Linux Platform Hardening Recommendations for VOLTTRON users

Platform Hardening for VOLTTRON

Rev. 0 | 1/29/2015 | Initial Document Development

Rev. 1 | 2/5/2015 | Integrate comments from extended VOLTTRON team.

Introduction

VOLTTRON is an agent-based application development platform for distributed control systems. VOLTTRON itself is built with modern security principles in mind [security-wp] and implements many security features for hosted agents. However, VOLTTRON is built on top of Linux and the underlying Linux platform also needs to be secured in order to declare the resulting control system as "secure." Any system is only as secure as its weakest link. The rest of this note is dedicated to making recommendations for hardening of the underlying Linux platform that VOLTTRON uses. Note that no system can be 100% secure and the cyber security strategy that is recommended in this document is based on risk management.

Linux System Hardening

Here are the non-exhaustive recommendations for Linux hardening from the VOLTTRON team:

• Physical Security: Keep the system in locked cabinets or a locked room. Limit physical access to systems and to the networks to which they are attached. The goal should be to avoid physical access by untrusted personnel. This could be extended to blocking or locking USB ports, removable media drives, etc. Drive encryption could be used to avoid access via alternate-media booting (off USB stick or DVD) if physical access can't be guaranteed. Downside of drive encryption would be needing to enter a passphrase to start system. Alternately, the Trusted Platform Module (TPM) may be used, but the drive might still be accessible to those with physical access. Enable chassis intrusion detection and reporting if supported. If available, use a physical tamper seal along with or in place of an interior switch.

• Low level device Security: Keep firmware of all devices (including BIOS) up-to-date. Password-protect the BIOS. Disable unneeded/unnecessary devices including serial, parallel, USB, Firewire, etc. ports; optical drives; wireless devices, such as Wi-Fi and Bluetooth. Leaving a USB port enabled may be helpful if a breach occurs to allow saving forensic data to an external drive.

• Boot security: Disable automounting of external devices. Restrict the boot device. Disable PXE and other network boot options (unless that is the primary boot method). Disable booting from USB and other removable drives. Secure the boot loader. Require an administrator password to do anything but start the default kernel. Do not allow editing of kernel parameters. Disable, remove, or password-protect emergency/recovery boot entries.

• Security Updates: First and foremost, configure the system to automatically download security updates. Most security updates can be installed without rebooting the system, but some updated (e.g. shared libraries, kernel, etc) require the system to be rebooted. If possible, configure the system to install the security updates automatically and reboot at a particular time. We also recommend reserving the reboot time (e.g. 1:30AM on a Saturday morning) using the Actuator Agent so that no control actions can happen during that time.

• System Access only via Secured Protocols: Disallow all clear text access to VOLTTRON systems. No telnet, no rsh, no ftp and no exceptions. Use ssh to gain console access, and scp/sftp to get files in and out of the system. Disconnect excessively idle SSH Sessions.

• Disable remote login for "root" users. Do not allow a user to directly access the system as the "root" user from a remote network location. Root access to privileged operations can be accomplished using “sudo” This adds an extra level of security by restricting access to privileged operations and tracking those operations through the system log.

• Manage users and usernames. Limit the number of user accounts. Use complex usernames rather than first names.

• Authentication. If possible, use two factor authentication to allow access to the system. Informally, two factor authentication uses a combination of "something you know" and "something you have" to allow access to the system. RSA SecurID tokens are commonly used for two factor authentication but other tools are available. When not using two-factor authentication, use strong passwords and do not share accounts.

• Scan for weak passwords. Use password cracking tools such as John the Ripper (http://www.openwall.com/john/) or nmap with password cracking modules (http://nmap.org) to look for weak passwords.

• Utilize Pluggable Authentication Modules (PAM) to strengthen passwords and the login process. We recommend:

  • pam_abl: Automated blacklisting on repeated failed authentication attempts
  • pam_captcha: A visual text-based CAPTCHA challenge module for PAM
  • pam_passwdqc: A password strength checking module for PAM-aware password changing programs
  • pam_cracklib: PAM module to check the password against dictionary words
  • pam_pwhistory: PAM module to remember last passwords

• Disable unwanted services. Most desktop and server Linux distributions come with many unnecessary services enabled. Disable all unnecessary services. Refer to your distribution's documentation to discover how to check and disable these services.

• Just as scanning for weak passwords is a step to more secure systems, regular network scans using Nmap (www.nmap.org) to find what network services are being offered is another step towards a more secure system. Note, use nmap or similar tools very carefully on BACnet and modbus environments. These scanning tools are known to crash/reset BACnet and modbus devices.

• Control incoming and outgoing network traffic. Use the built-in host-based firewall to control who/what can connect to this system. Many iptables frontends offer a set of predefined rules that provide a default deny policy for incoming connections and provide rules to prevent or limit other well known attacks (i.e. rules that limit certain responses that might amplify a DDoS attack). ufw (uncomplicated firewall) is a good example. For example, if the system administrators for the VOLTTRON device are all located in 10.10.10.0/24 subnetwork, then allow SSH and SCP logins from only that IP address range. If VOLTTRON system exports data to a historian at 10.20.20.1 using TCP port 443, allow outgoing traffic to that port on that server. The idea here is to limit the attack surface of the system. The smaller the surface, the better we can analyze the communication patterns of the system and detect anomalies. One word of caution. While some system administrators disable network-based diagnostic tools such as ICMP ECHO responses, VOLTTRON team believes that this hampers usability. As an example, monitoring which incoming and outgoing firewall rules are triggering can be accomplished with this command: watch --interval=5 'iptables -nvL | grep -v "0 0"' .

• Rate limit incoming connections to discourage brute force hacking attempts. Use a tool such as fail2ban (http://www.fail2ban.org/wiki/index.php/Main_Page) to dynamically manage firewall rules to rate limit incoming connections and discourage brute force hacking attempts. sshguard (http://www.sshguard.net/) is similar to fail2ban but only used for ssh connections. Further rate limiting can be accomplished at the firewall level. As an example, you can restrict the number of connections used by a single IP address to your server using iptables. Only allow 4 ssh connections per client system: iptables -A INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 4 –j DROP You can limit the number of connections per minute. The following example will drop incoming connections if an IP address makes more than 10 connection attempts to port 22 within 60 seconds: iptables -A INPUT -p tcp –dport 22 -i eth0 -m state --state NEW -m recent --set iptables -A INPUT -p tcp –dport 22 -i eth0 -m state --state NEW -m recent \ --update –-seconds 60 -–hitcount 10 –j DROP

• Use a file system integrity tool to monitor for unexpected file changes. Tools such as tripwire (http://sourceforge.net/projects/tripwire/) to monitor filesystem for changed files. Another file integrity checking tool to consider is AIDE (Advanced Intrusion Detect Environment) (http://aide.sourceforge.net/).

• Use filesystem scanning tools periodically to check for exploits. Available tools such as checkrootkit (http://www.chkrootkit.org), rkhunter (http://rkhunter.sourceforge.net) and others should be used to check for known exploits on a periodic basis and report their results.

• VOLTTRON does not use apache or require it. If Apache is being used, we recommend using mod_security and mod_evasive modules.

System Monitoring

• Monitor system state and resources. Use a monitoring tool such as Xymon (http://xymon.sourceforge.net) or big brother (http://www.bb4.org/features.html) to remotely monitor the system resources and state. Set the monitoring tools to alert the system administrators if anomalous use of resources (e.g. connections, memory, etc) are detected. An administrator can also use unix commands such as netstat to look for open connections periodically.

• Watch system logs and get logs off the system. Use a utility such as logwatch (http://sourceforge.net/projects/logwatch/files/) or logcheck (http://logcheck.org) to get daily summary of system activity via email. For Linux distributions that use systemd, use journalwatch (http://git.the-compiler.org/journalwatch/) to accomplish the same task. Additionally, use a remote syslog server to collect logs from all VOLTTRON systems in the field at a centralized location for analysis. A tool such as splunk is ideal for this task and comes with many built-in analysis applications. Another benefit of sending logs remotely off the platform is the ability to inspect the logs even when the platform may be compromised.

• An active intrusion sensor such as PSAD (http://cipherdyne.org/psad/) can be used to look for intrusions as well.

Security Testing

Every security control discussed in the previous sections must be tested to determine correct operation and impact. For example, if we inserted a firewall rule to ban connections from an IP address such as 10.10.10.2, then we need to test that the connections actually fail.

In addition to functional correctness testing, common security testing tools such as Nessus (http://www.tenable.com/products/nessus) and nmap (http://nmap.org) should be used to perform cyber security testing.

Conclusion

No system is 100% secure unless it is disconnected from the network and is in a physically secure location. VOLTTRON team recommends a risk-based cyber security approach that considers each risk, and the impact of an exploit. Mitigating technologies can then be used to mitigate the most impactful risks first. VOLTTRON is built with security in mind from the ground up. But it is only as secure as the operating system that it runs on top of. This document is intended to help VOLTTRON users to secure the underlying Linux operating system to further improve the robustness of the VOLTTRON platform. Any security questions should be directed to [email protected].