Detect Masquerade File Type in Elastic Security
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/
Elastic Detection Query
sequence by host.id with maxspan=5m
[file where event.action in ("creation", "modification") and
file.extension in ("jpg", "jpeg", "png", "gif", "bmp", "txt", "pdf", "doc", "mp3", "wav", "avi", "pub", "accdb") and
file.size > 50000 and
process.name in~ ("powershell.exe", "cmd.exe", "wscript.exe", "cscript.exe", "mshta.exe", "certutil.exe", "bitsadmin.exe", "rundll32.exe") and
(
file.path like~ "*\\temp\\*" or
file.path like~ "*\\tmp\\*" or
file.path like~ "*\\appdata\\*" or
file.path like~ "*\\downloads\\*"
)
] by process.entity_idDetects masqueraded file type drops where suspicious scripting or LOLBin processes write files with benign media or document extensions (jpg, png, pdf, etc.) that exceed 50KB into writable user directories. Sequences on host and process entity to correlate parent context.
Data Sources
Required Tables
False Positives & Tuning
- Certutil legitimately decodes base64 content to files with arbitrary extensions during PKI operations or package deployments
- PowerShell scripts that generate reports or exports as PDF/CSV may write large files to AppData or temp directories
- Software installers using cmd.exe or msiexec dropping binary blobs with document-like intermediate filenames during staged extraction
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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