Windows Rundll32 with Non-Standard File Extension
Description
This analytic identifies the instance of rundll32.exe process loading a non-standard Windows modules file extension. This behavior is not common and can be associated with malicious activities, such as the Gh0st RAT backdoor. This technique is to evade possible detection by security tools that monitors a suspicious dll loading activity.
Query · spl
| tstats `security_content_summariesonly`
count min(_time) as firstTime
max(_time) as lastTime
FROM datamodel=Endpoint.Processes WHERE
`process_rundll32`
(
Processes.parent_process_path IN (
"*:\\PerfLogs\\*",
"*:\\ProgramData\\*",
"*:\\Recycle.bin\\*",
"*:\\Users\\Administrator\\Music\\*",
"*:\\Users\\Public\\*",
"*:\\Windows\\Cursors\\*",
"*:\\Windows\\debug\\*",
"*:\\Windows\\fonts\\*",
"*:\\Windows\\INF\\*",
"*:\\Windows\\Media\\*",
"*:\\Windows\\Prefetch\\*",
"*:\\Windows\\repair\\*",
"*:\\Windows\\Tasks\\*",
"*\\Temp\\*"
)
OR
Processes.parent_process_name IN (
"*cmd.exe*",
"*cscript.exe*",
"*mshta.exe*",
"*powershell.exe*",
"*pwsh.exe*",
"*regsvr32.exe*",
"*wscript.exe*"
)
)
NOT Processes.process IN (
"*.cpl*",
"*.dll*",
"*.drv*",
"*.inf*",
"*.mui*",
"*.ocx*"
)
BY Processes.action Processes.dest Processes.original_file_name
Processes.parent_process_exec Processes.parent_process_guid
Processes.parent_process_id Processes.parent_process_path
Processes.process_exec Processes.process_guid Processes.process_hash
Processes.process_id Processes.process_integrity_level
Processes.process_path Processes.user Processes.user_id
Processes.vendor_product Processes.parent_process_name
Processes.parent_process Processes.process_name Processes.process
| `drop_dm_object_name(Processes)`
| rex field=process "^(?<cmd_base_process>[^\s]+)\s+\"?(?<cmd_base_first_param>[^,^\"^\s]+).*?,(?<cmd_args>.*)$"
| rex field=cmd_base_first_param "^(?<root>[^\\\\]+)\\\\(?<subdirs>[^\\\\]+)"
| eval folder_count = mvcount(split(cmd_base_first_param, "\\"))
| where (
folder_count = 3
AND
NOT lower(subdirs) IN (
"windows",
"program files",
"program files (x86)"
)
)
OR
like(cmd_base_first_param, "%:\\ProgramData\\%")
OR
like(cmd_base_first_param, "%:\\Users\\Public\\%")
OR
like(cmd_base_first_param, "%\\AppData\\Local\\Temp\\%")
OR
like(cmd_base_first_param, "%\\AppData\\Roaming\\%")
| `security_content_ctime(firstTime)`
| `security_content_ctime(lastTime)`
| `windows_rundll32_with_non_standard_file_extension_filter`
Implementation guide
The detection is based on data that originates from Endpoint Detection and Response (EDR) agents. These agents are designed to provide security-related telemetry from the endpoints where the agent is installed. To implement this search, you must ingest logs that contain the process GUID, process name, and parent process. Additionally, you must ingest complete command-line executions. These logs must be processed using the appropriate Splunk Technology Add-ons that are specific to the EDR product. The logs must also be mapped to the Processes node of the Endpoint data model. Use the Splunk Common Information Model (CIM) to normalize the field names and speed up the data modeling process.
Known false positives
- Certain legitimate Windows components, third-party applications, or administrative scripts may use rundll32.exe with non-standard or extensionless inputs during normal operation. Filtering and contextual analysis are required, focus on command-line arguments, parent process, file location, and prevalence across the environment before determining malicious intent.
Analyst notes
Known false positives: Certain legitimate Windows components, third-party applications, or administrative scripts may use rundll32.exe with non-standard or extensionless inputs during normal operation. Filtering and contextual analysis are required, focus on command-line arguments, parent process, file location, and prevalence across the environment before determining malicious intent.