Analyzing Scheduled Task Abuse Telemetry

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Analyzing Scheduled Task Abuse Telemetry: A Practical Threat Hunting Investigation

Why This Activity Stood Out

During our recent hunt session, we flagged some suspicious activity in our telemetry data. Specifically, there were multiple instances of unusual scheduled tasks being created and executed using binaries and scripts that didn’t seem to fit the usual business workflow. Given the context of a known threat actor group, Wizard Spider, this behavior caught my attention as it could be a sign of an adversary trying to establish persistence within our network.

Initial Hunt Logic

Hypothesis

Hypothesis: Abnormal execution behavior involving scheduled tasks may indicate that an adversary is using these techniques to bypass monitoring and gain persistent access.

MITRE ATT&CK Mapping

  • Technique: T1053 – Scheduled Task/Job
  • Tactic: Persistence

Available Telemetry

We had access to the following data:

  • 4698 Scheduled Task Events from Windows Event Logs
  • Task Scheduler Operational Logs
  • Sysmon logs
  • Process Creation Events

Initial Investigation

Why This Activity Stood Out

The activity was flagged due to several unusual patterns in the scheduled tasks being created. The task names were unusually long, contained encoded command lines, and pointed to non-standard binaries.

Suspicious Task Details

  1. Task Name: C:\Windows\Temp\[RandomName].exe
  2. Execution Frequency: Daily at 3:00 AM
  3. Scheduled Task Creation Event ID: 4698

Reviewing Process Lineage

To better understand the context, I started by reviewing the process lineage of these scheduled tasks.

KQL Hunting Reference

DeviceProcessEvents
| where ProcessCommandLine contains "-enc"
| project Timestamp, DeviceName, ProcessCommandLine

SPL Hunting Reference

index=sysmon Image="*rundll32.exe"
| stats count by Computer, CommandLine

Expanding the Investigation

Checking Task Details

Using Get-ScheduledTask and Get-Process, I gathered more information on these tasks. The task details revealed that they were created under the System account with high privileges.

Analysis of Executed Commands

The commands executed by the scheduled tasks contained encoded PowerShell scripts, which further indicated a sophisticated threat actor using obfuscation techniques to evade detection.

Detection Opportunities

Sigma Rule

To improve our detection capabilities, I developed a Sigma rule that triggers on specific command-line patterns indicating the use of Invoke-Expression or encoded PowerShell commands.

title: Suspicious Encoded PowerShell
logsource:
  category: process_creation

detection:
  selection:
    CommandLine|contains:
      - '-enc'
      - 'EncodedCommand'

condition: selection

level: high

KQL Hunting Reference

DeviceProcessEvents
| where ProcessCommandLine contains "-enc"
| project Timestamp, DeviceName, ProcessCommandLine

Analyst Notes

  1. Uncertainty: The initial suspicion was based on the unusual nature of the tasks and their timing. However, we needed to rule out legitimate use cases for such behavior.
  2. Operational Challenges: Some scheduled tasks are necessary for system maintenance, making it difficult to distinguish between benign and malicious activities.

Telemetry Gaps

Missing Context

  • The telemetry did not provide sufficient context on the source of these encoded commands or their intended target.
  • There was no detailed logging of network communications from the execution of these scripts.

Interesting Findings

  1. Network Activity: Further investigation revealed some suspicious outbound connections to rare domains, which we had flagged earlier as potential Command and Control (C2) servers.
  2. User Interaction: There were no user logs or other interactions indicating human involvement in creating or modifying these tasks.

Hypothesis Validation

Based on the telemetry and our experience with Wizard Spider’s tactics, I concluded that this activity was likely part of their tradecraft to establish persistence.

Detection Engineering Insights

  1. Improved Sigma Rules: We added more specific patterns to detect encoded commands and PowerShell usage.
  2. Custom Logs: Implementing custom logs for high-privilege task creation could help in better identifying such activities.

Operational Challenges

  1. Balancing Sensitivity: Ensuring that the detection rules are sensitive enough to catch malicious activities without generating too many false positives.
  2. Resource Constraints: Limited resources can make it challenging to investigate every alert, leading to potential oversight of critical activities.

Telemetry Improvement Ideas

  1. Enhanced Logging: Implementing more detailed logging for scheduled task creation and execution would provide better context.
  2. Correlation Engine: Developing a correlation engine that links task creation with network activity could help in identifying C2 communications.

Practical Detection Recommendations

  1. Regular Audits: Conduct regular audits of scheduled tasks to ensure they align with legitimate business needs.
  2. Behavioral Analysis: Integrate behavioral analysis tools to detect anomalies in task execution patterns.

Hashtags

WizardSpider #T1053 #Persistence #ScheduledTaskAbuse #ThreatHunting #SecurityOperationsCenter #Sysmon #SigmaDetection #TelemetryImprovement

By following these steps and implementing the recommended improvements, we can better detect and mitigate threats related to scheduled task abuse.

#ThreatHunting #CyberSecurity #BlueTeam #SOC #DetectionEngineering #MITREATTACK #Persistence #Windows #ScheduledTaskAbuse