defense_evasion_posh_compressed.toml

[metadata]
creation_date = "2021/10/19"
maturity = "production"
updated_date = "2022/05/21"

[rule]
author = ["Elastic"]
description = """
Identifies the use of .NET functionality for decompression and base64 decoding combined in PowerShell scripts, which 
malware and security tools heavily use to deobfuscate payloads and load them directly in memory to bypass defenses.
"""
false_positives = ["Legitimate PowerShell Scripts which makes use of compression and encoding."]
from = "now-9m"
index = ["winlogbeat-*", "logs-windows.*"]
language = "kuery"
license = "Elastic License v2"
name = "PowerShell Suspicious Payload Encoded and Compressed"
note = """## Triage and analysis

### Investigating PowerShell Suspicious Payload Encoded and Compressed

PowerShell is one of the main tools system administrators use for automation, report routines, and other tasks. This
makes it available for use in various environments, and creates an attractive way for attackers to execute code.

Attackers can embed compressed and encoded payloads in scripts to load directly into the memory without touching the
disk. This strategy can circumvent string and file-based security protections.

#### Possible investigation steps

- Examine the script content that triggered the detection; look for suspicious DLL imports, collection or exfiltration
capabilities, suspicious functions, encoded or compressed data, and other potentially malicious characteristics.
- Investigate the script 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.
- Examine file or network events from the involved PowerShell process for suspicious behavior.
- Investigate other alerts associated with the user/host during the past 48 hours.
- Evaluate whether the user needs to use PowerShell to complete tasks.
- Retrieve the script and determine if it is 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 activity is unlikely to happen legitimately outside engineering or IT business units. As long as the analyst did
not identify malware or suspicious activity related to the user or host, this alert can be dismissed.

### Related rules

- PowerShell PSReflect Script - 56f2e9b5-4803-4e44-a0a4-a52dc79d57fe
- Potential Process Injection via PowerShell - 2e29e96a-b67c-455a-afe4-de6183431d0d
- Suspicious .NET Reflection via PowerShell - e26f042e-c590-4e82-8e05-41e81bd822ad

### Response and remediation

- Initiate the incident response process based on the outcome of the triage.
- Isolate the involved hosts 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.
- Investigate credential exposure on systems compromised or used by the attacker to ensure all compromised accounts are
identified. Reset passwords for these accounts and other potentially compromised credentials, such as email, business
systems, and web services.
- Restrict PowerShell usage outside of IT and engineering business units using GPOs, AppLocker, Intune, or similar software.
- Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and
malware components.
- 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).

## Config

The 'PowerShell Script Block Logging' logging policy must be enabled.
Steps to implement the logging policy with with Advanced Audit Configuration:

```
Computer Configuration > 
Administrative Templates > 
Windows PowerShell > 
Turn on PowerShell Script Block Logging (Enable)
```

Steps to implement the logging policy via registry:

```
reg add "hklm\\SOFTWARE\\Policies\\Microsoft\\Windows\\PowerShell\\ScriptBlockLogging" /v EnableScriptBlockLogging /t REG_DWORD /d 1
```
"""
risk_score = 47
rule_id = "81fe9dc6-a2d7-4192-a2d8-eed98afc766a"
severity = "medium"
tags = ["Elastic", "Host", "Windows", "Threat Detection", "Defense Evasion"]
timestamp_override = "event.ingested"
type = "query"

query = '''
event.category:process and 
  powershell.file.script_block_text : (
    (
      "System.IO.Compression.DeflateStream" or
      "System.IO.Compression.GzipStream" or
      "IO.Compression.DeflateStream" or
      "IO.Compression.GzipStream"
    ) and
    FromBase64String
  )
'''


[[rule.threat]]
framework = "MITRE ATT&CK"
[[rule.threat.technique]]
id = "T1140"
name = "Deobfuscate/Decode Files or Information"
reference = "https://attack.mitre.org/techniques/T1140/"

[[rule.threat.technique]]
id = "T1027"
name = "Obfuscated Files or Information"
reference = "https://attack.mitre.org/techniques/T1027/"

[rule.threat.tactic]
id = "TA0005"
name = "Defense Evasion"
reference = "https://attack.mitre.org/tactics/TA0005/"


[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1059"
reference = "https://attack.mitre.org/techniques/T1059/"
name = "Command and Scripting Interpreter"

  [[rule.threat.technique.subtechnique]]
  id = "T1059.001"
  reference = "https://attack.mitre.org/techniques/T1059/001/"
  name = "PowerShell"

[rule.threat.tactic]
id = "TA0002"
reference = "https://attack.mitre.org/tactics/TA0002/"
name = "Execution"