Detect Masquerade File Type in CrowdStrike LogScale
Adversaries may masquerade malicious payloads as legitimate files through changes to the payload's formatting, including the file's signature, extension, icon, and contents. Various file types have a typical standard format, including how they are encoded and organized. For example, a file's signature (also known as header or magic bytes) is the beginning bytes of a file and is often used to identify the file's type. Adversaries may edit the header's hex code and/or the file extension of a malicious payload in order to bypass file validation checks and/or input sanitization. This behavior is commonly used when payload files are transferred and stored so that adversaries may move their malware without triggering detections. Polyglot files, which function differently based on the application that executes them, may also be used to disguise malicious capabilities.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1036 Masquerading
- Sub-technique
- T1036.008 Masquerade File Type
- Canonical reference
- https://attack.mitre.org/techniques/T1036/008/
LogScale Detection Query
#event_simpleName=WriteFile
| TargetFileName = /\.(gif|jpg|jpeg|png|bmp|txt|pdf|mp3|wav|pub|accdb)$/i
| ImageFileName = /(?i)(powershell\.exe|cmd\.exe|wscript\.exe|cscript\.exe|mshta\.exe|certutil\.exe|bitsadmin\.exe|rundll32\.exe)$/
| TargetFileName = /(?i)(\\temp\\|\\tmp\\|\\appdata\\|\\downloads\\)/
| eval proc_name = replace(ImageFileName, /^.*\\([^\\]+)$/, "$1")
| eval file_ext = replace(TargetFileName, /^.*(\.\w+)$/, "$1")
| table([_time, ComputerName, UserName, proc_name, TargetFileName, file_ext, TargetProcessId, CommandLine])
| sort(field=_time, order=desc)CrowdStrike LogScale query against WriteFile telemetry matching Falcon sensor events where a LOLBin process writes a file with a benign media or document extension to a high-risk user-writable directory. Surfaces the process name, command line, target path, and user for rapid analyst triage.
Data Sources
Required Tables
False Positives & Tuning
- Bitsadmin.exe or certutil.exe used by SCCM or Intune to stage pre-deployment packages as encoded blobs with image extensions
- PowerShell-based log collection scripts exporting structured data to .txt or .pdf report files in the user downloads directory
- Security tooling (e.g., DLP agents) using cmd.exe to create shadow copies of sensitive documents in AppData with renamed extensions for inspection
Other platforms for T1036.008
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Masquerade EXE as GIF File (Volt Typhoon Pattern)
Expected signal: Sysmon Event ID 11: FileCreate with TargetFilename 'data_export.gif'. The file hash will match calc.exe despite the .gif extension. DeviceFileEvents with FileName=data_export.gif, ActionType=FileCreated, InitiatingProcessFileName=cmd.exe.
- Test 2Create Polyglot HTML/DLL File (StrelaStealer Pattern)
Expected signal: Sysmon Event ID 11: FileCreate with TargetFilename 'invoice_polyglot.html'. Sysmon Event ID 1: Process Create for powershell.exe with Set-Content command. PowerShell ScriptBlock Log Event ID 4104 with the polyglot content creation.
- Test 3Rename DLL to Image Extension
Expected signal: Sysmon Event ID 11: FileCreate with TargetFilename 'screenshot.png' in Temp directory. File hash will match version.dll. DeviceFileEvents with FileName=screenshot.png created by cmd.exe.
- Test 4Certutil Download with Extension Masquerade
Expected signal: Sysmon Event ID 1: Process Create for certutil.exe with '-encode' in CommandLine. Sysmon Event ID 11: FileCreate with TargetFilename 'payload.txt'. DeviceProcessEvents with ProcessCommandLine containing 'certutil -encode'.
References (7)
- https://attack.mitre.org/techniques/T1036/008/
- https://unit42.paloaltonetworks.com/polyglot-file-icedid-payload
- https://www.secureworks.com/research/bronze-silhouette-targets-us-government-and-defense-organizations
- https://learn.microsoft.com/en-us/defender-endpoint/advanced-hunting-devicefileevents-table
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1036.008/T1036.008.md
- https://www.withsecure.com/en/research/publications/kapeka
- https://www.netskope.com/blog/lumma-stealer
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