How to apply PCI data security standards to Linux data centers

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The Payment Card Industry (PCI) data security standards are a set of best practices and requirements established to protect sensitive data such as payment card information. Following these standards is mandatory for merchants dealing with payment cards, but any responsible organization can benefit by using them to enhance information security.

Secure your network

To meet the PCI requirement to secure your network you should have a dedicated router/firewall that by default denies all incoming and outgoing connectivity. You should allow connections only for explicit needs.
CentOS has a strong default firewall that denies all incoming connections except those to port 22 (ssh). You can improve on its rules in two ways. First, allow only your own organization's IP addresses to connect via ssh. Edit the file /etc/sysconfig/iptables and changing the line -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT to -A INPUT -s YOURIP -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT, then restart iptables with the command service iptables restart.
You should also deny all outgoing connections except those you need. Limiting outgoing connections can limit the impact of a security compromise. Use these commands:
/sbin/iptables -A OUTPUT -o lo -j ACCEPT #accept all outgoing connections to the loopback interface, which are usually internal service calls
/sbin/iptables -A OUTPUT -p tcp \! --syn -j ACCEPT #accept any outgoing connection except new ones
/sbin/iptables -A OUTPUT -p UDP --dport 53 -j ACCEPT #accept outgoing DNS requests on UDP ports. Similarly you should add other needed services.
/sbin/iptables -A OUTPUT -j DROP #drop all connections
The above commands create rules that take effect immediately. To save them permanently in the file /etc/sysconfig/iptables, run the command service iptables save.

Protect sensitive data

The next tool you should use to protect your sensitive data is encryption. Truecrypt is an excellent open source tool for encrypting data on disk.
On CentOS you can install Truecrypt easily. First, install its only requirement, fuse-libs, with the command yum install fuse-libs. Next, download the console-only installation package for Linux, extract the package, and run the installer with the command ./truecrypt-7.1a-setup-console-x86. When it finishes you can use binary /usr/bin/truecrypt to encrypt and decrypt your sensitive files.
Suppose you want to encrypt the directory /media/encrypted. A good option is to use only a single file for storing the encrypted content so that you don't have to change your current partition table, nor your disk layout. To do this, first create a Truecrypt file with the command truecrypt -t -c /root/truecrypt.tc. You have to answer a few questions, namely:

• Volume type – normal type is fine; the other alternative is hidden file, which is more practical for personal use than for server setup.
• Encryption algorithm – Choices are AES-256, Serpent, and Twofish. All of them are strong and reliable. You may even want to use a combination of them so you can apply multiple layers of encryption. Thus if you chose the combination AES-Twofish-Serpent, an intruder would have to break first the AES encryption, then Twofish, and finally Serpent in order to read your data. However, the more complex the encryption, the slower the read and write response you will get from the encrypted data.
• Hash algorithm – Choices are RIPEMD-160, SHA-512, and Whirlpool. The last is the best choice here because it's world-recognized and even adopted in the standard ISO/IEC 10118-3:2004.
• Filesystem – with CentOS, choose a native Linux filesystem such as Linux ext4. The encrypted file's filesystem can be different from the operating system's filesystem.
• Password – this is the most important choice. You should pick up a password that's strong (all kinds of characters) and long (more than 15 characters) to make the one that's hard to crack by brute-force attacks.
• Keyfile path – A keyfile contains random content used for decrypting your data. It is an extra protection against brute force attacks, but it is not needed as long as you choose a strong password.

Are you with me so far? If you're not familiar with Truecrypt or encryption as a whole you may be confused by the difference between an encryption algorithm and a hash algorithm. Hashing allows you to generate the same shortened reference result every time from some given data. The result is useful for validating that the original data has not changed, and cannot be used to regenerate the original data. By contrast, encryption changes the original data in such a way that it can be restored if you have the encryption key. Truecrypt uses both hashing and encryption to protect your data.
After you complete the wizard you should have the file /root/truecrypt.tc. Create a mount point for it with the command mkdir /media/encrypted, then mount the Truecrypt file by running /usr/bin/truecrypt /root/truecrypt.tc /media/encrypted/. To dismount it run /usr/bin/truecrypt -d; you don't have to specify the mount point. The file will also be dismounted automatically when the operating system is restarted.
Truecrypt protects your data only while the Truecrypt file is not mounted. Once the file is mounted your data is readable and you have to rely on the security and permissions provided by the operating system for the data protection. That's why you should dismount the file as soon as possible after you have accessed any files you need in the encrypted file/directory.
Unfortunately, Truecrypt is not suitable if your sensitive data is stored in a database such as MySQL. MySQL requires constant access to its data files and thus it's not practical to constantly mount and dismount encrypted volumes. Instead, to encrypt MySQL data you should use MySQL's encryption functions.
By using encryption you protect your data in case of a physical theft of media. Also, if your system is compromised, encryption makes it harder for an intruder to read your data.

Manage vulnerabilities

PCI standards also require you to mitigate threats and vulnerabilities in a timely fashion. You must patch critical vulnerabilities as soon as possible and no later than one month of their discovery.
In CentOS, system updates are relatively easy and safe because of the famous Red Hat backporting update process, in which essential fixes are extracted from new versions and ported to old versions. You should regularly run yum -y update command to update your CentOS operating system and applications, but bear in mind that there is always a risk of making complex systems hiccup when you update a production system, even with backported fixes.
You should also run antivirus software. A good open source antivirus solution is ClamAV, though it lacks the real-time protection found in commercial antivirus programs.
You can install ClamAV on CentOS from the EPEL repository. First add EPEL on your CentOS source files with the command rpm -ivh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm. Then install ClamAV with the command yum install clamav.
After you first install ClamAV, update its antivirus database with the command /usr/bin/freshclam. It's best to set this command as a cron task that runs daily, with a line such as 3 3 * * * /usr/bin/freshclam --quiet in your crontab file to run it every day at 3:03 a.m.
You should perform regular antivirus scans on directories that are exposed to external services. For example, if you have an Apache web server, you should scan its default document root /var/www/html and the /tmp directory, where temporary files may be uploaded.
Two hints here: First, run this scan automatically as a cron job. Second, email yourself the output so you can see whether there were scanning errors or viruses. You can do both with a crontab entry such as 4 4 * * * /usr/bin/clamscan /var/www/html /tmp --log /var/log/clamav/scan.log || mail -s 'Virus Report' yourmail@example.org < /var/log/clamav/scan.log. Here, if clamscan does not detect a virus or error, it exits with status 0 and no mail is sent. Otherwise, you will receive a message with the scan log.
Viruses aren't the only threat to your systems. In addition to ClamAV it's a good idea to run an auditing and hardening tool such as Lynis. Lynis checks your system for misconfiguration and security errors, and searches for popular rootkits and any evidence of your system being compromised. Once you download and extract it it's ready for use. When you run it manually you should use the argument -c to perform all of its checks, with a command like /root/lynis-1.3.5/lynis -c. Going through all the checks does not take much time or resources. If you want to schedule the command as a cron job you should use the -q option for a quiet run, which throws only warnings: /root/lynis-1.3.5/lynis -q.

Perform audits and control access

The PCI standards require from you to track every user's actions with sensitive (cardholder) data and also every action performed by privileged users. On the system level this usually means running Linux's auditd daemon such as described in the article Linux auditing 101.
Another good practice from the PCI standards is the requirement to restrict access to only those who need it. With Linux you may have situations where the usual user/group/other permissions are not sufficient to provide the required granular access control.
For example, imagine that the web file /var/www/html/config.php is owned by the user apache but needs to be read by user admin1 from the admins group and user qa1 from the QA group. To avoid granting "other" read permission you can use Linux access control lists (ACL) by using the command setfacl with the -m argument (modify) like this:

setfacl -m u:qa1:r /var/www/html/config.php
setfacl -m u:admin1:r /var/www/html/config.php

You can check the results with the command getfacl: getfacl /var/www/html/config.php. The output should be similar to this:

getfacl: Removing leading '/' from absolute path names
# file: var/www/html/config.php
# owner: apache
# group: apache
user::rw-
user:admin1:r--
user:qa1:r--
group::r--
mask::r--
other::---

As you can see, the user admin1 and qa1 here have the needed read permissions set, while others don't have any permissions and thus other users cannot read the file.

Scan the network

PCI requires you to scan your system and network for vulnerabilities. Such remote scans are to be performed by external security auditors every three months, but you can adopt this good practice and scan your network by yourself.
To learn how to scan your network, read the article BackTrack and its tools can protect your environment from remote intrusions. It explains not only how to perform a remote security scan but also how to resolve the most common vulnerabilities that such a scan may detect.

Maintain an information security policy

PCI improves information security by formalizing security roles and responsibilities in an information security policy document. Obviously, clear resource ownership ensures better care for resources. Unfortunately, many organizations neglect this practice and muddle along with unclear responsibilities for resources.
Part of this requirement is that personnel be regularly exposed to security awareness programs. This helps people remember to use information security best practices in everyday work. SANS Institute provides daily security awareness tips that you can use for this purpose.
Finally, you should create scenarios for handling security incidents. Sooner or later such incidents happen, and you should be prepared to resolve them swiftly. Security incidents may include data being stolen or a whole system being compromised. Make sure to prepare for every such unpleasant scenario specific to your organization.
As you can see, the PCI data security standards are comprehensive and versatile, and you can use them to improve the information security of your organization even if you never handle payment cards, because they are designed to protect an organization's most sensitive resources.

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