26 Common Criteria #

Refer to the , the limitations of cryptography are briefly outlined.

Be aware that cryptography is certainly useful, but only for the specific purposes that it is good for. Using cryptography is not a generic recipe for better security in a system, its use may even impose additional risk on the system. Make informed decisions about the use of cryptography, and feel obliged to have a reason for your decisions. A false sense of security can be more harmful than the weakness itself.

SUSE Linux Enterprise Server supports encryption for:

It is useful to restrict the installed packages in your system to a minimum. Binaries not installed cannot be executed.

During installation of the system, you can limit the set of packages that is installed. For example, you can deselect all packages and select only those that you want to use. For example, the selection of the apache2-mod_perl package in YaST would automatically cause all packages to be selected for installation that are needed for the Apache package to operate. Dependencies have often been artificially cut down to handle the system's dependency tree more flexibly. You can chose the minimal system, and build the dependency tree from there with your (leaf) package selection.

Whenever possible, a server should be dedicated to serving exactly one service or application. This limits the number of other services that could be compromised if an attacker can successfully exploit a software flaw in one service (assuming that flaw allows access to others).

The use of AppArmor for services that are provided on a system is an effective means of containment. For more information, see Part V, “Confining Privileges with AppArmor” and the man page of apparmor.

The use of virtualization technology is supported with SUSE Linux Enterprise Server. While virtualization is generally designed for server consolidation purposes, it is also usefulness for service isolation. However, virtualization technology cannot match or substitute the separation strength that is given by running services on different physical machines! Be aware that the capability of the hypervisor to separate virtual machines is not higher or stronger than the Linux kernel's capability to separate processes and their address spaces.

Doing regular backups and having a fingerprint of your system is vital, especially in the case of a successful attack against your system. Make it an integral part of your security routine to verify that your backups work.

A fast and directly accessible backup adds confidence about the integrity of your system. However, it is important that the backup mechanism/solution has adequate versioning support so that you can trace changes in the system. As an example: The installation times of packages (rpm -q --queryformat='%{INSTALLTIME} %{NAME}\n' PACKAGE NAME) must correspond to the changed files in the backup log files.

Several tools exist on SUSE Linux Enterprise Server 15 SP2 which can be used for the detection of unknown, yet successful attacks. It does not take much effort to configure them.

In particular, we recommend using the file and directory integrity checker AIDE (Advanced Intrusion Detection Environment). When run for initialization, it creates a hash database of all files in the system that are listed in its configuration file. This allows verifying the integrity of all cataloged files at a later time.

Warning: Backdoors

If you use AIDE, copy the hash database to a place that is inaccessible for potential attackers. Otherwise, the attacker may modify the integrity database after planting a backdoor, thereby defeating the purpose of the integrity measurement.

An attacker may also have planted a backdoor in the kernel. Apart from being very hard to detect, the kernel-based backdoor can effectively remove all traces of the system compromise, so system alterations become almost invisible. Consequently, an integrity check needs to be done from a rescue system (or any other independent system with the target system's file systems mounted manually).

Be aware that the application of security updates invalidates the integrity database. rpm -qlv packagename lists the files that are contained in a package. The RPM subsystem is very powerful with the data that it maintains. It is accessible with the --queryformat command line option. A differential update of integrity database with the changed files becomes more manageable with some fine-grained usage of RPM.