Linux Server Hardening

The Linux Paradigm

The beauty of Linux is that it is so accessible and freely available that it is easy to get up and running with very little training or knowledge. The web-based support community places all the tips and tutorials you’ll ever need to carry out any Linux set-up task or troubleshoot issues you may experience.

Finding and interpreting the right hardening checklist for your Linux hosts may still be a challenge so this guide gives you a concise checklist to work from, encompassing the highest priority hardening measures for a typical Linux server.

NNT Change Tracker Enterprise has been certified by the CIS to 100% accurately audit all RHEL, CentOS, Ubuntu, SUSE and other Linux, identifying where vulnerable configuration settings are present and explaining in plain English, how to mitigate them.

Account Policies

Enforce password history – 365 days
Maximum Password Age - 42 days
Minimum password length – 8 characters
Password Complexity - Enable
Account Lockout Duration - 30 minutes
Account Lockout Threshold – 5 attempts
Reset Account Lockout Counter - 30 minutes

Edit the /etc/pam.d/common-password to define password policy parameters for your host.

Access Security

Ensure SSH version 2 is in use
Disable remote root logons
Enable AllowGroups to permitted Group names only
Allow access to valid devices only
Restrict the number of concurrent root sessions to 1 or 2 only

Edit sshd.config to define SSHD policy parameters for your host and /etc/hosts.allow and /etc/hosts.deny to control access. Use /etc/securetty to restrict root access to tty1 or tty1 and tty2 only.

Secure Boot Only

Remove options to boot from CD or USB devices and password protect the computer to prevent the BIOS options from being edited.

Password protect the /boot/grub/menu.lst file, then remove the rescue-mode boot entry.

Disable All Unnecessary Processes, Services, and Daemons

Each system is unique so it is important to review which processes and services are unnecessary for your server to run your applications.

Assess your server by running the ps –ax command and see what is running currently.

Similarly, assess the startup status of all processes by running a chkconfig –list command.

Disable any unnecessary services using the sysv-rc-conf service-name off

Restrict Permissions on Sensitive Files and Folders to root Only

Ensure the following sensitive programs are root executable only

/etc/fstab
/etc/passwd
/bin/ping
/usr/bin/who
/usr/bin/w
/usr/bin/locate
/usr/bin/whereis
/sbin/ifconfig
/bin/nano
/usr/bin/vi
/usr/bin/which
/usr/bin/gcc
/usr/bin/make
/usr/bin/apt-get
/usr/bin/aptitude

Ensure the following folders are root access only

/etc
/usr/etc
/bin
/usr/bin
/sbin
/usr/sbin
/tmp
/var/tmp

Disable SUID and SGID Binaries

Identify SUID and SGID files on the system: find / \( -perm -4000 -o -perm -2000 \) –print.

Render these files safe by removing the SUID or SGID bits using chmod –s filename

You should also restrict access to all compilers on the system by adding them to a new ‘compilers’ group.

chgrp compilers *cc*
chgrp compilers *++*
chgrp compilers ld
chgrp compilers as

Once added to the group, restrict permissions using a chmod 750 compiler

Implement Regular/Real-Time FIM on Sensitive Folders and Files

File integrity should be monitored for all files and folders to ensure permissions and files do not change without approval.

Standard guidance is to implement AIDE on Linux but a commercial Linux FIM solution like NNT Change Tracker can provide real-time FIM with Intelligent Change Control to reduce unwanted 'change noise'

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Configure Auditing on the Linux Server

Ensure key security events are being audited and are forwarded to your Syslog or SIEM server. Edit the Syslog.conf file accordingly.

General Hardening of Kernel Variables

Edit the /etc/sysctl.conf file to set all kernel variables to secure settings in order to prevent spoofing, syn flood, and DOS attacks.

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