Guide to the Secure Configuration of Debian 8

with profile Profile for ANSSI DAT-NT28 Average (Intermediate) Level
This profile contains items for GNU/Linux installations already protected by multiple higher level security stacks.

The SCAP Security Guide Project
https://www.open-scap.org/security-policies/scap-security-guide

This guide presents a catalog of security-relevant configuration settings for Debian 8. 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 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 Debian 8, which provides required settings for US Department of Defense systems, is one example of a baseline created from this guidance.

Do not attempt to implement any of the settings in this guide without first testing them in a non-operational environment. The creators of this guidance assume no responsibility whatsoever for its use by other parties, and makes no guarantees, expressed or implied, about its quality, reliability, or any other characteristic.

Profile Title	Profile for ANSSI DAT-NT28 Average (Intermediate) Level
Profile ID	xccdf_org.ssgproject.content_profile_anssi_np_nt28_average

Revision History

Current version: 0.1.31

draft (as of 2017-08-11)

Platforms

cpe:/o:debianproject:debian:8

System Settings

File Permissions and Masks
Hardening the filesystem
Configure Syslog

Services

Deprecated services
Generic required services
SSH Server

Checklist

contains 31 rules

System Settings [ref]group

Contains rules that check correct system settings.

contains 20 rules

File Permissions and Masks [ref]group

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.

contains 7 rules

Verify Permissions on Important Files and Directories [ref]group

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.

contains 5 rules

Verify permissions on files containing sensitive informations about the system [ref]group

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

contains 1 rule

Verify that local System.map file (if exists) is readable only by root [ref]rule

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-*

Rationale:

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

Severity: low

References: NT28(R13)

Verify Permissions on Files with Local Account Information and Credentials [ref]group

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.

contains 4 rules

Verify Permissions and ownership on shadow File [ref]rule

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

Rationale:

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.

Severity: medium

References: AC-6, http://iase.disa.mil/stigs/cci/Pages/index.aspx, Req-8.7.c, NT28(R36)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

chmod 0640 /etc/shadow

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

- name: Ensure permission 0640 on /etc/shadow
  file:
    path="{{item}}"
    mode=0640
  with_items:
    - /etc/shadow

Verify Permissions and ownership on gshadow File [ref]rule

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

Rationale:

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

Severity: medium

References: AC-6, http://iase.disa.mil/stigs/cci/Pages/index.aspx, NT28(R36)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

chmod 0640 /etc/gshadow

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

- name: Ensure permission 0640 on /etc/gshadow
  file:
    path="{{item}}"
    mode=0640
  with_items:
    - /etc/gshadow

Verify Permissions and ownership on passwd File [ref]rule

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

Rationale:

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

Severity: medium

References: AC-6, http://iase.disa.mil/stigs/cci/Pages/index.aspx

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

chmod 0644 /etc/passwd

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

- name: Ensure permission 0644 on /etc/passwd
  file:
    path="{{item}}"
    mode=0644
  with_items:
    - /etc/passwd

Verify Permissions and ownership on group File [ref]rule

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

Rationale:

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

Severity: medium

References: AC-6, http://iase.disa.mil/stigs/cci/Pages/index.aspx

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

chmod 0644 /etc/group

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	configure

- name: Ensure permission 0644 on /etc/group
  file:
    path="{{item}}"
    mode=0644
  with_items:
    - /etc/group

Restrict Programs from Dangerous Execution Patterns [ref]group

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.

contains 2 rules

Disable Core Dumps [ref]group

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.

contains 1 rule

Disable Core Dumps for SUID programs [ref]rule

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

Rationale:

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.

Severity: low

References: SI-11, 1.6.1, NT28(R23)

Enable ExecShield [ref]group

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.

contains 1 rule

Enable Randomized Layout of Virtual Address Space [ref]rule

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

Rationale:

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.

Severity: medium

References: SC-30(2), 1.6.1, NT28(R23)

Hardening the filesystem [ref]group

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.

contains 7 rules

Partitioning [ref]group

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: Filesystem Hierarchy Standard

contains 5 rules

Ensure /tmp Located On Separate Partition [ref]rule

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.

Rationale:

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.

Severity: low

References: SC-32, NT28(R12)

Ensure /var Located On Separate Partition [ref]rule

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.

Rationale:

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.

Severity: low

References: SC-32, NT28(R12)

Ensure /var/log Located On Separate Partition [ref]rule

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.

Rationale:

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

Severity: low

References: AU-9, SC-32, http://iase.disa.mil/stigs/cci/Pages/index.aspx, NT28(R12), NT28(R47)

Ensure /var/log/audit Located On Separate Partition [ref]rule

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.

Rationale:

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.

Severity: low

References: AU-4, AU-9, SC-32, http://iase.disa.mil/stigs/cci/Pages/index.aspx

Ensure /home Located On Separate Partition [ref]rule

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.

Rationale:

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.

Severity: low

References: SC-32, 1208, NT28(R12)

filesystem rights management [ref]group

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.

contains 2 rules

Disallow creating symlinks to a file you not own [ref]rule

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

$ sudo sysctl -w fs.protected_symlinks=1

If this is not the system's default value, add the following line to /etc/sysctl.conf:

fs.protected_symlinks = 1

Rationale:

Disallowing such symlink mitigate vulnerabilities based on insecure file system accessed by privilegied programs, avoiding an exploitation vector exploiting unsafe use of open() or creat().

Severity: low

References: SI-11, 1.6.1, NT28(R23)

Disallow creating symlinks to a file you not own [ref]rule

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

$ sudo sysctl -w fs.protected_hardlinks=1

If this is not the system's default value, add the following line to /etc/sysctl.conf:

fs.protected_hardlinks = 1

Rationale:

Disallowing such hardlink mitigate vulnerabilities based on insecure file system accessed by privilegied programs, avoiding an exploitation vector exploiting unsafe use of open() or creat().

Severity: low

References: SI-11, 1.6.1, NT28(R23)

Configure Syslog [ref]group

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 Debian Jessie, 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.

contains 6 rules

Ensure Proper Configuration of Log Files [ref]group

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 Debian 8 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

contains 3 rules

Ensure Log Files Are Owned By Appropriate User [ref]rule

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

Rationale:

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.

Severity: medium

References: AC-6, SI-11, 1314, Req-10.5.1, Req-10.5.2, NT28(R46)

Ensure Log Files Are Owned By Appropriate Group [ref]rule

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

Rationale:

Severity: medium

References: AC-6, SI-11, 1314, Req-10.5.1, Req-10.5.2, NT28(R46)

Ensure System Log Files Have Correct Permissions [ref]rule

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

Rationale:

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.

Severity: medium

References: SI-11, 1314, Req-10.5.1, Req-10.5.2, 5.1.4, NT28(R36)

Ensure All Logs are Rotated by logrotate [ref]group

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.

contains 1 rule

Ensure Logrotate Runs Periodically [ref]rule

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

Rationale:

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.

Severity: low

References: AU-9, 366, Req-10.7

Ensure rsyslog is Installed [ref]rule

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

# apt-get install rsyslog

Rationale:

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

Severity: medium

References: AU-9(2), 1311, 1312, 5.1.1, NT28(R46)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# Include source function library.

apt-get install rsyslog

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	enable

- name: Ensure rsyslog is installed
  package:
    name="{{item}}"
    state=present
  with_items:
    - rsyslog

Enable rsyslog Service [ref]rule

The rsyslog service provides syslog-style logging by default on Debian 8. The rsyslog service can be enabled with the following command:

$ sudo chkconfig --level 2345 rsyslog on

Rationale:

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

Severity: medium

References: AU-4(1), AU-12, 1311, 1312, 1557, 1851, 5.1.2, NT28(R46)

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	enable

- name: Enable service rsyslog
  service:
    name="{{item}}"
    enabled="yes"
    state="started"
  with_items:
    - rsyslog

Services [ref]group

The best protection against vulnerable software is running less software. This section describes how to review the software which Debian 8 installs on a system and disable software which is not needed. It then enumerates the software packages installed on a default Debian 8 system and provides guidance about which ones can be safely disabled.

Debian 8 provides a convenient minimal install option that essentially installs the bare necessities for a functional system. When building Debian 8 systems, it is highly recommended to select the minimal packages and then build up the system from there.

contains 11 rules

Deprecated services [ref]group

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.

contains 5 rules

Uninstall the telnet server [ref]rule

The telnet daemon should be uninstalled.

Rationale:

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.

Severity: high

Identifiers: CCE-

References: AC-17(8), CM-7, http://iase.disa.mil/stigs/cci/Pages/index.aspx, NT007(R03)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# 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

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

- name: Ensure telnetd is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - telnetd

Uninstall the inet-based telnet server [ref]rule

The inet-based telnet daemon should be uninstalled.

Rationale:

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.

Severity: high

Identifiers: CCE-

References: AC-17(8), CM-7, http://iase.disa.mil/stigs/cci/Pages/index.aspx, NT007(R03)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# 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

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

- name: Ensure inetutils-telnetd is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - inetutils-telnetd

Uninstall the ssl compliant telnet server [ref]rule

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

Rationale:

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

Severity: high

Identifiers: CCE-

References: AC-17(8), CM-7, NT007(R02)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# 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

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

- name: Ensure telnetd-ssl is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - telnetd-ssl

Uninstall the nis package [ref]rule

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

Rationale:

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.

Severity: low

Identifiers: CCE-

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# 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

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

- name: Ensure nis is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - nis

Uninstall the ntpdate package [ref]rule

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

Rationale:

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.

Severity: low

Identifiers: CCE-

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# 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

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

- name: Ensure ntpdate is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - ntpdate

Generic required services [ref]group

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.

contains 1 rule

Install the ntp service [ref]rule

The ntpd service should be installed.

Rationale:

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.

Severity: high

Identifiers: CCE-

References: AU-8(1), 160, Req-10.4, NT012(R03)

Remediation Shell script: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	disable

# Include source function library.

apt-get install ntp

Remediation Ansible snippet: (show)

Complexity:	low
Disruption:	low
Reboot:	false
Strategy:	enable

- name: Ensure ntp is installed
  package:
    name="{{item}}"
    state=present
  with_items:
    - ntp

SSH Server [ref]group

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 Debian package for server side implementation is called openssh-server.

contains 5 rules

Configure OpenSSH Server if deployed [ref]group

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.

contains 5 rules

Allow Only SSH Protocol 2 [ref]rule

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

Rationale:

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

Severity: high

Identifiers: CCE-

References: AC-17(7), IA-5(1)(c), NT007(R1)

Set SSH Idle Timeout Interval [ref]rule

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.

Rationale:

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

Severity: low

Identifiers: CCE-

References: AC-2(5), SA-8, Req-8.1.8

Set SSH Client Alive Count [ref]rule

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

ClientAliveCountMax 0

Rationale:

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

Severity: low

Identifiers: CCE-

References: AC-2(5), SA-8

Disable SSH Root Login [ref]rule

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

Rationale:

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.

Severity: medium

Identifiers: CCE-

References: AC-3, AC-6(2), IA-2(1), NT007(R21)

Disable SSH Access via Empty Passwords [ref]rule

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.

Rationale:

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.

Severity: high

Identifiers: CCE-

References: AC-3, NT007(R17)

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