provider to another

Threat Model + Attack Surface

Attack surface on a typical Debian server:

Layer	Attack vector	Primary defence
Boot	Single-user mode, evil maid, kernel tampering	GRUB password, Secure Boot, LUKS
Kernel	Privilege escalation via syscall, unsigned module	sysctl, lockdown mode, MAC, module signing
Filesystem	Path traversal, setuid abuse, world-writable	perms, ACLs, mount options (nosuid,noexec)
Users	Brute force, privilege abuse, shared accounts	PAM, sudo least-priv, MFA, password policy
Services	Unpatched CVE, default creds, exposed port	unattended-upgrades, service minimization
Process	Arbitrary code execution in an app	AppArmor, systemd sandboxing, capabilities
Monitoring	Attacker erases logs	auditd off-host, FIM, log integrity

Assume breach. Layer defences so a single failure does not equal game over.

The 10 Hardening Domains

#	Domain	Reference
1	Users, groups, sudo	references/users-groups-sudo.md
2	File permissions + ACLs	references/file-permissions-acls.md
3	PAM authentication	references/pam-authentication.md
4	Mandatory Access Control (AppArmor)	references/selinux-apparmor.md
5	auditd + system logging	references/auditd-logging.md
6	Kernel sysctl hardening	references/kernel-sysctl-hardening.md
7	File integrity monitoring	references/file-integrity.md
8	Rootkit detection	references/rootkit-detection.md
9	Patch management	references/patch-management.md
10	Boot security (GRUB, UEFI, LUKS)	references/boot-security.md

Compliance: references/cis-benchmark-checklist.md (~60 items aligned to CIS Debian 12 benchmark).

Baseline Hardening Checklist

Run this against every fresh Debian 12 / Ubuntu 24.04 server before accepting production traffic.

1. System baseline

apt update && apt full-upgrade -y
apt install -y \
  ufw nftables fail2ban \
  auditd audispd-plugins \
  unattended-upgrades needrestart apt-listchanges \
  libpam-pwquality libpam-google-authenticator \
  rkhunter chkrootkit aide aide-common \
  apparmor apparmor-utils apparmor-profiles \
  lynis debsecan

Enable unattended security updates with auto-reboot at a quiet hour (see references/patch-management.md).

2. Disable unused services and kernel modules

# Disable services
for svc in avahi-daemon cups rpcbind nfs-server bluetooth; do
  systemctl disable --now "$svc" 2>/dev/null || true
done

# Blacklist rare filesystem / network modules
cat > /etc/modprobe.d/blacklist-hardening.conf <<EOF
install cramfs /bin/true
install freevxfs /bin/true
install jffs2 /bin/true
install hfs /bin/true
install hfsplus /bin/true
install udf /bin/true
install dccp /bin/true
install sctp /bin/true
install rds /bin/true
install tipc /bin/true
EOF
update-initramfs -u

3. User and sudo hardening

passwd -l root                              # disable root password login
# Add each admin to sudo group; no shared accounts.
usermod -aG sudo deploy
# Edit sudoers to require pty and log IO
visudo  # add: Defaults use_pty, log_input, log_output

Full detail: references/users-groups-sudo.md

4. File permissions

chmod 640 /etc/shadow /etc/gshadow
chmod 644 /etc/passwd /etc/group
chmod 700 /root
# Find unexpected setuid binaries
find / -xdev -type f -perm -4000 2>/dev/null
# Find world-writable
find / -xdev -type f -perm -0002 2>/dev/null

Set umask 027 globally via /etc/login.defs. Mount /tmp, /var/tmp, /dev/shm with nodev,nosuid,noexec. Detail: references/file-permissions-acls.md.

5. PAM password policy + lockout

Edit /etc/security/pwquality.conf:

minlen = 12
dcredit = -1
ucredit = -1
lcredit = -1
ocredit = -1
retry = 3
enforce_for_root

Enable faillock in /etc/pam.d/common-auth and /etc/security/faillock.conf (deny=5, unlock_time=900). Full stack: references/pam-authentication.md.

6. AppArmor enforce

systemctl enable --now apparmor
aa-status                                   # check active profiles
# Put any complain-mode profiles into enforce
for p in /etc/apparmor.d/*; do
  aa-enforce "$p" 2>/dev/null
done

Custom profiles for your own apps: see references/selinux-apparmor.md.

7. auditd rules

systemctl enable --now auditd
# Load CIS-style rules (edit to /etc/audit/rules.d/audit.rules)
augenrules --load
auditctl -l                                 # verify loaded

Reference ruleset covering identity changes, sudo, SSH config, kernel module loads, and mount operations: references/auditd-logging.md.

8. Kernel hardening

Write /etc/sysctl.d/99-hardening.conf with the baseline from references/kernel-sysctl-hardening.md, then:

sysctl --system

Key values:

kernel.randomize_va_space = 2
kernel.kptr_restrict = 2
kernel.dmesg_restrict = 1
kernel.yama.ptrace_scope = 2
kernel.unprivileged_bpf_disabled = 1
net.core.bpf_jit_harden = 2
fs.suid_dumpable = 0
fs.protected_hardlinks = 1
fs.protected_symlinks = 1

9. File integrity baseline

aideinit                                    # build baseline DB
mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
# Schedule daily checks via systemd timer

Ship output off-host. Detail: references/file-integrity.md.

10. Rootkit baseline

rkhunter --propupd                          # build baseline
rkhunter --update                           # update signature DB
chkrootkit                                  # initial clean scan

Schedule daily + alert on INFECTED. Detail: references/rootkit-detection.md.

11. Patch automation

dpkg-reconfigure -plow unattended-upgrades
systemctl status unattended-upgrades

Configure security-only updates + auto-reboot at 02:00. Detail: references/patch-management.md.

12. Verify with Lynis

lynis audit system --profile /etc/lynis/default.prf
# Target: hardening index >= 80 at Level 1

Compliance Mapping (Brief)

Control family	CIS	ISO 27001	PCI-DSS	Primary reference
Access control	5.x	A.9	7, 8	users-groups-sudo.md, pam-authentication.md
Cryptography	-	A.10	3, 4	boot-security.md, network-security/tls-pki.md
Operations security	4.x	A.12	6	patch-management.md, file-integrity.md
Communications security	3.x	A.13	1	network-security/*
Logging / monitoring	4.x	A.12.4	10	auditd-logging.md, rootkit-detection.md
System acquisition / dev	-	A.14	6	cicd-devsecops

Full CIS Debian 12 checklist: references/cis-benchmark-checklist.md.

Audit Runbook

Weekly or quarterly, run this sequence on each production host:

1. lynis audit system — quick hardening score
2. debsecan --only-fixed — list unpatched CVEs
3. rkhunter --check --skip-keypress — rootkit scan
4. aide --check — file integrity delta
5. aureport --summary — audit log summary
6. fail2ban-client status — see active jails and bans
7. last -a | head -20 — recent logins
8. sudo grep -i 'COMMAND' /var/log/auth.log | tail -50 — recent privileged commands
9. systemctl --failed — failed services
10. apt list --upgradable — pending updates

Log findings and track remediation in a ticket.

Anti-Patterns

Do not:

• Rely on DAC (rwx) alone — add AppArmor for any internet-facing daemon.
• Disable AppArmor "to make the app work" — write a profile instead.
• Run services as root when a system user and DynamicUser=true in systemd would suffice.
• Skip patching because "uptime is important" — schedule maintenance windows with live kernel patching where needed.
• Edit /etc/sudoers directly without visudo — syntax errors lock you out.
• Use NOPASSWD: ALL in sudoers — the whole point of sudo is the audit trail.
• Give every admin the same SSH key — per-person keys enable revocation and accountability.
• Treat AIDE / rkhunter as checkboxes — they must alert off-host, or they are paperweights.
• Disable auditd because "logs fill the disk" — rotate, ship, don't disable.
• Leave AppArmor profiles in complain mode forever — tune and enforce.
• Leave umask 0022 in /etc/login.defs — use 027 so others can't read new files by default.
• Forget /boot — it often ends up 755 and world-readable.
• Leave kernel.dmesg_restrict=0 — kernel messages leak addresses useful for exploitation.
• Skip the post-upgrade AIDE re-baseline — every legit update generates false positives until rebased.
• Run Lynis once, fix nothing, forget. Track hardening score as a metric over time.

References Index

Domain references:

• references/users-groups-sudo.md — account model, sudo hardening, password aging
• references/file-permissions-acls.md — DAC, setuid audit, ACLs, mount options
• references/pam-authentication.md — PAM stack, pwquality, faillock, MFA
• references/selinux-apparmor.md — MAC, AppArmor profiles, systemd sandboxing
• references/auditd-logging.md — rules, ausearch, off-host shipping
• references/kernel-sysctl-hardening.md — sysctls, modprobe blacklist, lockdown
• references/file-integrity.md — AIDE baseline, alerting, re-baselining
• references/rootkit-detection.md — rkhunter, chkrootkit, incident runbook
• references/patch-management.md — unattended-upgrades, debsecan, CVE SLAs
• references/boot-security.md — GRUB password, Secure Boot, LUKS, TPM

Compliance:

• references/cis-benchmark-checklist.md — ~60-item CIS-aligned audit checklist

Related skills:

• network-security — firewall, WAF, TLS, VPN, IDS (network layer)
• cicd-devsecops — secrets, dependency scanning, pipeline hardening
• cicd-jenkins-debian — Debian server provisioning for CI/CD
• database-reliability — DB-specific hardening, backup, failover
• web-app-security-audit — application-layer vulnerabilities