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Description

This detection identifies instances of the Windows utility mshta.exe being used to write files to world-writable directories, a technique commonly leveraged by adversaries to execute malicious scripts or payloads. Starting from 26 February 2024, APT29 has been observed distributing phishing attachments that lead to the download and execution of the ROOTSAW dropper via a compromised website. The ROOTSAW payload, utilizing obfuscated JavaScript, downloads a file named invite.txt to the C:\Windows\Tasks directory. This file is then decoded and decompressed to execute a malicious payload, often leveraging legitimate Windows binaries for malicious purposes, as seen with SqlDumper.exe in this campaign.
The analytic is designed to detect the initial file write operation by mshta.exe to directories that are typically writable by any user, such as C:\Windows\Tasks, C:\Windows\Temp, and others. This behavior is indicative of an attempt to establish persistence, execute code, or both, as part of a multi-stage infection process. The detection focuses on the use of mshta.exe to write to these locations, which is a deviation from the utility's legitimate use cases and thus serves as a strong indicator of compromise (IoC).
The ROOTSAW campaign associated with APT29 utilizes a sophisticated obfuscation technique and leverages multiple stages of payloads, ultimately leading to the execution of the WINELOADER malware. This detection aims to catch the early stages of such attacks, enabling defenders to respond before full compromise occurs.

  • Type: TTP

  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud

  • Last Updated: 2024-03-26
  • Author: Michael Haag, Splunk
  • ID: efbcf8ee-bc75-47f1-8985-a5c638c4faf0

Annotations

ATT&CK

ATT&CK

ID Technique Tactic
T1218.005 Mshta Defense Evasion
Kill Chain Phase
  • Exploitation
NIST
  • DE.CM
CIS20
  • CIS 10
CVE 


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`sysmon` EventCode=11 Image="*\\mshta.exe" TargetFilename IN ("*\\Windows\\Tasks\\*", "*\\Windows\\Temp\\*", "*\\Windows\\tracing\\*", "*\\Windows\\PLA\\Reports\\*", "*\\Windows\\PLA\\Rules\\*", "*\\Windows\\PLA\\Templates\\*", "*\\Windows\\PLA\\Reports\\en-US\\*", "*\\Windows\\PLA\\Rules\\en-US\\*", "*\\Windows\\Registration\\CRMLog\\*", "*\\Windows\\System32\\Tasks\\*", "*\\Windows\\System32\\Com\\dmp\\*", "*\\Windows\\System32\\LogFiles\\WMI\\*", "*\\Windows\\System32\\Microsoft\\Crypto\\RSA\\MachineKeys\\*", "*\\Windows\\System32\\spool\\PRINTERS\\*", "*\\Windows\\System32\\spool\\SERVERS\\*", "*\\Windows\\System32\\spool\\drivers\\color\\*", "*\\Windows\\System32\\Tasks\\Microsoft\\Windows\\RemoteApp and Desktop Connections Update\\*", "*\\Windows\\SysWOW64\\Tasks\\*", "*\\Windows\\SysWOW64\\Com\\dmp\\*", "*\\Windows\\SysWOW64\\Tasks\\Microsoft\\Windows\\PLA\\*", "*\\Windows\\SysWOW64\\Tasks\\Microsoft\\Windows\\RemoteApp and Desktop Connections Update\\*", "*\\Windows\\SysWOW64\\Tasks\\Microsoft\\Windows\\PLA\\System\\*") 
| rename Computer as dest, User as user 
| stats count min(_time) as firstTime max(_time) as lastTime by dest, user, Image, TargetFilename 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `windows_mshta_writing_to_world_writable_path_filter`

Macros

The SPL above uses the following Macros:

:information_source: windows_mshta_writing_to_world_writable_path_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

  • dest
  • user
  • Image
  • TargetFilename

How To Implement

The analytic is designed to be run against Sysmon event logs collected from endpoints. The analytic requires the Sysmon event logs to be ingested into Splunk. The search focuses on EventCode 11 where the Image is mshta.exe and the TargetFilename is within world-writable directories such as C:\Windows\Tasks, C:\Windows\Temp, and others. The detection is designed to catch the initial file write operation by mshta.exe to these locations, which is indicative of an attempt to establish persistence or execute malicious code. The analytic can be modified to include additional world-writable directories as needed.

Known False Positives

False positives may occur if legitimate processes are writing to world-writable directories. It is recommended to investigate the context of the file write operation to determine if it is malicious or not. Modify the search to include additional known good paths for mshta.exe to reduce false positives.

Associated Analytic Story

RBA

Risk Score Impact Confidence Message
64.0 80 80 An instance of $Image$ writing to $TargetFilename$ was detected on $dest$.

:information_source: The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

Reference

Test Dataset

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

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