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Applies to SUSE Linux Enterprise Server 15

1 Security and Confidentiality Edit source

This chapter introduces basic concepts of computer security. Threats and basic mitigation techniques are described. The chapter also provides references to other chapters, guides and websites with further information.

1.1 Overview Edit source

One main characteristic of Linux is its ability to handle multiple users at the same time (multiuser) and to allow these users to simultaneously perform tasks (multitasking) on the same computer. To users, there is no difference between working with data stored locally and data stored in the network.

Due to the multiuser capability, data from different users has to be stored separately to guarantee security and privacy. Also important is the ability to keep data available in spite of a lost or damaged data medium, for example a hard disk.

This chapter is primarily focused on confidentiality and privacy. But a comprehensive security concept includes a regularly updated, workable, and tested backup. Without a backup, restoring data after it has been tampered with or after a hardware failure is very hard.

Use a defense-in-depth approach to security: Assume that no single threat mitigation can fully protect your systems and data, but multiple layers of defense will make an attack much harder. Components of a defense-in-depth strategy can be the following:

  • Hashing passwords (for example with PBKDF2, bcrypt, or scrypt) and salting them

  • Encrypting data (for example with AES)

  • Logging, monitoring, and intrusion detection

  • Firewall

  • Antivirus scanner

  • Defined and documented emergency procedures

  • Backups

  • Physical security

  • Audits, security scans, and intrusion tests

SUSE Linux Enterprise Server includes software that addresses the requirements of the list above. The following sections provide starting points for securing your system.

1.2 Passwords Edit source

On a Linux system, only hashes of passwords are stored. Hashes are one-way algorithms that make it easy to encrypt data. At the same time, hash algorithms make it very hard to compute the original secret from the hash.

The hashes are stored in the file /etc/shadow, which cannot be read by normal users. Because restoring passwords is possible with powerful computers, hashed passwords should not be visible to regular users.

The National Institute of Standards and Technology (NIST) publishes a guideline for passwords, which is available at https://pages.nist.gov/800-63-3/sp800-63b.html#sec5

For details about how to set a password policy, see Section 16.3, “Password Settings. For general information about authentication on Linux, see Part I, “Authentication”.

1.3 Backups Edit source

If your system is compromised, backups can be used to restore a prior system state. When bugs or accidents occur, backups can also be used to compare the current system against an older version. For production systems, it is very important to take some backups off-site for cases like disasters (for example off-site storage of tapes/recordable media, or off-site initiated).

For legal reasons, some firms and organizations must be careful about backing up too much information and holding it too long. If your environment has a policy regarding the destruction of old paper files, you might need to extend this policy to Linux backup tapes as well.

The rules about physical security of servers apply to backups as well. Additionally it is advisable to encrypt backup data. This can be done either per individual backup archive or for the complete backup file system, if applicable. Should a backup medium ever be lost, for example during transportation, the data will be protected against unauthorized access. The same applies if a backup system itself is compromised. To some extent encryption also ensures the integrity of the backups. Keep in mind, however, that the appropriate people need to be able to decrypt backups in emergency situations. Also, the case that an encryption key itself is compromised and needs to be replaced should be considered.

If a system is known to be compromised or suspected to be compromised, then it is vital to determine the integrity status of backups. If a system compromise has not been detected for a longer time, then it is possible that backups already include manipulated configuration files or malicious programs. Keeping a long enough history of backups allows to inspect for possible unwarranted differences.

Even in the absence of any known security breach, a regular inspection of differences among important configuration files in backups can help with finding security issues (maybe even accidental misconfigurations). This approach is best suited for files and environments where the content doesn't change too frequently.

1.4 System Integrity Edit source

If it is possible to physically access a computer, the firmware and boot process can be manipulated to gain access as soon as an authorized person boots the machine. While not all computers can be locked into inaccessible rooms, your first step should be physically locking the server room.

Also remember that disposing of old equipment must be handles in a secure manner. Securing the boot loader and restricting removable media also provide useful physical security, See Chapter 10, Physical Security for more information.

Consider taking the following additional measures:

  • Configure your system so it cannot be booted from a removable device.

  • Protect the boot process with a UEFI password, Secure Boot, and a GRUB2 password.

  • Linux systems are started by a boot loader that usually allows passing additional options to the booted kernel. You can prevent others from using such parameters during boot by setting an additional password for the boot loader. This is crucial to system security. Not only does the kernel itself run with root permissions, but it is also the first authority to grant root permissions at system start-up.

    For more information about setting a password in the boot loader, see Section 14.2.6, “Setting a Boot Password”.

  • Enable hard disk encryption. For more information, see Chapter 14, Encrypting Partitions and Files.

  • Use AIDE to detect any changes in your system configuration. For more information, see Chapter 20, Intrusion Detection with AIDE.

1.5 File Access Edit source

Because of the everything is a file approach in Linux, file permissions are important for controlling access to most resources. This means that by using file permissions, you can define access to regular files and directories as well as hardware devices. By default, most hardware devices are only accessible for root. However, some devices, for example serial ports, can be accessible for normal users.

As a general rule, always work with the most restrictive privileges possible for a given task. For example, it is definitely not necessary to be root to read or write e-mail. If the mail program has a bug, this bug could be exploited for an attack that acts with exactly the permissions of the program at the time of the attack. By following the above rule, minimize the possible damage.

For details, see Section 18.1, “Traditional File Permissions” and Section 18.2, “Advantages of ACLs”.

AppArmor and SELinux allow you to set constraints for applications and users. For details, see Part IV, “Confining Privileges with AppArmor and Part V, “SELinux”.

If there is a chance that hard disks could be accessed outside of the installed operating system, for example by booting a live system or removing the hardware, encrypt the data. SUSE Linux Enterprise Server allows you to encrypt partitions containing data and the operating system. For details, see Chapter 14, Encrypting Partitions and Files.

1.6 Networking Edit source

Securing network services is a crucial task. Aim to secure as many layers of the OSI model as possible.

All communication should be authenticated and encrypted with up-to-date cryptographic algorithms on the transport or application layer. Use a Virtual Private Network (VPN) as an additional secure layer on physical networks.

SUSE Linux Enterprise Server provides many options for securing your network:

  • Use openssl to create X509 certificates. These certificates can be used for encryption and authentication of many services. You can set up your own certificate authority (CA) and use it as a source of trust in your network. For details, see man openssl.

  • Usually, at least parts of networks are exposed to the public Internet. Reduce attack surfaces by closing ports with firewall rules and by uninstalling or at least disabling unrequired services. For details, see Chapter 23, Masquerading and Firewalls.

  • Use OpenVPN to secure communication channels over insecure physical networks. For details, see Chapter 24, Configuring a VPN Server.

  • Use strong authentication for network services. For details, see Part I, “Authentication”.

1.7 Software Vulnerabilities Edit source

Software vulnerabilities are issues in software that can be exploited to obtain unauthorized access or misuse systems. Vulnerabilities are especially critical if they affect remote services, such as HTTP servers. Computer systems are very complex, therefore they always include certain vulnerabilities.

When such issues become known, they must usually be fixed in the software by software developers. The resulting update must then be installed by system administrators in a timely and safe manner on affected systems.

Vulnerabilities are usually announced on centralized databases, for example the National Vulnerability Database, which is maintained by the US government. You can subscribe to feeds to stay informed about newly discovered vulnerabilities. In some cases the problems induced by the bugs can be mitigated until a software update is provided. Vulnerabilities are assigned a Common Vulnerabilities and Exposures (CVE) number and a Common Vulnerability Scoring System (CVSS) score. The score helps identify the severity of vulnerabilities.

SUSE provides a feed of security advisories. It is available at https://www.suse.com/en-us/support/update/. There is also a list of security updates by CVE number available at https://www.suse.com/en-us/security/cve/.

Note
Note: Backports and Version Numbers

SUSE employs the practice of applying the important source code fixes onto older stable versions of software (backporting). Therefore, even if the version number of a software in SUSE Linux Enterprise Server is lower than that of the latest version number from the upstream project, the software version in SUSE Linux Enterprise Server may already contain the latest fixes for vulnerabilities.

For more information, see Chapter 6, Backports of Source Code.

In general, administrators should be prepared for severe vulnerabilities in their systems. This includes hardening all computers as far as possible. Also, we recommend to have predefined procedures in place for quickly installing updates for severe vulnerabilities.

To reduce the damage of possible attacks, use restrictive file permissions. See Section 18.1, “Traditional File Permissions”.

Other useful links:

1.8 Malware Edit source

Malware is software that is intended to interrupt the normal functioning of a computer or steal data. This includes viruses, worms, ransomware, or rootkits. Sometimes malware uses software vulnerabilities to attack a computer. However, in many cases it is accidentally executed by a user, especially when installing third-party software from unknown sources. SUSE Linux Enterprise Server provides an extensive list of programs (packages) in its download repositories. This reduces the need to download third-party software. All packages provided by SUSE are signed. The package manager of SUSE Linux Enterprise Server checks the signatures of packages after the download to verify their integrity.

The command rpm --checksig RPM_FILE shows whether the checksum and the signature of a package are correct. You can find the signing key on the first DVD of SUSE Linux Enterprise Server and on most key servers worldwide.

You can use the ClamAV antivirus software to detect malware on your system. ClamAV can be integrated into several services, for example mail servers and HTTP proxies. This can be used to filter malware before it reaches the user.

Restrictive user privileges can reduce the risk of accidental code execution.

1.9 Important Security Tips Edit source

The following tips are a quick summary of the sections above:

  • Stay informed about the latest security issues. Get and install the updated packages recommended by security announcements as quickly as possible.

  • Avoid using root privileges whenever possible. Set restrictive file permissions.

  • Only use encrypted protocols for network communication.

  • Disable any network services you do not absolutely require.

  • Conduct regular security audits. For example, scan your network for open ports.

  • Monitor the integrity of files on your systems with AIDE (Advanced Intrusion Detection Environment).

  • Take proper care when installing any third-party software.

  • Check all your backups regularly.

  • Check your log files, for example with logwatch.

  • Configure the firewall to block all ports that are not explicitly whitelisted.

  • Design your security measures to be redundant.

  • Use encryption where possible, for example for hard disks of mobile computers.

1.10 Reporting Security Issues Edit source

If you discover a security-related problem, first check the available update packages. If no update is available, write an e-mail to <>. Include a detailed description of the problem and the version number of the package concerned. We encourage you to encrypt e-mails with GPG.

You can find a current version of the SUSE GPG key at https://www.suse.com/support/security/contact/.

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