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Advanced topic: lists

Policy rules can use lists for even more control over when and how alerts are fired. Lists are particularly useful for configuring policies to block or allow certain behavior.

List types

Sophos Linux Sensor (SLS) supports five data types for lists: names, hosts, paths, and numbers, along with an experimental lineage type. The list data type corresponds to the data type of the field to which it's being compared. For example, the following rule will expect the Process_Injection-programName-allowList to be of data type paths, as programName is a path field.

ignore programName IN $Process_Injection-programName-allowList

Names

Name lists store strings and includes support for glob formatting. For example, both "alpine" and "al*" are valid entries in a name list.

Hosts

Host lists store either IP addresses as CIDR blocks or domain names (which are resolved to an IP address). For example, both "8.8.8.8/8" and "www.sophos.com" are valid entries in a host list.

Paths

Path lists store program or file paths. You may write paths with the wildcard operator '*', which will match all paths that fit the pattern. For example, both "/bin/bash" and "/bin/*/bash" are valid entries in a path list.

Numbers

Numbers lists store integers. For example, 10 is a valid entry in a number list.

Lineage (experimental)

Lineage lists store lists of strings that are indicative of process lineage sequences. As lineage is currently an experimental feature, Sophos doesn't recommended using lineage lists.

Adjusting allowlists and blocklists

Most SLS detections incorporate several lists - allowlists and blocklists, specifically - as an extensible way to describe wanted and unwanted system behavior. Detections have lists built-in for common allowlist and blocklist actions. For instance, if a certain detection generates false positives in your environment, you can add (or remove) actions to your list to better tailor detection for your environment's standard operation.

These lists enable customization of allowlists for alert elements such as program name, username, container image name, as well as parent program name (the program that executed the program currently being inspected by the detection) and ancestor program name (the same as parent program, but will match the parent's parent, grandparent, etc. - all the way back to the system boot process).

The lists included with SLS's content are named consistently to facilitate tweaking. The standard naming of these lists is a combination of the detection the list applies to, the alert element that should be allowed/blocked, and the action (allow or block), as demonstrated below:

$Snake_Cased_Detection-$alertElement-$actionList

As far as modifying lists goes, there are three different supported behaviors:

  • `add`: For adding an item to a list.
  • `remove`: For removing an item from a list.
  • `replace`: For replacing an entire list's contents. Use this when you want to both add and remove.

When modifying lists, you specify the behavior with the behavior: attribute, as we’ll show in some examples shortly.

For content, lists can be of two different types: Shared lists and Local lists.

Shared lists

Shared lists allow you to allow or block an application running across all enabled detections. These lists should rarely be used, because they can introduce a significant coverage gap and serve as a method for attackers to evade detection.

As shared lists apply to all detections, they take the format Global-$alertElement-$actionList.

For example, customers commonly want to allowlist actions performed by a user associated with vulnerability management tooling, such as Tenable's Nessus vulnerability scanner. Nessus regularly connects to vast portions of an organization's network and executes commands to determine patch levels and identify the presence of known vulnerabilities. This activity could generate a lot of alerts - so it may be desirable to add the nessus operating system user to the shared allowlist to suppress any alerts it might generate. To add nessus, you must add the following to the host's configuration file (/etc/sophos/runtimedetections-rules.yaml):

  Global-userName-allowList:
    behavior: add
    list:
    - "nessus"

The shared list is empty by default, nothing is allowlisted. However, that list is still used in most detections by default. Suppose you add an item to the shared list, like the userName "nessus" in the previous example. In that case, any detection that can filter on userName will automatically ignore alerts from that user.

Another commonly shared list is SystemUpdatePrograms. By default, this list consists of package management programs (like yum and dpkg) and some related helper programs used by package managers. However, many organizations use configuration management tools such as Puppet to manage fleet configurations. In this case, it would be a good idea to mark the Puppet agent (puppet) as being allowed to perform tasks related to system configuration management. You can achieve this by adding puppet to the SystemUpdatePrograms list, per the following example:

  SystemUpdatePrograms:
    behavior: add
    list:
    - "/opt/puppetlabs/bin/puppet"

These lines ensure that any detections focused on system changes won't generate alerts if the program /opt/puppet/puppet-agent was involved.

Detection-local lists

In addition to shared lists, most detections include their own lists that only apply to a single detection policy. Detection-local lists provide fine-grained control over what each detection considers malicious or benign and can be very helpful for tuning out false-positive alerts.

Detection-local lists are named consistently to facilitate tweaking. The standard naming of these lists is a combination of the detection name, the element of the event to be allowed or blocked, and the action (allow or block), as demonstrated below:

$Snake_Cased_Detection_Name-$alertElementName-$actionList

For example, the Process Injection detection has the following lists:

Process_Injection-programName-allowList
Process_Injection-userName-allowList
Process_Injection-imageName-allowList
Process_Injection-parentProgramName-allowList
Process_Injection-ancestorProgramName-allowList

The full list of the detections and predicates that can be modified is in the content file - /var/lib/sophos/content/runtimedetections-content.yaml.

You can add to each list so that alerts aren't generated. These lists enable customization of allowlists for program name, username, container image name, parent program name, and ancestor program name.

For example, the Process Injection detection monitors for programs using debugging techniques on other processes. Some Application Performance Management (APM) tools use debugging techniques to profile applications and provide information regarding system call usage. This could generate a significant number of alerts for standard system operation. Assuming the APM program's name is /opt/myAPM/ap-monitor, the following snippet should be added to the host's configuration file (/etc/sophos/runtimedetections-rules.yaml):

Process_Injection-programName-allowList:
    behavior: add
    list:
    - "/opt/myAPM/ap-monitor"

Creating Custom Lists

When creating custom detections, defining a custom list that SLS will refer to in the ruleset may be useful.

List names must be unique within a configuration file across all strategy and list names (note: unlike strategy names, list names may not contain spaces). Aside from the name, the following table outlines the attributes necessary to define a list:

List Attribute Type Required Description
type string yes The data type of the list.
description string no The description for this list.
list list yes actual list of elements, which must conform to the type of the list. Each element may optionally have a description

For example, a custom list of authorized shell programs might look like this:

AuthorizedShellPrograms:
  type: paths
  description: allowlist of authorized programs
  list:
    - "/bin/bash"
    - "/bin/sh"
    - "/usr/sbin/*sh":

The final line uses a wildcard to match all programs beginning with /usr/sbin/" and ending in "sh".

You must now add this custom list to the host's /etc/sophos/runtimedetections-rules.yaml configuration file, along with the detection that refers to it.

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