Windows Defender Exclusions Added via PowerShell

## Triage and analysis

### Investigating Windows Defender Exclusions Added via PowerShell

Microsoft Windows Defender is an antivirus product built into Microsoft Windows. Since this software product is
used to prevent and stop malware, it's important to monitor what specific exclusions are made to the product's configuration
settings. These can often be signs of an adversary or malware trying to bypass Windows Defender's capabilities. One of
the more notable [examples](https://www.cyberbit.com/blog/endpoint-security/latest-trickbot-variant-has-new-tricks-up-its-sleeve/)
was observed in 2018 where Trickbot incorporated mechanisms to disable Windows Defender to avoid detection.

#### Possible investigation steps

- Investigate the process execution chain (parent process tree) for unknown processes. Examine their executable files
for prevalence, whether they are located in expected locations, and if they are signed with valid digital signatures.
- Identify the user account that performed the action and whether it should perform this kind of action.
- Contact the account owner and confirm whether they are aware of this activity.
- Examine the exclusion in order to determine the intent behind it.
- Assess whether this behavior is prevalent in the environment by looking for similar occurrences across hosts.
- If the exclusion specifies a suspicious file or path, retrieve the file(s) and determine if malicious:
  - Use a private sandboxed malware analysis system to perform analysis.
    - Observe and collect information about the following activities:
      - Attempts to contact external domains and addresses.
      - File and registry access, modification, and creation activities.
      - Service creation and launch activities.
      - Scheduled tasks creation.
  - Use the PowerShell Get-FileHash cmdlet to get the files' SHA-256 hash values.
    - Search for the existence and reputation of the hashes in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.

### False positive analysis

- This rule has a high chance to produce false positives due to how often network administrators legitimately configure
exclusions. In order to validate the activity further, review the specific exclusion and its intent. There are many
legitimate reasons for exclusions, so it's important to gain context.

### Related rules

- Windows Defender Disabled via Registry Modification - 2ffa1f1e-b6db-47fa-994b-1512743847eb
- Disabling Windows Defender Security Settings via PowerShell - c8cccb06-faf2-4cd5-886e-2c9636cfcb87

### Response and remediation

- Initiate the incident response process based on the outcome of the triage.
- Isolate the involved host to prevent further post-compromise behavior.
- If the triage identified malware, search the environment for additional compromised hosts.
  - Implement temporary network rules, procedures, and segmentation to contain the malware.
  - Stop suspicious processes.
  - Immediately block the identified indicators of compromise (IoCs).
  - Inspect the affected systems for additional malware backdoors like reverse shells, reverse proxies, or droppers that
  attackers could use to reinfect the system.
- Remove and block malicious artifacts identified during triage.
- Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and
malware components.
- Exclusion lists for antimalware capabilities should always be routinely monitored for review.
- Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
- Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the
mean time to respond (MTTR).

## 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.