Creating custom SCA policies

You need to consider the following four sections when creating custom policy files, although not all of them are required.

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

An SCA policy looks like the following:

# Security Configuration Assessment
# Audit for UNIX systems
# Copyright (C) 2015, ThreatLockDown Inc.
#
# This program is free software; you can redistribute it
# and/or modify it under the terms of the GNU General Public
# License (version 2) as published by the FSF - Free Software
# Foundation

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/

requirements:
  title: "Check that the SSH service 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'

variables:
  $sshd_file: /etc/ssh/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

checks:
  - id: 3000
    title: "SSH Hardening: Port should not be 22"
    description: "The ssh daemon should not be listening on port 22 (the default value) for incoming connections."
    rationale: "Changing the default port you may reduce the number of successful attacks from zombie bots, an attacker or bot doing port-scanning can quickly identify your SSH port."
    remediation: "Change the Port option value in the sshd_config file."
    compliance:
      - pci_dss: ["2.2.4"]
      - nist_800_53: ["CM.1"]
    condition: all
    rules:
      - 'f:$sshd_file -> !r:^# && r:Port && !r:\s*\t*22$'

  - id: 3001
    title: "SSH Hardening: Protocol should be set to 2"

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, and Checks section.

Policy section
Field	Mandatory	Type	Allowed values	Description
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 (empty by default)	Links to references
regex_type	No	String	"osregex" (default), "pcre2"	Policy regex engine

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

The id field under 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 scan to be performed in the endpoint. The checks contain fields that define what actions the agent should take to scan the endpoint, and how to evaluate the scan results. Each check is composed of several fields as described in the table below:

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
regex_type	No	String	"pcre2" or "osregex"

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.

Note

If you set a regex_type, it overrides the regex engine type defined in the policy.

Condition

The result of each SCA check is governed by the conditions set in the condition field, and the results of the evaluation of its rules. 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 is evaluated as Passed if all of its rules are satisfied and as Failed as soon as one rule is not satisfied.
any: The check is evaluated as Passed as soon as any of its rules are satisfied.
none: The check is evaluated as Passed if none of its rules are satisfied and as Failed as soon as one rule is satisfied.

There are certain situations in which the aforementioned aggregators are evaluated as Not applicable.

all: If any rule returns Not applicable, and no rule returns Failed, the result is Not applicable.
any: The check is evaluated as Not applicable if no rule is evaluated as Passed and any rule returns Not applicable.
none: The check is evaluated as Not applicable if no rule is evaluated as Passed and any rule returns Not applicable.

Condition / rule evaluation
Condition \ Rule evaluation	Passed	Failed	Not applicable	Result
`all`	yes	no	no	Passed
`all`	*	no	yes	Not applicable
`all`	*	yes	*	Failed
`any`	yes	*	*	Passed
`any`	no	yes	no	Failed
`any`	no	*	yes	Not applicable
`none`	yes	*	*	Failed
`none`	no	*	yes	Not applicable
`none`	no	yes	no	Passed

* This result does not affect the final result.

Rules

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 is the target of the test, followed by the actual test specification. Such tests fall into two categories: existence and content checks. The type of location is listed in the Rule types table below, and the location could be a file name, directory, process name, command, or a registry key.

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 shown in the content comparison operators table below.

Content comparison operators
Operation	Operator	Example
Literal comparison, exact match	by omission (the absence of an operator signifies a literal comparison or exact match)	`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`

The operators for numeric comparison are shown in the table below.

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`

You can place not at the beginning of a rule to negate it. For example:

not f:/some_file -> some_text

The SCA rule above fails 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 /etc/sshd_config file.
d:/etc checks the existence of the /etc directory.
not p:sshd tests the presence of processes called sshd and fails if one is found.
r:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa checks for the existence of the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa key.
r:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa -> LimitBlankPasswordUse checks for the existence of LimitBlankPasswordUse value 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.
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 Not 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 makes them evaluate as Passed for anything that does not match with the check specified. For example, rule f:/etc/ssh_config -> !r:PermitRootLogin is 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:

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