Screensaver Plist File Modified by Unexpected Process

Identifies when a screensaver plist file is modified by an unexpected process. An adversary can maintain persistence on a macOS endpoint by creating a malicious screensaver (.saver) file and configuring the screensaver plist file to execute code each time the screensaver is activated.

Rule type: eql

Rule indices:

auditbeat-*
logs-endpoint.events.*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

Elastic
Host
macOS
Threat Detection
Persistence

Version: 5

Rule authors:

Elastic

Rule license: Elastic License v2

Investigation guide

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## Triage and analysis

- Analyze the plist file modification event to identify whether the change was expected or not
- Investigate the process that modified the plist file for malicious code or other suspicious behavior
- Identify if any suspicious or known malicious screensaver (.saver) files were recently written to or modified on the host

## Setup

If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2, events will not define `event.ingested` and default fallback for EQL rules was not added until 8.2, so you will need to add a custom pipeline to populate `event.ingested` to @timestamp for this rule to work.

Rule query

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file where event.type != "deletion" and
  file.name: "com.apple.screensaver.*.plist" and
  file.path : (
    "/Users/*/Library/Preferences/ByHost/*",
    "/Library/Managed Preferences/*",
    "/System/Library/Preferences/*"
    ) and
  /* Filter OS processes modifying screensaver plist files */
  not process.executable : (
    "/usr/sbin/cfprefsd",
    "/usr/libexec/xpcproxy",
    "/System/Library/CoreServices/ManagedClient.app/Contents/Resources/MCXCompositor",
    "/System/Library/CoreServices/ManagedClient.app/Contents/MacOS/ManagedClient"
    )

Framework: MITRE ATT&CK^TM

Tactic:
- Name: Persistence
- ID: TA0003
- Reference URL: https://attack.mitre.org/tactics/TA0003/
Technique:
- Name: Event Triggered Execution
- ID: T1546
- Reference URL: https://attack.mitre.org/techniques/T1546/

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