Linux Server Hardening

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'

Try it on your Linux Systems Now - Click Here

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.