## Guide to the Secure Configuration of Debian 8

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 ID | (default)

## Revision History

Current version: 0.1.31

- draft (as of 2017-08-11)

## Platforms

- cpe:/o:debianproject:debian:8

## Table of Contents

1. Remediation functions used by the SCAP Security Guide Project
2. Introduction

1. General Principles
2. How to Use This Guide

3. System Settings

1. Installing and Maintaining Software
2. File Permissions and Masks
3. Hardening the filesystem
4. Account and Access Control
5. Configure Syslog

4. Services

1. Deprecated services
2. Generic required services
3. SSH Server

## Checklist

### Remediation functions used by the SCAP Security Guide Project [ref]group

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

### Introduction [ref]group

The purpose of this guidance is to provide security configuration recommendations and baselines for the Debian 8 operating system. Recommended settings for the basic operating system are provided, as well as for many network services that the system can provide to other systems. The guide is intended for system administrators. Readers are assumed to possess basic system administration skills for Unix-like systems, as well as some familiarity with the product's documentation and administration conventions. Some instructions within this guide are complex. All directions should be followed completely and with understanding of their effects in order to avoid serious adverse effects on the system and its security.

### General Principles [ref]group

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

### Encrypt Transmitted Data Whenever Possible [ref]group

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 Debian 8 machines can and should be configured so that no unencrypted authentication data is ever transmitted between machines.

### Minimize Software to Minimize Vulnerability [ref]group

The simplest way to avoid vulnerabilities in software is to avoid installing that software. On Debian 8, the Package Manager (originally apt) allows for careful management of the set of software packages installed on a system. Installed software contributes to system vulnerability in several ways. Packages that include setuid programs may provide local attackers a potential path to privilege escalation. Packages that include network services may give this opportunity to network-based attackers. Packages that include programs which are predictably executed by local users (e.g. after graphical login) may provide opportunities for trojan horses or other attack code to be run undetected. The number of software packages installed on a system can almost always be significantly pruned to include only the software for which there is an environmental or operational need.

### Run Different Network Services on Separate Systems [ref]group

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.

### Configure Security Tools to Improve System Robustness [ref]group

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.

### Least Privilege [ref]group

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.

### How to Use This Guide [ref]group

Readers should heed the following points when using the guide.

### Read Sections Completely and in Order [ref]group

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.

### Test in Non-Production Environment [ref]group

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.

### Root Shell Environment Assumed [ref]group

Most of the actions listed in this document are written with the assumption that they will be executed by the root user running the `/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.

### Formatting Conventions [ref]group

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.

### Reboot Required [ref]group

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.

### System Settings [ref]group

Contains rules that check correct system settings.

### Installing and Maintaining Software [ref]group

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

### File Permissions and Masks [ref]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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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`.

### 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.

### 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

### 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.

### Account and Access Control [ref]group

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 Debian 8.

### Protect Accounts by Restricting Password-Based Login [ref]group

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.

### 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.

### 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

### Rsyslog Logs Sent To Remote Host [ref]group

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.

### 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.

### 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.

### 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.

### 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.

### 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.

### 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`.

### 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.

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