The following analytic identifies suspicious PowerShell script execution via EventCode 4104 that is using memory stream as new object backstore. The malicious PowerShell script will contain stream flate data and will be decompressed in memory to run or drop the actual payload. During triage, review parallel processes within the same timeframe. Review the full script block to identify other related artifacts.
- Type: TTP
Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
- Last Updated: 2023-04-14
- Author: Teoderick Contreras, Splunk
- ID: c396a0c4-c9f2-11eb-b4f5-acde48001122
Kill Chain Phase
- CIS 10
1 2 3 4 5 6 7 `powershell` EventCode=4104 ScriptBlockText = *New-Object* ScriptBlockText = *IO.MemoryStream* | stats count min(_time) as firstTime max(_time) as lastTime by EventCode ScriptBlockText Computer UserID | rename Computer as dest | rename UserID as user | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `powershell_using_memory_as_backing_store_filter`
The SPL above uses the following Macros:
powershell_using_memory_as_backing_store_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.
List of fields required to use this analytic.
How To Implement
To successfully implement this analytic, you will need to enable PowerShell Script Block Logging on some or all endpoints. Additional setup here https://docs.splunk.com/Documentation/UBA/22.214.171.124/GetDataIn/AddPowerShell#Configure_module_logging_for_PowerShell.
Known False Positives
powershell may used this function to store out object into memory.
Associated Analytic Story
|40.0||50||80||A PowerShell script contains memorystream command in $ScriptBlockText$ as new object backstore with EventCode $EventCode$ on host $dest$.|
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.
source | version: 2