PSAsyncShell - Asynchronous TCP Reverse Shell
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In this page
Rule name | Rule type | Log sources | MITRE ATT&CK tags | Severity |
PSAsyncShell - Asynchronous TCP Reverse Shell | Standard | Windows | Execution: Command and Scripting Interpreter - PowerShell (T1059.001) | Critical |
About the rule
Rule Type
Standard
Rule Description
PSAsyncShell is an asynchronous TCP reverse shell written in PowerShell that allows remote attackers to maintain control over a compromised system without constant interaction. It opens a back channel from the victim machine to a command-and-control (C2) server, enabling the attacker to run commands, exfiltrate data, or stage further attacks asynchronously.
Severity
Trouble
Rule journey
Attack chain scenario
Initial access via vulnerable web app → Command injection using PowerShell → Reverse shell established via PSAsyncShell → Remote commands issued for persistence → Tools and payloads downloaded for further actions
Impact
- Ransomware deployment
- System compromise
- Compliance breach
Rule Requirement
Prerequisites
Logon to Group Policy Management Console with administrative privileges and enable Module Logging for Windows PowerShell in the Group Policy Management Editor. Ensure to enter * in the Module Names window to record all modules. Similarly enable PowerShell Script Block Logging for Windows PowerShell. Finally, create a new registry key "Microsoft-Windows-Powershell/Operational" in the directory "Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EventLog\".
Criteria
Action1: actionname = "PowerShell Script Block Logged" AND SCRIPTEXECUTED contains "PSAsyncShell" select Action1.HOSTNAME,Action1.MESSAGE,Action1.SCRIPTEXECUTED
Detection
Execution Mode
realtime
Log Sources
Windows
MITRE ATT&CK
Execution: Command and Scripting Interpreter - PowerShell (T1059.001)
Security standard:
Enabling this rule will help you meet the security standard's requirement listed below:
DE.CM-09: Computing hardware and software, runtime environments, and their data are monitored to find potentially adverse events.
When this rule is triggered, you're notified of execution of PowerShell scripts containing references to psasyncshell. This enables you to monitor runtime environments like PowerShell, identify potential credential compromises, and detect attempts to create AD snapshots.
Author
Nasreddine Bencherchali (Nextron Systems)
Future actions
Known False Positives
Unlikely
Next Steps
When this rule is triggered, the following measures can be implemented:
- Identification: Identify if the flagged event is a new incident or part of an existing incident.
- Analysis: Analyze the impact and extent of the incident to comprehend the severity of the attack using the Incident Workbench.
- Response: Respond promptly by initiating an automated workflow to interrupt the network connections and cease the malicious process.
- Audit PowerShell activities: Enable PowerShell script block logging and monitor for known reverse shell patterns.
Mitigation
Mitigation ID | Mitigation Name | Mitigation description |
M1049 | Implement antivirus or antimalware scanning to isolate suspicious files. | |
M1045 | Configure policies that allow PowerShell to execute only signed scripts. | |
M1042 | Restrict or disable PowerShell on systems where it is not required. | |
M1038 | Restrict the execution of scripts that contain sensitive language elements i.e., malicious codes using the PowerShell Constrained Language mode. | |
M1026 | Restrict privileges to execute PowerShell scripts to administrators and enforce limitations on the commands that can be executed via remote PowerShell sessions. |
