Guide to the Secure Configuration of Debian 8

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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.
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Revision History

Current version: 0.1.38

  • draft (as of 2018-03-05)

Platforms

  • cpe:/o:debianproject:debian:8

Table of Contents

  1. System Settings
    1. File Permissions and Masks
    2. Access Control using sudo
    3. Configure Syslog
  2. Services
    1. Deprecated services
    2. APT service configuration

Checklist

contains 16 rules

System Settings   [ref]group

Contains rules that check correct system settings.

contains 10 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 4 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 4 rules
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, Req-8.7.c, NT28(R36)

Remediation Shell script:   (show)

Complexity:low
Disruption:low
Strategy:configure
chmod 0640 /etc/shadow
Remediation Ansible snippet:   (show)

Complexity:low
Disruption:low
Strategy:configure
- name: Ensure permission 0640 on /etc/shadow
  file:
    path="{{item}}"
    mode=0640
  with_items:
    - /etc/shadow
  tags:
    - file_permissions_etc_shadow
    - medium_severity
    - configure_strategy
    - low_complexity
    - low_disruption
    - NIST-800-53-AC-6
    - PCI-DSS-Req-8.7.c

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, NT28(R36)

Remediation Shell script:   (show)

Complexity:low
Disruption:low
Strategy:configure
chmod 0640 /etc/gshadow
Remediation Ansible snippet:   (show)

Complexity:low
Disruption:low
Strategy:configure
- name: Ensure permission 0640 on /etc/gshadow
  file:
    path="{{item}}"
    mode=0640
  with_items:
    - /etc/gshadow
  tags:
    - file_permissions_etc_gshadow
    - medium_severity
    - configure_strategy
    - low_complexity
    - low_disruption
    - NIST-800-53-AC-6

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

Remediation Shell script:   (show)

Complexity:low
Disruption:low
Strategy:configure
chmod 0644 /etc/passwd
Remediation Ansible snippet:   (show)

Complexity:low
Disruption:low
Strategy:configure
- name: Ensure permission 0644 on /etc/passwd
  file:
    path="{{item}}"
    mode=0644
  with_items:
    - /etc/passwd
  tags:
    - file_permissions_etc_passwd
    - medium_severity
    - configure_strategy
    - low_complexity
    - low_disruption
    - NIST-800-53-AC-6

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

Remediation Shell script:   (show)

Complexity:low
Disruption:low
Strategy:configure
chmod 0644 /etc/group
Remediation Ansible snippet:   (show)

Complexity:low
Disruption:low
Strategy:configure
- name: Ensure permission 0644 on /etc/group
  file:
    path="{{item}}"
    mode=0644
  with_items:
    - /etc/group
  tags:
    - file_permissions_etc_group
    - medium_severity
    - configure_strategy
    - low_complexity
    - low_disruption
    - NIST-800-53-AC-6

Access Control using sudo   [ref]group

Sudo, which stands for "su 'do'", provides the ability to delegate authority to certain users, groups of users, or system administrators. When configured for system users and/or groups, Sudo can allow a user or group to execute privileged commands that normally only root is allowed to execute.

For more information on Sudo and addition Sudo configuration options, see https://www.sudo.ws

contains 2 rules

Ensure Users Re-Authenticate for Privilege Escalation - sudo NOPASSWD   [ref]rule

The sudo NOPASSWD tag, when specified, allows a user to execute commands using sudo without having to authenticate. This should be disabled by making sure that the NOPASSWD tag does not exist in /etc/sudoers configuration file or any sudo configuration snippets in /etc/sudoers.d/.

Rationale:

Without re-authentication, users may access resources or perform tasks for which they do not have authorization.

When operating systems provide the capability to escalate a functional capability, it is critical that the user re-authenticate.

Severity:  medium

Ensure Users Re-Authenticate for Privilege Escalation - sudo !authenticate   [ref]rule

The sudo !authenticate option, when specified, allows a user to execute commands using sudo without having to authenticate. This should be disabled by making sure that the !authenticate option does not exist in /etc/sudoers configuration file or any sudo configuration snippets in /etc/sudoers.d/.

Rationale:

Without re-authentication, users may access resources or perform tasks for which they do not have authorization.

When operating systems provide the capability to escalate a functional capability, it is critical that the user re-authenticate.

Severity:  medium

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. Yet, it is also possible to install syslog-ng to manage local and server-side logging services. Both are tested here, to let the administrator choose.

contains 4 rules

Configure rsyslogd to Accept Remote Messages If Acting as a Log Server   [ref]group

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.

contains 2 rules

Ensure syslog-ng is Installed   [ref]rule

syslog-ng can be installed in replacement of rsyslog. The syslog-ng-core package can be installed with the following command:

# apt-get install syslog-ng-core

Rationale:

The syslog-ng-core package provides the syslog-ng daemon, which provides system logging services.

Severity:  medium

Enable syslog-ng Service   [ref]rule

The syslog-ng service (in replacement of rsyslog) provides syslog-style logging by default on Debian 8. The syslog-ng service can be enabled with the following command:

$ sudo chkconfig --level 2345 syslog-ng on

Rationale:

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

Severity:  medium

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

Remediation Shell script:   (show)

Complexity:low
Disruption:low
Strategy:enable
# Include source function library.

apt-get install rsyslog
Remediation Ansible snippet:   (show)

Complexity:low
Disruption:low
Strategy:enable
- name: Ensure rsyslog is installed
  package:
    name="{{item}}"
    state=present
  with_items:
    - rsyslog
  tags:
    - package_rsyslog_installed
    - medium_severity
    - enable_strategy
    - low_complexity
    - low_disruption
    - NIST-800-53-AU-9(2)

Remediation Puppet snippet:   (show)

Complexity:low
Disruption:low
Strategy:enable
include install_rsyslog

class install_rsyslog {
  package { 'rsyslog':
    ensure => 'installed',
  }
}

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

Remediation Ansible snippet:   (show)

Complexity:low
Disruption:low
Strategy:enable
- name: Enable service rsyslog
  service:
    name="{{item}}"
    enabled="yes"
    state="started"
  with_items:
    - rsyslog
  tags:
    - service_rsyslog_enabled
    - medium_severity
    - enable_strategy
    - low_complexity
    - low_disruption
    - NIST-800-53-AU-4(1)
    - NIST-800-53-AU-12

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 6 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 4 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, NT007(R03)

Remediation Shell script:   (show)

Complexity:low
Disruption:low
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
Strategy:disable
- name: Ensure telnetd is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - telnetd
  tags:
    - package_telnetd_removed
    - high_severity
    - disable_strategy
    - low_complexity
    - low_disruption
    - CCE-
    - NIST-800-53-AC-17(8)
    - NIST-800-53-CM-7

Remediation Puppet snippet:   (show)

Complexity:low
Disruption:low
Strategy:disable
include remove_telnetd

class remove_telnetd {
  package { 'telnetd':
    ensure => 'purged',
  }
}

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, NT007(R03)

Remediation Shell script:   (show)

Complexity:low
Disruption:low
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
Strategy:disable
- name: Ensure inetutils-telnetd is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - inetutils-telnetd
  tags:
    - package_inetutils-telnetd_removed
    - high_severity
    - disable_strategy
    - low_complexity
    - low_disruption
    - CCE-
    - NIST-800-53-AC-17(8)
    - NIST-800-53-CM-7

Remediation Puppet snippet:   (show)

Complexity:low
Disruption:low
Strategy:disable
include remove_inetutils-telnetd

class remove_inetutils-telnetd {
  package { 'inetutils-telnetd':
    ensure => 'purged',
  }
}

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
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
Strategy:disable
- name: Ensure telnetd-ssl is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - telnetd-ssl
  tags:
    - package_telnetd-ssl_removed
    - high_severity
    - disable_strategy
    - low_complexity
    - low_disruption
    - CCE-
    - NIST-800-53-AC-17(8)
    - NIST-800-53-CM-7

Remediation Puppet snippet:   (show)

Complexity:low
Disruption:low
Strategy:disable
include remove_telnetd-ssl

class remove_telnetd-ssl {
  package { 'telnetd-ssl':
    ensure => 'purged',
  }
}

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
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
Strategy:disable
- name: Ensure nis is removed
  package:
    name="{{item}}"
    state=absent
  with_items:
    - nis
  tags:
    - package_nis_removed
    - low_severity
    - disable_strategy
    - low_complexity
    - low_disruption
    - CCE-

Remediation Puppet snippet:   (show)

Complexity:low
Disruption:low
Strategy:disable
include remove_nis

class remove_nis {
  package { 'nis':
    ensure => 'purged',
  }
}

APT service configuration   [ref]group

The 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.

contains 2 rules

Disable unauthenticated repositories in APT configuration   [ref]rule

Unauthenticated repositories should not be used for updates.

Rationale:

Repositories hosts all packages that will be intsalled on the system during update. If a repository is not authenticated, the associated packages can't be trusted, and then should not be installed localy.

Severity:  low

References:  NT28(R15)

Ensure that official distribution repositories are used   [ref]rule

Check that official Debian repositories, including security repository, are configured in apt.

Rationale:

The Debian distribution deliver DSA (Debian Security Announce), through the official Debian security repository, to correct various vulnerabilities impacting the Debian packages. Using the official repositories is the best way to ensure that the Debian updates are integrated soon enough.

Severity:  low

References:  NT28(R15)

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