Creating custom SCA policies

Table of Contents

Creating custom SCA policies
- Variables
- Checks
  - Condition
  - Rules

An SCA policy looks like the following:

policy:
  id: "unix_audit"
  file: "sca_unix_audit.yml"
  name: "System audit for Unix based systems"
  description: "Guidance for establishing a secure configuration for Unix based systems."
  references:
    - https://www.ssh.com/ssh/

variables:
  $sshd_file: /etc/ssh/sshd_config,/opt/ssh/etc/sshd_config
  $pam_d_files: /etc/pam.d/common-password,/etc/pam.d/password-auth,/etc/pam.d/system-auth,/etc/pam.d/system-auth-ac,/etc/pam.d/passwd

requirements:
  title: "Check that the SSH service is installed on the system and password-related files are present on the system"
  description: "Requirements for running the SCA scan against the Unix based systems policy."
  condition: any
  rules:
    - 'f:$sshd_file'
    - 'f:/etc/passwd'
    - 'f:/etc/shadow'

checks:
  - id: 4004
    title: "SSH Hardening - 5: Password Authentication should be disabled"
    description: "The option PasswordAuthentication should be set to no."
    rationale: "The option PasswordAuthentication specifies whether we should use password-based authentication. Use public key authentication instead of passwords."
    remediation: "Change the PasswordAuthentication option value in the sshd_config file."
    compliance:
      - pci_dss: ["2.2.4"]
      - nist_800_53: ["CM.1"]
    condition: all
    rules:
     - 'f:$sshd_file -> r:^\s*PasswordAuthentication\s*\t*no'

  - id: [...]

As shown in this example, policy files are comprised by four sections, although not all of them are required, as detailed in the Policy file Sections table.

Policy file Sections
Section	Required
policy	Yes
requirements	No
variables	No
checks	Yes

Note

If the requirements aren't satisfied for a specific policy file, the scan for that file won't start.

Each section has its fields as described in the tables Policy section, Requirements section, Variables section, Checks section.

Policy section
Field	Mandatory	Type	Allowed values	Allowed values
id	Yes	String	Any string	Policy ID
file	Yes	String	Any string	Policy filename
name	Yes	String	Any string	Policy title
description	Yes	String	Any string	Brief description
references	No	Array of strings	Any string	Any string

Requirements section
Field	Mandatory	Type	Allowed values
title	Yes	String	Any string
description	Yes	String	Any string
condition	Yes	String	Any string
rules	Yes	Array of strings	Any string

Variables section
Field	Mandatory	Type	Allowed values
variable_name	Yes	Array of strings	Any string

Note

Fields id from policy and checks must be unique across policy files.

Variables

Variables are set in the variables section. Their names are preceded by $. For instance,

$list_of_files: /etc/ssh/sshd_config,/etc/sysctl.conf,/var/log/dmesg
$list_of_folders: /etc,/var,/tmp
$program_name: apache2

Variables can be placed anywhere in the left part of the rule. Therefore, regarding the variables above, the following rules could be built:

f:$list_of_files -> r:^Content to be found
c:systemctl is-enabled $program_name -> r:^enabled

There is no limit on the number of variables to add within a rule.

Checks

Checks are the core of an SCA policy, as they describe the checks to be performed in the system. Each check is comprised by several fields as described in the table Checks section.

Checks section
Field	Mandatory	Type	Allowed values
id	Yes	Numeric	Any integer number
title	Yes	String	Any string
description	No	String	Any string
rationale	No	String	Any string
remediation	No	String	Any string
compliance	No	Array of arrays of strings	Any string
references	No	Array of strings	Any string
condition	Yes	String	all, any, none
rules	Yes	Array of strings	Any string

Check evaluation is governed by its rule result aggregation strategy, as set in its condition field, and the results of the evaluation of its rules.

Condition

The condition field specifies how rule results are aggregated in order to calculate the final value of a check. There are three options:

all: the check will be evaluated as passed if all of its rules are satisfied and as failed as soon as one evaluates to failed,
any: the check will be evaluated as passed as soon as any of its rules is satisfied,
none: the check will be evaluated as passed if none of its rules are satisfied and as failed as soon as one evaluates to passed.

Special mention deserves how rules evaluated as non-applicable are treated by the aforementioned aggregators.

all: If any rule returns non-applicable, and no rule returns failed, the result will be non-applicable.
any: The check will be evaluated as non-applicable if no rule evaluates to passed and any returns non-applicable.
none: The check will be evaluated as non-applicable if no rule evaluates to passed and any returns non-applicable.

Condition truth-table
Condition \ Rule evaluation	passed(s)	failed(s)	non-applicable(s)	Result
`all`	yes	no	no	passed
`all`	indifferent	no	yes	non-applicable
`all`	indifferent	yes	indifferent	failed
`any`	yes	indifferent	indifferent	passed
`any`	no	yes	no	failed
`any`	no	indifferent	yes	non-applicable
`none`	yes	indifferent	indifferent	failed
`none`	no	indifferent	yes	non-applicable
`none`	no	yes	no	passed

Rules can check for the existence of files, directories, registry keys and values, running processes, and recursively test for the existence of files inside directories. When it comes to content checking, they are able to check for file contents, recursively check for the contents of files inside directories, command output and registry value data.

Abstractly, rules start with a location (and a type of location), that will be the target of the test, followed by the actual test specification. Such tests fall into two categories: existence and content checks.

There are five main types of rules as described below:

Rule types
Type	Character
File	`f`
Directory	`d`
Process	`p`
Commands	`c`
Registry (Windows Only)	`r`

The operators for content checking are:

Content comparison operators
Operation	Operator	Example
Literal comparison, exact match	by omission	`f:/file -> CONTENT`
Lightweight Regular expression match	`r:`	`f:/file -> r:REGEX`
Numeric comparison (integers)	`n:`	`f:/file -> n:REGEX_WITH_CAPTURE_GROUP compare <= VALUE`

Numeric comparison operators
Arithmetic relational operator	Operator	Example
less than	`<`	`n:SomeProperty (\d) compare < 42`
less than or equal to	`<=`	`n:SomeProperty (\d) compare <= 42`
equal to	`==`	`n:SomeProperty (\d) compare == 42`
not equal to	`!=`	`n:SomeProperty (\d) compare != 42`
greater than or equal to	`>=`	`n:SomeProperty (\d) compare >= 42`
greater than	`>`	`n:SomeProperty (\d) compare > 42`

A whole rule can be negated using the operator not, which is placed at the beginning of the rule.

not RULE

Example: not f:/some_file -> some_text will fail if some_text is found within the contents of some_file.

By combining the aforementioned rule types and operators, both existence and content checking can be performed.

Note

Process rules only allow existence checks.
Command rules only allow content (output) checks.

Existence checking rules

Existence checks are created by setting rules without a content operator, the general form is as follows:

RULE_TYPE:target

Examples of existence checks:

f:/etc/sshd_config checks the existence of file /etc/ssh_config
d:/etc checks the existence of directory /etc
not p:sshd will test the presence of processes called sshd and fail if one is found.
r:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa checks for the existence of that key.
r:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa -> LimitBlankPasswordUse checks for the existence of value LimitBlankPasswordUse in the key.

Content checking rules

The general form of a rule testing for contents is as follows:

RULE_TYPE:target -> CONTENT_OPERATOR:value

Warning

The context of a content check is limited to a line.
Content checks are case-sensitive.
It is mandatory to respect the spaces around the -> and compare separators.
If the target of a rule that checks for contents does not exist, the result will be non-applicable as it could not be checked.

Content check operator results can be negated by adding a ! before then, for example:

f:/etc/ssh_config -> !r:PermitRootLogin

Warning

Be careful when negating content operators as that will make them evaluate as passed for anything that does not match with the check specified. For example rule `f:/etc/ssh_config -> !r:PermitRootLogin` will be evaluated as passed if it finds any line that does not contain PermitRootLogin.

Content check operators can be chained using the operator && (AND) as follows:

f:/etc/ssh_config -> !r:^# && r:Protocol && r:2

This rule reads as Pass if there's a line whose first character is not "#" and contains "Protocol" and "2".

Warning

It is mandatory to respect the spaces around the && operator.
There's no particular order of evaluation between tests chained using the && operator.

Examples of content checks:

systemctl is-enabled cups -> r:^enabled checks that the output of the command contains a line starting by enabled.

f:$sshd_file -> n:^\s*MaxAuthTries\s*\t*(\d+) compare <= 4 checks that MaxAuthTries is less or equal to 4.

r:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa -> LimitBlankPasswordUse -> 1 checks that value of LimitBlankPasswordUse is 1.

Examples

The following sections cover each rule type, illustrating them with several examples. It is also recommended to check the actual policies and, for minimalistic although complete examples, the SCA test suite policies.

Rule syntax for files

Check that a file exists: f:/path/to/file
Check that a file does not exist: not f:/path/to/file
Check file contains (whole line literal match): f:/path/to/file -> content
Check file contents against regex: f:/path/to/file -> r:REGEX
Check a numeric value: f:/path/to/file -> n:REGEX(\d+) compare <= Number

Rule syntax for directories

Check if a directory exists: d:/path/to/directory
Check if a directory contains a file: d:/path/to/directory -> file
Check if a directory contains files that match a regex: d:/path/to/directory -> r:^files
Check files matching file_name for content: d:/path/to/directory -> file_name -> content

Rule syntax for processes

Check if a process is running p:process_name
Check if a process is not running not p:process_name

Rule syntax for commands

Check the output of a command c:command -> output
Check the output of a command using regex c:command -> r:REGEX
Check a numeric value c:command -> n:REGEX_WITH_A_CAPTURE_GROUP compare >= number

Rule syntax for Windows Registry

Check if a registry exists r:path/to/registry
Check if a registry key exists r:path/to/registry -> key
Check registry key contents r:path/to/registry -> key -> content

Composite rules

Check if there is a line that does not begin with # and contains Port 22 f:/etc/ssh/sshd_config -> !r:^# && r:Port\.+22
Check if there is no line that does not begin with # and contains Port 22 not f:/etc/ssh/sshd_config -> !r:^# && r:Port\.+22

Other examples

Check for file contents, whole line match: f:/proc/sys/net/ipv4/ip_forward -> 1
Check if a file exists: f:/proc/sys/net/ipv4/ip_forward
Check if a process is running: p:avahi-daemon
Check value of registry: r:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Netlogon\Parameters -> MaximumPasswordAge -> 0
Check if a directory contains files: d:/home -> ^.mysql_history$
Check if a directory exists: d:/etc/mysql
Check the running configuration of sshd for the maximum authentication tries allowed: c:sshd -T -> !r:^\s*maxauthtries\s+4\s*$
Check if root is the only account with UID 0: f:/etc/passwd -> !r:^# && !r:^root: && r:^\w+:\w+:0: