IR 6: Persistence, Powershell and Advanced Forensics

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Introduction

One of the goals for incident response is not just stopping an attack but making sure attackers can’t get back in. Attackers often use persistence techniques to maintain access, setting up mechanisms that survive reboots or system updates. Powershell has also become a common vector for threats, especially for delivering scripts and payloads that avoid traditional detection. This post covers practical approaches for handling persistence, investigating Powershell based threats, and applying advanced forensics to identify and contain these threats effectively.

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Detecting and Removing Persistence Techniques

Persistence techniques allow attackers to retain access, often hiding in plain sight or as disguised components of a system. Here’s how to detect and counter these techniques.

• Common Persistence Techniques:
  • Registry Keys: Attackers frequently use the Windows Registry to execute payloads on startup. Key locations include HKCU\Software\Microsoft\Windows\CurrentVersion\Run and HKLM\Software\Microsoft\Windows\CurrentVersion\Run.
  • Scheduled Tasks: Task Scheduler can be abused to run malicious scripts or executables at set intervals. Attackers often create tasks that mimic legitimate names to avoid detection.
  • Services: Attackers may install malicious services that start automatically with the system, blending in with legitimate services.
• Detection Strategies:
  • Regular Audits of Startups and Scheduled Tasks: Routine checks of startup locations and scheduled tasks help spot anything that doesn’t belong. Look for unusual task names, executable paths, or unrecognized service names.
  • Hunt for Anomalous Registry Entries: Use Powershell scripts to scan for suspicious entries in common persistence locations. For instance, filtering out known, legitimate entries can help analysts zero in on anomalies.
  • Use EDR Tools: Endpoint Detection and Response (EDR) tools like CrowdStrike and Microsoft Defender are valuable for identifying and tracking persistence mechanisms. They provide insights into process creation, registry modifications, and other activity patterns.
• Removal:
  • Manual Removal: Once you’ve confirmed a persistence method, remove or disable the associated registry key, scheduled task, or service.
  • Restore from Known Good State: When feasible, restoring or reimaging affected systems from a base or known good state can help ensure no residual persistence mechanisms remain.
  • Enhanced Monitoring: After removal, monitor for signs of reinfection, as persistence mechanisms are often part of broader campaigns.

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Investigating Powershell Based Threats

Powershell has evolved as a double edged tool in incident response: it’s powerful for defenders, but it’s also a favorite among attackers. Threats delivered via Powershell are often obfuscated and can be difficult to detect. Here’s how to identify and manage these threats.

• Identifying Malicious Powershell:
  • Obfuscation Patterns: Attackers use obfuscation techniques, like Base64 encoding and variable manipulation, to hide the true intent of Powershell scripts. Commands like -EncodedCommand or random string concatenation in scripts are red flags.
  • Logging and Monitoring: Enable Powershell script block logging and module logging in your environment. These logs capture the contents of Powershell commands and modules, providing visibility into script activity.
  • Check Network Connections: Malicious Powershell scripts often establish command and control (C2) connections. Reviewing network logs for unusual connections originating from Powershell processes can help identify these attempts.
• Decoding Obfuscated Powershell Scripts:
  • Use Built In Decoding: Powershell can decode Base64 encoded commands directly. For example, to decode a Base64 encoded command, run:

[System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String("encoded_string"))

• Containment and Mitigation:
  • Isolate Affected Systems: If a malicious Powershell script is detected, isolate the system immediately to prevent further spread.
  • Disable Powershell for Non Admin Users: Where feasible, restrict Powershell access to only administrators. This limits attackers’ ability to exploit Powershell as a persistence method.
  • Monitor for Reuse of Script Patterns: If you identify a specific Powershell attack pattern, update detection rules in your SIEM to alert on similar script behavior across the environment.

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Advanced Forensics Techniques for Complex Cases

Advanced forensics provides insights into deep system level activity and hidden traces left by attackers. Here’s how to approach advanced forensics for persistence and Powershell based threats.

• Memory Forensics:
  • Memory Dumps: Capture a memory dump of affected systems. Memory forensics tools like Volatility or Rekall can help identify malicious processes, loaded modules, and suspicious network connections that would otherwise be hard to detect.
  • Analyze Running Processes and DLLs: Attackers often inject code into legitimate processes to avoid detection. Use memory analysis to identify processes with injected code or suspicious DLLs loaded.
  • Network Artifacts in Memory: Memory analysis can also reveal network artifacts, such as C2 IPs, DNS queries, and encrypted connections, which may not be recorded in traditional network logs.
• File System Analysis:
  • Look for Hidden Files and Directories: Attackers sometimes hide malware in alternate data streams (ADS) or hidden directories. Using tools like Autopsy, you can analyze file systems for unusual file locations and attributes.
  • Check File Metadata: Metadata, such as timestamps, can reveal anomalies. For instance, finding files with creation dates outside of normal working hours or timestamps that don’t align with other system activity can indicate tampering.
• Registry and Event Log Analysis:
  • Registry Analysis: The Windows Registry stores a wealth of information that attackers may modify or leverage for persistence. Focus on common persistence keys and any recent modifications.
  • Event Logs: Event logs, particularly Security and Powershell logs, are invaluable for tracing an attacker’s activity. Logs can reveal login attempts, script execution, and modifications to critical system files.

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Integrating These Techniques into SOC Playbooks

Persistence, Powershell threats, and advanced forensics require precise actions that can be standardized in SOC playbooks. Here’s how to integrate them effectively:

• Persistence Playbook:
  • Include steps for checking common persistence locations (registry, scheduled tasks, services).
  • Define procedures for removing confirmed persistence mechanisms and for monitoring high risk systems post removal.
• Powershell Threat Playbook:
  • Outline detection, decoding, and isolation procedures specific to Powershell based threats.
  • Establish criteria for obfuscated script detection and steps for decoding commands before executing containment.
• Advanced Forensics Playbook:
  • Detail steps for capturing memory dumps, analyzing file systems, and examining event logs.
  • Create guidelines for escalating to advanced forensics in complex incidents, such as cases involving sophisticated persistence or stealthy C2 channels.

Developing these playbooks with clearly defined steps allows SOC analysts to handle persistence, Powershell, and advanced forensic cases consistently and efficiently.

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Conclusion

Dealing with persistence techniques, Powershell threats, and advanced forensic investigations requires a mix of practical skills and specialized tools. By establishing reliable detection methods, decoding obfuscated scripts, and using advanced forensics where needed, SOC teams can effectively identify and neutralize even the more sophisticated threats. Standardizing these processes in playbooks provides a repeatable response model, allowing analysts to address these complex threats with a well structured approach.