Guide to the Secure Configuration of Ubuntu 1604
https://www.open-scap.org/security-policies/scap-security-guide
scap-security-guide
package which is developed at
https://www.open-scap.org/security-policies/scap-security-guide.
Providing system administrators with such guidance informs them how to securely configure systems under their control in a variety of network roles. Policy makers and baseline creators can use this catalog of settings, with its associated references to higher-level security control catalogs, in order to assist them in security baseline creation. This guide is a catalog, not a checklist, and satisfaction of every item is not likely to be possible or sensible in many operational scenarios. However, the XCCDF format enables granular selection and adjustment of settings, and their association with OVAL and OCIL content provides an automated checking capability. Transformations of this document, and its associated automated checking content, are capable of providing baselines that meet a diverse set of policy objectives. Some example XCCDF Profiles, which are selections of items that form checklists and can be used as baselines, are available with this guide. They can be processed, in an automated fashion, with tools that support the Security Content Automation Protocol (SCAP). The DISA STIG for Ubuntu 1604, which provides required settings for US Department of Defense systems, is one example of a baseline created from this guidance.
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Revision History
Current version: 0.1.31
- draft (as of 2017-03-31)
Platforms
- cpe:/o:canonical:ubuntu_linux:16.04
Table of Contents
- Remediation functions used by the SCAP Security Guide Project
- Introduction
- System Settings
- Hardening the hardware usage
- Installing and Maintaining Software
- File Permissions and Masks
- Hardening the filesystem
- Account and Access Control
- Configure Syslog
- Services
Checklist
Remediation functions used by the SCAP Security Guide Project [ref]groupXCCDF form of the various remediation functions as used by remediation scripts from the SCAP Security Guide Project |
Introduction [ref]groupThe purpose of this guidance is to provide security configuration recommendations and baselines for the Ubuntu 1604 operating system. Recommended settings for the basic operating system are provided, as well as for many network services that the system can provide to other systems. The guide is intended for system administrators. Readers are assumed to possess basic system administration skills for Unix-like systems, as well as some familiarity with the product's documentation and administration conventions. Some instructions within this guide are complex. All directions should be followed completely and with understanding of their effects in order to avoid serious adverse effects on the system and its security. |
General Principles [ref]groupThe following general principles motivate much of the advice in this guide and should also influence any configuration decisions that are not explicitly covered. |
Encrypt Transmitted Data Whenever Possible [ref]groupData transmitted over a network, whether wired or wireless, is susceptible to passive monitoring. Whenever practical solutions for encrypting such data exist, they should be applied. Even if data is expected to be transmitted only over a local network, it should still be encrypted. Encrypting authentication data, such as passwords, is particularly important. Networks of Ubuntu 1604 machines can and should be configured so that no unencrypted authentication data is ever transmitted between machines. |
Minimize Software to Minimize Vulnerability [ref]groupThe simplest way to avoid vulnerabilities in software is to avoid installing that software. On Ubuntu 1604, allows for careful management of the set of software packages installed on a system. Installed software contributes to system vulnerability in several ways. Packages that include setuid programs may provide local attackers a potential path to privilege escalation. Packages that include network services may give this opportunity to network-based attackers. Packages that include programs which are predictably executed by local users (e.g. after graphical login) may provide opportunities for trojan horses or other attack code to be run undetected. The number of software packages installed on a system can almost always be significantly pruned to include only the software for which there is an environmental or operational need. |
Run Different Network Services on Separate Systems [ref]groupWhenever possible, a server should be dedicated to serving exactly one network service. This limits the number of other services that can be compromised in the event that an attacker is able to successfully exploit a software flaw in one network service. |
Configure Security Tools to Improve System Robustness [ref]groupSeveral tools exist which can be effectively used to improve a system's resistance to and detection of unknown attacks. These tools can improve robustness against attack at the cost of relatively little configuration effort. In particular, this guide recommends and discusses the use of host-based firewalling, SELinux for protection against vulnerable services, and a logging and auditing infrastructure for detection of problems. |
Least Privilege [ref]group
Grant the least privilege necessary for user accounts and software to perform tasks.
For example, |
How to Use This Guide [ref]groupReaders should heed the following points when using the guide. |
Read Sections Completely and in Order [ref]groupEach section may build on information and recommendations discussed in prior sections. Each section should be read and understood completely; instructions should never be blindly applied. Relevant discussion may occur after instructions for an action. |
Test in Non-Production Environment [ref]groupThis guidance should always be tested in a non-production environment before deployment. This test environment should simulate the setup in which the system will be deployed as closely as possible. |
Root Shell Environment Assumed [ref]group
Most of the actions listed in this document are written with the
assumption that they will be executed by the root user running the
|
Formatting Conventions [ref]group
Commands intended for shell execution, as well as configuration file text,
are featured in a |
Reboot Required [ref]groupA system reboot is implicitly required after some actions in order to complete the reconfiguration of the system. In many cases, the changes will not take effect until a reboot is performed. In order to ensure that changes are applied properly and to test functionality, always reboot the system after applying a set of recommendations from this guide. |
System Settings [ref]groupContains rules that check correct system settings. |
Hardening the hardware usage [ref]groupHardware dependent, but efficient against various risks. |
Installing and Maintaining Software [ref]groupThe following sections contain information on security-relevant choices during the initial operating system installation process and the setup of software updates. |
File Permissions and Masks [ref]groupTraditional Unix security relies heavily on file and
directory permissions to prevent unauthorized users from reading or
modifying files to which they should not have access.
$ mount -t xfs | awk '{print $3}'For any systems that use a different local filesystem type, modify this command as appropriate. |
Verify Permissions on Important Files and Directories [ref]groupPermissions for many files on a system must be set restrictively to ensure sensitive information is properly protected. This section discusses important permission restrictions which can be verified to ensure that no harmful discrepancies have arisen. |
Verify permissions on files containing sensitive informations about the system [ref]groupVarious files contains sensitive informations that can leads to specific weaknesses or give structural informations for local exploits. |
Verify Permissions on Files with Local Account Information and Credentials [ref]groupThe default restrictive permissions for files which act as
important security databases such as |
Restrict Programs from Dangerous Execution Patterns [ref]groupThe recommendations in this section are designed to ensure that the system's features to protect against potentially dangerous program execution are activated. These protections are applied at the system initialization or kernel level, and defend against certain types of badly-configured or compromised programs. |
Disable Core Dumps [ref]groupA core dump file is the memory image of an executable
program when it was terminated by the operating system due to
errant behavior. In most cases, only software developers
legitimately need to access these files. The core dump files may
also contain sensitive information, or unnecessarily occupy large
amounts of disk space.
|
Enable ExecShield [ref]groupExecShield describes kernel features that provide
protection against exploitation of memory corruption errors such as buffer
overflows. These features include random placement of the stack and other
memory regions, prevention of execution in memory that should only hold data,
and special handling of text buffers. These protections are enabled by default
on 32-bit systems and controlled through |
Hardening the filesystem [ref]groupHardening the filesystem and its usage is an efficient way to ensure an efficient separation of services, data and configurations while ensuring a more precise management of filesystem level access rights, enabling deactivation of some specific rights at the filesystem level. Moreover, the Linux Virtual file system support various hardening mechanisms that can be set using sysctl. |
Partitioning [ref]groupSeparating various locations of the file systems in different partitions allows a more restrictive segregation, distinctly from one location to another. Moreover, some native restrictions can be made by partitioning, such as no hard link between different filesystems, and reduce the corruption impact to the affected filesystem instead of the entire system. The last gain is to allow a differenciated usage of storage media, depending on the operational needs (speed, resilience, etc.). References: Filesystem Hierarchy Standard |
filesystem rights management [ref]groupAdding filesystem specific hardening seriously limits various exploitation vectors based on filesystem invalid usage, such as invalid file types in invalid places (devices or setuid root files in external media, executable file in insecure filesystems, etc.). Some of these hardening require an efficient system partitioning. |
Account and Access Control [ref]groupIn traditional Unix security, if an attacker gains shell access to a certain login account, they can perform any action or access any file to which that account has access. Therefore, making it more difficult for unauthorized people to gain shell access to accounts, particularly to privileged accounts, is a necessary part of securing a system. This section introduces mechanisms for restricting access to accounts under Ubuntu 1604. |
Protect Accounts by Restricting Password-Based Login [ref]groupConventionally, Unix shell accounts are accessed by
providing a username and password to a login program, which tests
these values for correctness using the |
Configure Syslog [ref]groupThe syslog service has been the default Unix logging mechanism for
many years. It has a number of downsides, including inconsistent log format,
lack of authentication for received messages, and lack of authentication,
encryption, or reliable transport for messages sent over a network. However,
due to its long history, syslog is a de facto standard which is supported by
almost all Unix applications.
|
Ensure Proper Configuration of Log Files [ref]group
The file auth,authpriv.* /var/log/auth.log *.*;auth,authpriv.none -/var/log/syslog daemon.* -/var/log/daemon.log kern.* -/var/log/kern.log lpr.* -/var/log/lpr.log mail.* -/var/log/mail.log user.* -/var/log/user.log mail.info -/var/log/mail.info mail.warn -/var/log/mail.warn mail.err /var/log/mail.err news.crit /var/log/news/news.crit news.err /var/log/news/news.err news.notice -/var/log/news/news.noticeSee the man page rsyslog.conf(5) for more information.
Note that the rsyslog daemon is configured to use traditional timestamping
to be understood by any log processing program. For high precision timestamping,
comment the following line in /etc/rsyslog.conf :
$ ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat |
Ensure All Logs are Rotated by logrotate [ref]groupEdit the file /var/log/messages /var/log/secure /var/log/maillog /var/log/spooler \ /var/log/boot.log /var/log/cron {Edit this line so that it contains a one-space-separated listing of each log file referenced in /etc/rsyslog.conf .
All logs in use on a system must be rotated regularly, or the log files will consume disk space over time, eventually interfering with system operation. The file /etc/logrotate.d/syslog is the
configuration file used by the logrotate program to maintain all
log files written by syslog . By default, it rotates logs weekly and
stores four archival copies of each log. These settings can be
modified by editing /etc/logrotate.conf , but the defaults are
sufficient for purposes of this guide.
Note that logrotate is run nightly by the cron job
/etc/cron.daily/logrotate . If particularly active logs need to be
rotated more often than once a day, some other mechanism must be
used. |
Services [ref]group
The best protection against vulnerable software is running less software. This section describes how to review
the software which Ubuntu 1604 installs on a system and disable software which is not needed. It
then enumerates the software packages installed on a default Ubuntu 1604 system and provides guidance about which
ones can be safely disabled.
|
Deprecated services [ref]groupSome deprecated software services impact the overall system security due to their behavior (leak of confidentiality in network exchange, usage as uncontrolled communication channel, risk associated with the service due to its old age, etc. |
Generic required services [ref]groupSome services need to be deployed in order to ensure basic verifications and reporting on GNU/Linux operating systems. Each of these service take part in the administrability of the system. |
APT service configuration [ref]groupThe apt service manage the package management and update of the whole system. Its configuration need to be properly defined to ensure efficient security updates, packages and repository authentication and proper lifecycle management. |
SSH Server [ref]groupThe SSH protocol is recommended for remote access (remote login and secure remote file transfer).
SSH provides both confidentiality and integrity for exchanged data but needs to be configured properly in term of: |
Configure OpenSSH Server if deployed [ref]groupIf the system needs to act as an SSH server, then
certain changes should be made to the OpenSSH daemon configuration
file |