This guide presents a catalog of security-relevant configuration settings for Ubuntu 1604. It is a rendering of content structured in the eXtensible Configuration Checklist Description Format (XCCDF) in order to support security automation. The SCAP content is is available in the scap-security-guide package which is developed at

.

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.

Applicable platforms

  • cpe:/o:canonical:ubuntu_linux:16.04

Version: 0.1.31

Revision history

  • draft (as of 2017-04-10)

1. Remediation functions used by the SCAP Security Guide Project

XCCDF form of the various remediation functions as used by remediation scripts from the SCAP Security Guide Project

2. Introduction

The 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.

2.1. General Principles

The following general principles motivate much of the advice in this guide and should also influence any configuration decisions that are not explicitly covered.

2.1.1. Encrypt Transmitted Data Whenever Possible

Data 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.

2.1.2. Minimize Software to Minimize Vulnerability

The 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.

2.1.3. Run Different Network Services on Separate Systems

Whenever 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.

2.1.4. Configure Security Tools to Improve System Robustness

Several 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.

2.1.5. Least Privilege

Grant the least privilege necessary for user accounts and software to perform tasks. For example, sudo can be implemented to limit authorization to super user accounts on the system only to designated personnel. Another example is to limit logins on server systems to only those administrators who need to log into them in order to perform administration tasks. Using SELinux also follows the principle of least privilege: SELinux policy can confine software to perform only actions on the system that are specifically allowed. This can be far more restrictive than the actions permissible by the traditional Unix permissions model.

2.2. How to Use This Guide

Readers should heed the following points when using the guide.

2.2.1. Read Sections Completely and in Order

Each 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.

2.2.2. Test in Non-Production Environment

This 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.

2.2.3. Root Shell Environment Assumed

Most of the actions listed in this document are written with the assumption that they will be executed by the root user running the /bin/bash shell. Commands preceded with a hash mark (#) assume that the administrator will execute the commands as root, i.e. apply the command via sudo whenever possible, or use su to gain root privileges if sudo cannot be used. Commands which can be executed as a non-root user are are preceded by a dollar sign ($) prompt.

2.2.4. Formatting Conventions

Commands intended for shell execution, as well as configuration file text, are featured in a monospace font. Italics are used to indicate instances where the system administrator must substitute the appropriate information into a command or configuration file.

2.2.5. Reboot Required

A 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.

3. System Settings

Contains rules that check correct system settings.

Table of Contents

3.1. Installing and Maintaining Software

The following sections contain information on security-relevant choices during the initial operating system installation process and the setup of software updates.

3.2. File Permissions and Masks

Traditional 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.

Several of the commands in this section search filesystems for files or directories with certain characteristics, and are intended to be run on every local partition on a given system. When the variable PART appears in one of the commands below, it means that the command is intended to be run repeatedly, with the name of each local partition substituted for PART in turn.

The following command prints a list of all xfs partitions on the local system, which is the default filesystem for Red Hat Enterprise Linux 7 installations:

$ mount -t xfs | awk '{print $3}'
For any systems that use a different local filesystem type, modify this command as appropriate.

3.2.1. Verify Permissions on Important Files and Directories

Permissions 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.

3.2.1.1. Verify permissions on files containing sensitive informations about the system

Various files contains sensitive informations that can leads to specific weaknesses or give structural informations for local exploits.

3.2.1.1.a. Verify that local System.map file (if exists) is readable only by root

Files containing sensitive informations should be protected by restrictive permissions. Most of the time, there is no need that these files need to be read by any non-root user To properly set the permissions of /boot/System.map-*, run the command:

$ sudo chmod 0600 /boot/System.map-*
To properly set the owner of /boot/System.map-*, run the command:
$ sudo chown root /boot/System.map-*

The System.map file contains information about kernel symbols and can give some hints to generate local exploitation.

3.2.1.2. Verify Permissions on Files with Local Account Information and Credentials

The default restrictive permissions for files which act as important security databases such as passwd, shadow, group, and gshadow files must be maintained. Many utilities need read access to the passwd file in order to function properly, but read access to the shadow file allows malicious attacks against system passwords, and should never be enabled.

3.2.1.2.a. Verify Permissions and ownership on shadow File

To properly set the permissions of /etc/shadow, run the command:

$ sudo chmod 0640 /etc/shadow
To properly set the owner of /etc/shadow, run the command:
$ sudo chown root /etc/shadow
To properly set the group owner of /etc/shadow, run the command:
$ sudo chgrp shadow /etc/shadow

The /etc/shadow file contains the list of local system accounts and stores password hashes. Protection of this file is critical for system security. Failure to give ownership of this file to root provides the designated owner with access to sensitive information which could weaken the system security posture.

Remediation script

                      chmod 0640 /etc/shadow

                    

3.2.1.2.b. Verify Permissions and ownership on gshadow File

To properly set the permissions of /etc/gshadow, run the command:

$ sudo chmod 0640 /etc/gshadow
To properly set the owner of /etc/gshadow, run the command:
$ sudo chown root /etc/gshadow
To properly set the group owner of /etc/gshadow, run the command:
$ sudo chgrp shadow /etc/gshadow

The /etc/shadow file contains group password hashes. Protection of this file is critical for system security.

Remediation script

                      chmod 0640 /etc/gshadow

                    

3.2.1.2.c. Verify Permissions and ownership on passwd File

To properly set the permissions of /etc/passwd, run the command:

$ sudo chmod 0644 /etc/passwd
To properly set the owner of /etc/passwd, run the command:
$ sudo chown root /etc/passwd
To properly set the group owner of /etc/passwd, run the command:
$ sudo chgrp root /etc/passwd

The /etc/shadow file contains information about the users that are configured on the system. Protection of this file is critical for system security.

Remediation script

                      chmod 0644 /etc/passwd

                    

3.2.1.2.d. Verify Permissions and ownership on group File

To properly set the permissions of /etc/passwd, run the command:

$ sudo chmod 0644 /etc/passwd
To properly set the owner of /etc/passwd, run the command:
$ sudo chown root /etc/passwd
To properly set the group owner of /etc/passwd, run the command:
$ sudo chgrp root /etc/passwd

The /etc/shadow file contains information about the groups that are configured on the system. Protection of this file is critical for system security.

Remediation script

                      chmod 0644 /etc/group

                    

3.2.2. Restrict Programs from Dangerous Execution Patterns

The 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.

3.2.2.1. Disable Core Dumps

A 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.

Once a hard limit is set in /etc/security/limits.conf, a user cannot increase that limit within his or her own session. If access to core dumps is required, consider restricting them to only certain users or groups. See the limits.conf man page for more information.

The core dumps of setuid programs are further protected. The sysctl variable fs.suid_dumpable controls whether the kernel allows core dumps from these programs at all. The default value of 0 is recommended.

3.2.2.1.a. Disable Core Dumps for SUID programs

To set the runtime status of the fs.suid_dumpable kernel parameter, run the following command:

$ sudo sysctl -w fs.suid_dumpable=0
If this is not the system's default value, add the following line to /etc/sysctl.conf:
fs.suid_dumpable = 0

The core dump of a setuid program is more likely to contain wve data, as the program itself runs with greater privileges than the user who initiated execution of the program. Disabling the ability for any setuid program to write a core file decreases the risk of unauthorized access of such data.

3.2.2.2. Enable ExecShield

ExecShield 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 sysctl variables kernel.exec-shield and kernel.randomize_va_space. On the latest 64-bit systems, kernel.exec-shield cannot be enabled or disabled with sysctl.

3.2.2.2.a. Enable Randomized Layout of Virtual Address Space

To set the runtime status of the kernel.randomize_va_space kernel parameter, run the following command:

$ sudo sysctl -w kernel.randomize_va_space=2
If this is not the system's default value, add the following line to /etc/sysctl.conf:
kernel.randomize_va_space = 2

Address space layout randomization (ASLR) makes it more difficult for an attacker to predict the location of attack code they have introduced into a process's address space during an attempt at exploitation. Additionally, ASLR makes it more difficult for an attacker to know the location of existing code in order to re-purpose it using return oriented programming (ROP) techniques.

3.2.2.2.b. Restrict exposed kernel pointers addresses access

To set the runtime status of the kernel.kptr_restrict kernel parameter, run the following command:

$ sudo sysctl -w kernel.kptr_restrict=1
If this is not the system's default value, add the following line to /etc/sysctl.conf:
kernel.kptr_restrict = 1

Exposing kernel pointers (through procfs or seq_printf()) exposes kernel writeable structures that can contain functions pointers. If a write vulnereability occurs in the kernel allowing a write access to any of this structure, the kernel can be compromise. This option disallow any program withtout the CAP_SYSLOG capability from getting the kernel pointers addresses, replacing them with 0.

3.3. Hardening the filesystem

Hardening 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.

3.3.1. Partitioning

Separating 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

  1. Filesystem Hierarchy Standard. URL: <http://www.pathname.com/fhs/>.

3.3.1.a. Ensure /tmp Located On Separate Partition

The /tmp directory is a world-writable directory used for temporary file storage. Ensure it has its own partition or logical volume at installation time, or migrate it using LVM (when non-ephemeral is needed) or use tmpfs if possible.

The /tmp partition is used as temporary storage by many programs. Placing /tmp in its own partition enables the setting of more restrictive mount options, which can help protect programs which use it.

3.3.1.b. Ensure /var Located On Separate Partition

The /var directory is used by daemons and other system services to store frequently-changing data. Ensure that /var has its own partition or logical volume at installation time, or migrate it using LVM.

Ensuring that /var is mounted on its own partition enables the setting of more restrictive mount options. This helps protect system services such as daemons or other programs which use it. It is not uncommon for the /var directory to contain world-writable directories installed by other software packages.

3.3.1.c. Ensure /var/log Located On Separate Partition

System logs are stored in the /var/log directory. Ensure that it has its own partition or logical volume at installation time, or migrate it using LVM.

Placing /var/log in its own partition enables better separation between log files and other files in /var/.

3.3.1.d. Ensure /var/log/audit Located On Separate Partition

Audit logs are stored in the /var/log/audit directory. Ensure that it has its own partition or logical volume at installation time, or migrate it later using LVM. Make absolutely certain that it is large enough to store all audit logs that will be created by the auditing daemon.

Placing /var/log/audit in its own partition enables better separation between audit files and other files, and helps ensure that auditing cannot be halted due to the partition running out of space.

3.3.1.e. Ensure /home Located On Separate Partition

If user home directories will be stored locally, create a separate partition for /home at installation time (or migrate it later using LVM). If /home will be mounted from another system such as an NFS server, then creating a separate partition is not necessary at installation time, and the mountpoint can instead be configured later.

Ensuring that /home is mounted on its own partition enables the setting of more restrictive mount options, and also helps ensure that users cannot trivially fill partitions used for log or audit data storage.

3.3.1.f. Ensure /srv Located On Separate Partition

If a file server (FTP, TFTP...) is hosted locally, create a separate partition for /srv at installation time (or migrate it later using LVM). If /srv will be mounted from another system such as an NFS server, then creating a separate partition is not necessary at installation time, and the mountpoint can instead be configured later.

Srv deserves files for local network file server such as FTP. Ensuring that /srv is mounted on its own partition enables the setting of more restrictive mount options, and also helps ensure that users cannot trivially fill partitions used for log or audit data storage.

3.3.2. filesystem rights management

Adding 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.

3.4. Account and Access Control

In 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.

3.4.1. Protect Accounts by Restricting Password-Based Login

Conventionally, Unix shell accounts are accessed by providing a username and password to a login program, which tests these values for correctness using the /etc/passwd and /etc/shadow files. Password-based login is vulnerable to guessing of weak passwords, and to sniffing and man-in-the-middle attacks against passwords entered over a network or at an insecure console. Therefore, mechanisms for accessing accounts by entering usernames and passwords should be restricted to those which are operationally necessary.

3.5. Configure Syslog

The 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.

In Ubuntu 1604, rsyslog has replaced syslog as the syslog daemon of choice, and it includes some additional security features such as reliable, connection-oriented (i.e. TCP) transmission of logs, the option to log to database formats, and the encryption of log data en route to a central logging server. This section discusses how to configure rsyslog for best effect, and how to use tools provided with the system to maintain and monitor logs.

3.5.a. Ensure rsyslog is Installed

Rsyslog is installed by default. The rsyslog package can be installed with the following command:

# apt-get install rsyslog

The rsyslog package provides the rsyslog daemon, which provides system logging services.

Remediation script

                  # Include source function library.

apt-get install rsyslog

                

3.5.b. Enable rsyslog Service

The rsyslog service provides syslog-style logging by default on Ubuntu 1604. The rsyslog service can be enabled with the following command:

$ sudo chkconfig --level 2345 rsyslog on

The rsyslog service must be running in order to provide logging services, which are essential to system administration.

3.5.3. Ensure Proper Configuration of Log Files

The file /etc/rsyslog.conf controls where log message are written. These are controlled by lines called rules, which consist of a selector and an action. These rules are often customized depending on the role of the system, the requirements of the environment, and whatever may enable the administrator to most effectively make use of log data. The default rules in Ubuntu 1604 are:

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.notice
See 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

3.5.3.a. Ensure Log Files Are Owned By Appropriate User

The owner of all log files written by rsyslog should be root. These log files are determined by the second part of each Rule line in /etc/rsyslog.conf and typically all appear in /var/log. For each log file LOGFILE referenced in /etc/rsyslog.conf, run the following command to inspect the file's owner:

$ ls -l LOGFILE
If the owner is not root, run the following command to correct this:
$ sudo chown root LOGFILE

The log files generated by rsyslog contain valuable information regarding system configuration, user authentication, and other such information. Log files should be protected from unauthorized access.

3.5.3.b. Ensure Log Files Are Owned By Appropriate Group

The group-owner of all log files written by rsyslog should be root. These log files are determined by the second part of each Rule line in /etc/rsyslog.conf and typically all appear in /var/log. For each log file LOGFILE referenced in /etc/rsyslog.conf, run the following command to inspect the file's group owner:

$ ls -l LOGFILE
If the owner is not adm, run the following command to correct this:
$ sudo chgrp adm LOGFILE

The log files generated by rsyslog contain valuable information regarding system configuration, user authentication, and other such information. Log files should be protected from unauthorized access.

3.5.3.c. Ensure System Log Files Have Correct Permissions

The file permissions for all log files written by rsyslog should be set to 640, or more restrictive. These log files are determined by the second part of each Rule line in /etc/rsyslog.conf and typically all appear in /var/log. For each log file LOGFILE referenced in /etc/rsyslog.conf, run the following command to inspect the file's permissions:

$ ls -l LOGFILE
If the permissions are not 640 or more restrictive, run the following command to correct this:
$ sudo chmod 0640 LOGFILE

Log files can contain valuable information regarding system configuration. If the system log files are not protected unauthorized users could change the logged data, eliminating their forensic value.

3.5.4. Rsyslog Logs Sent To Remote Host

If system logs are to be useful in detecting malicious activities, it is necessary to send logs to a remote server. An intruder who has compromised the root account on a machine may delete the log entries which indicate that the system was attacked before they are seen by an administrator.

However, it is recommended that logs be stored on the local host in addition to being sent to the loghost, especially if rsyslog has been configured to use the UDP protocol to send messages over a network. UDP does not guarantee reliable delivery, and moderately busy sites will lose log messages occasionally, especially in periods of high traffic which may be the result of an attack. In addition, remote rsyslog messages are not authenticated in any way by default, so it is easy for an attacker to introduce spurious messages to the central log server. Also, some problems cause loss of network connectivity, which will prevent the sending of messages to the central server. For all of these reasons, it is better to store log messages both centrally and on each host, so that they can be correlated if necessary.

3.5.4.a. Ensure Logs Sent To Remote Host

To configure rsyslog to send logs to a remote log server, open /etc/rsyslog.conf and read and understand the last section of the file, which describes the multiple directives necessary to activate remote logging. Along with these other directives, the system can be configured to forward its logs to a particular log server by adding or correcting one of the following lines, substituting loghost.example.com appropriately. The choice of protocol depends on the environment of the system; although TCP and RELP provide more reliable message delivery, they may not be supported in all environments.
To use UDP for log message delivery:

*.* @loghost.example.com

To use TCP for log message delivery:
*.* @@loghost.example.com

To use RELP for log message delivery:
*.* :omrelp:loghost.example.com

A log server (loghost) receives syslog messages from one or more systems. This data can be used as an additional log source in the event a system is compromised and its local logs are suspect. Forwarding log messages to a remote loghost also provides system administrators with a centralized place to view the status of multiple hosts within the enterprise.

3.5.5. Configure rsyslogd to Accept Remote Messages If Acting as a Log Server

By default, rsyslog does not listen over the network for log messages. If needed, modules can be enabled to allow the rsyslog daemon to receive messages from other systems and for the system thus to act as a log server. If the machine is not a log server, then lines concerning these modules should remain commented out.

3.5.5.a. Enable rsyslog to Accept Messages via TCP, if Acting As Log Server

The rsyslog daemon should not accept remote messages unless the system acts as a log server. If the system needs to act as a central log server, add the following lines to /etc/rsyslog.conf to enable reception of messages over TCP:

$ModLoad imtcp
$InputTCPServerRun 514

If the system needs to act as a log server, this ensures that it can receive messages over a reliable TCP connection.

3.5.5.b. Enable rsyslog to Accept Messages via UDP, if Acting As Log Server

The rsyslog daemon should not accept remote messages unless the system acts as a log server. If the system needs to act as a central log server, add the following lines to /etc/rsyslog.conf to enable reception of messages over UDP:

$ModLoad imudp
$UDPServerRun 514

Many devices, such as switches, routers, and other Unix-like systems, may only support the traditional syslog transmission over UDP. If the system must act as a log server, this enables it to receive their messages as well.

3.5.6. Ensure All Logs are Rotated by logrotate

Edit the file /etc/logrotate.d/rsyslog. Find the first line, which should look like this (wrapped for clarity):

/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.

3.5.6.a. Ensure Logrotate Runs Periodically

The logrotate utility allows for the automatic rotation of log files. The frequency of rotation is specified in /etc/logrotate.conf, which triggers a cron task. To configure logrotate to run daily, add or correct the following line in /etc/logrotate.conf:

# rotate log files frequency
daily

Log files that are not properly rotated run the risk of growing so large that they fill up the /var/log partition. Valuable logging information could be lost if the /var/log partition becomes full.

4. Services

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.

Ubuntu 1604 provides a convenient minimal install option that essentially installs the bare necessities for a functional system. When building Ubuntu 1604 systems, it is highly recommended to select the minimal packages and then build up the system from there.

4.1. Deprecated services

Some 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.

4.1.a. Uninstall the telnet server

The telnet daemon should be uninstalled.

telnet allows clear text communications, and does not protect any data transmission between client and server. Any confidential data can be listened and no integrity checking is made.

Remediation script

                  # CAUTION: This remediation script will remove telnetd
#	   from the system, and may remove any packages
#	   that depend on telnetd. Execute this
#	   remediation AFTER testing on a non-production
#	   system!

apt-get remove --purge telnetd

                

Security identifiers

  • CCE-

4.1.b. Uninstall the inet-based telnet server

The inet-based telnet daemon should be uninstalled.

telnet allows clear text communications, and does not protect any data transmission between client and server. Any confidential data can be listened and no integrity checking is made.

Remediation script

                  # CAUTION: This remediation script will remove inetutils-telnetd
#	   from the system, and may remove any packages
#	   that depend on inetutils-telnetd. Execute this
#	   remediation AFTER testing on a non-production
#	   system!

apt-get remove --purge inetutils-telnetd

                

Security identifiers

  • CCE-

4.1.c. Uninstall the ssl compliant telnet server

The telnet daemon, even with ssl support, should be uninstalled.

telnet, even with ssl support, should not be installed. When remote shell is required, up-to-date ssh daemon can be used.

Remediation script

                  # CAUTION: This remediation script will remove telnetd-ssl
#	   from the system, and may remove any packages
#	   that depend on telnetd-ssl. Execute this
#	   remediation AFTER testing on a non-production
#	   system!

apt-get remove --purge telnetd-ssl

                

Security identifiers

  • CCE-

4.1.d. Uninstall the nis package

The support for Yellowpages should not be installed unless it is required.

NIS is the historical SUN service for central account management, more and more replaced by LDAP. NIS does not support efficiently security constraints, ACL, etc. and should not be used.

Remediation script

                  # CAUTION: This remediation script will remove nis
#	   from the system, and may remove any packages
#	   that depend on nis. Execute this
#	   remediation AFTER testing on a non-production
#	   system!

apt-get remove --purge nis

                

Security identifiers

  • CCE-

4.1.e. Uninstall the ntpdate package

ntpdate is a historical ntp synchronization client for unixes. It sould be uninstalled.

ntpdate is an old not security-compliant ntp client. It should be replaced by modern ntp clients such as ntpd, able to use cryptographic mechanisms integrated in NTP.

Remediation script

                  # CAUTION: This remediation script will remove ntpdate
#	   from the system, and may remove any packages
#	   that depend on ntpdate. Execute this
#	   remediation AFTER testing on a non-production
#	   system!

apt-get remove --purge ntpdate

                

Security identifiers

  • CCE-

4.2. Generic required services

Some 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.

4.2.a. install the auditd service

The auditd service should be installed.

The auditd service is an access monitoring and accounting daemon, watching system calls to audit any access, in comparision with potential local access control policy such as SELinux policy.

Remediation script

                  # Include source function library.

apt-get install auditd

                

Security identifiers

  • CCE-

4.2.b. Enable the auditd service

The auditd service should be enabled.

The auditd service is an access monitoring and accounting daemon, watching system calls to audit any access, in comparision with potential local access control policy such as SELinux policy.

Security identifiers

  • CCE-

References

  1. NT28(R50). URL: <http://www.ssi.gouv.fr/administration/bonnes-pratiques/>.
  2. AC-17(1). URL: <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf>.
  3. AU-1(b). URL: <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf>.
  4. AU-10. URL: <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf>.
  5. AU-12(a). URL: <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf>.
  6. AU-12(c). URL: <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf>.
  7. IR-5. URL: <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r4.pdf>.
  8. 347. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  9. 157. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  10. 172. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  11. 880. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  12. 1353. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  13. 1462. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  14. 1487. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  15. 1115. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  16. 1454. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  17. 067. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  18. 158. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  19. 831. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  20. 1190. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  21. 1312. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  22. 1263. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  23. 130. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  24. 120. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  25. 1589. URL: <http://iase.disa.mil/stigs/cci/Pages/index.aspx>.
  26. Req-10. URL: <https://www.pcisecuritystandards.org/documents/PCI_DSS_v3-1.pdf>.

4.2.c. Install the cron service

The Cron service should be installed.

The cron service allow periodic job execution, needed for almost all administrative tasks and services (software update, log rotating, etc.). Access to cron service should be restricted to administrative accounts only.

Remediation script

                  # Include source function library.

apt-get install cron

                

Security identifiers

  • CCE-

4.2.d. Enable the cron service

The Cron service should be enabled.

The cron service allow periodic job execution, needed for almost all administrative tasks and services (software update, log rotating, etc.). Access to cron service should be restricted to administrative accounts only.

Security identifiers

  • CCE-

4.2.e. Install the ntp service

The ntpd service should be installed.

Time synchronization (using NTP) is required by almost all network and administrative tasks (syslog, cryptographic based services (authentication, etc.), etc.). Ntpd is regulary maintained and updated, supporting security features such as RFC 5906.

Remediation script

                  # Include source function library.

apt-get install ntp

                

Security identifiers

  • CCE-

4.2.f. Enable the ntpd service

The ntpd service should be enabled.

Time synchronization (using NTP) is required by almost all network and administrative tasks (syslog, cryptographic based services (authentication, etc.), etc.). Ntpd is regulary maintained and updated, supporting security features such as RFC 5906.

Security identifiers

  • CCE-

4.3. SSH Server

The 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:
Cryptography usage, according to the current CVEs associated to the various cryptographic modes
Authentication and autorization, depending on your needs but requiring some specific initial generic security
consideration in the OpenSSH configuration writing More detailed information is available from the OpenSSH project's website http://www.openssh.org. The Ubuntu package for server side implementation is called openssh-server.

4.3.a. Disable SSH Server if possible (unusual cases)

Most of the time, the SSH server is needed. However, it can be disabled, do so. The ssh service can be disabled with the following command:

$ sudo chkconfig ssh off
This is unusual, as SSH is a common method for encrypted and authenticated remote access.

4.3.2. Configure OpenSSH Server if deployed

If the system needs to act as an SSH server, then certain changes should be made to the OpenSSH daemon configuration file /etc/ssh/sshd_config. The following recommendations can be applied to this file. See the sshd_config(5) man page for more detailed information.

4.3.2.a. Allow Only SSH Protocol 2

Only SSH protocol version 2 connections should be permitted. The default setting in /etc/ssh/sshd_config is correct, and can be verified by ensuring that the following line appears:

Protocol 2

SSH protocol version 1 suffers from design flaws that result in security vulnerabilities and should not be used.

Security identifiers

  • CCE-

4.3.2.b. Set SSH Idle Timeout Interval

SSH allows administrators to set an idle timeout interval. After this interval has passed, the idle user will be automatically logged out.

To set an idle timeout interval, edit the following line in /etc/ssh/sshd_config as follows:

ClientAliveInterval interval
The timeout interval is given in seconds. To have a timeout of 15 minutes, set interval to 900.

If a shorter timeout has already been set for the login shell, that value will preempt any SSH setting made here. Keep in mind that some processes may stop SSH from correctly detecting that the user is idle.

Causing idle users to be automatically logged out guards against compromises one system leading trivially to compromises on another.

Security identifiers

  • CCE-

4.3.2.c. Set SSH Client Alive Count

To ensure the SSH idle timeout occurs precisely when the ClientAliveCountMax is set, edit /etc/ssh/sshd_config as follows:

ClientAliveCountMax 0

This ensures a user login will be terminated as soon as the ClientAliveCountMax is reached.

Security identifiers

  • CCE-

4.3.2.d. Disable SSH Root Login

The root user should never be allowed to login to a system directly over a network. To disable root login via SSH, add or correct the following line in /etc/ssh/sshd_config:

PermitRootLogin no

Permitting direct root login reduces auditable information about who ran privileged commands on the system and also allows direct attack attempts on root's password.

Security identifiers

  • CCE-

4.3.2.e. Disable SSH Access via Empty Passwords

To explicitly disallow remote login from accounts with empty passwords, add or correct the following line in /etc/ssh/sshd_config:

PermitEmptyPasswords no
Any accounts with empty passwords should be disabled immediately, and PAM configuration should prevent users from being able to assign themselves empty passwords.

Configuring this setting for the SSH daemon provides additional assurance that remote login via SSH will require a password, even in the event of misconfiguration elsewhere.

Security identifiers

  • CCE-

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