The following analytic identifies a
.dll being written by
spoolsv.exe. This was identified during our testing of CVE-2021-34527 previously(CVE-2021-1675) or PrintNightmare. Typically, this is not normal behavior for
spoolsv.exe to write a
.dll. Current POC code used will write the suspicious DLL to disk within a path of
\spool\drivers\x64\. During triage, isolate the endpoint and review for source of exploitation. Capture any additional file modification events.
- Type: TTP
Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
- Last Updated: 2021-07-01
- Author: Mauricio Velazco, Michael Haag, Splunk
- ID: 347fd388-da87-11eb-836d-acde48001122
Kill Chain Phase
- CIS 10
1 2 3 4 5 `sysmon` EventID=11 process_name=spoolsv.exe file_path="*\\spool\\drivers\\x64\\*" file_name=*.dll | stats count min(_time) as firstTime max(_time) as lastTime by dest, UserID, process_name, file_path, file_name, TargetFilename, process_id | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `spoolsv_writing_a_dll___sysmon_filter`
The SPL above uses the following Macros:
spoolsv_writing_a_dll_-_sysmon_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 search, you need to be ingesting logs with the process name, parent process, and command-line executions from your endpoints. If you are using Sysmon, you must have at least version 6.0.4 of the Sysmon TA. Tune and filter known instances where renamed rundll32.exe may be used.
Known False Positives
Limited false positives. Filter as needed.
Associated Analytic Story
|72.0||80||90||$process_name$ has been identified writing dll's to $file_path$ on endpoint $dest$. This behavior is suspicious and related to PrintNightmare.|
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.
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