Detect Double File Extension in Google Chronicle
Adversaries may abuse a double extension in the filename as a means of masquerading the true file type. A file name may include a secondary file type extension that may cause only the first extension to be displayed (ex: File.txt.exe may render in some views as just File.txt). However, the second extension is the true file type that determines how the file is opened and executed. The real file extension may be hidden by the operating system in the file browser (ex: explorer.exe), as well as in any software configured using or similar to the system's policies. Adversaries may abuse double extensions to attempt to conceal dangerous file types of payloads, commonly tricking a user into opening what they think is a benign file type but is actually executable code. Such files often pose as email attachments and allow an adversary to gain Initial Access via Spearphishing Attachment then User Execution.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1036 Masquerading
- Sub-technique
- T1036.007 Double File Extension
- Canonical reference
- https://attack.mitre.org/techniques/T1036/007/
YARA-L Detection Query
rule double_file_extension_masquerading {
meta:
author = "Argus Detection Engineering"
description = "Detects file creation or rename events with double file extensions where a benign extension precedes an executable extension, indicating T1036.007 masquerading."
severity = "HIGH"
priority = "HIGH"
mitre_attack_tactic = "Defense Evasion"
mitre_attack_technique = "T1036.007"
false_positives = "AV quarantine renaming, build artifacts, backup utilities"
version = "1.0"
created = "2024-01-01"
events:
$e.metadata.event_type = "FILE_CREATION" or $e.metadata.event_type = "FILE_MOVE"
re.regex($e.target.file.full_path, `(?i)\.(txt|doc|docx|pdf|jpg|jpeg|png|gif|xls|xlsx|ppt|pptx|csv|rtf|bmp|mp3|mp4)\.(exe|scr|bat|cmd|com|pif|hta|lnk|vbs|vbe|js|jse|wsh|wsf|msi|ps1)$`)
condition:
$e
}Chronicle YARA-L 2.0 rule detecting FILE_CREATION and FILE_MOVE UDM events where the target file path matches a double extension pattern with a benign decoy extension followed by an executable extension. Leverages the UDM target.file.full_path field with a regex match.
Data Sources
Required Tables
False Positives & Tuning
- Backup and archiving tools that rename files by appending the original extension before compression or transfer
- Security products (EDR, AV) that quarantine files by renaming them with an appended extension, resulting in apparent double extensions
- Legitimate software deployment or patch management systems that stage installer packages with temporary compound extensions
Other platforms for T1036.007
Testing Methodology
Validate this detection against 3 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 1Create Double Extension EXE File
Expected signal: Sysmon Event ID 11: FileCreate with TargetFilename containing 'report.pdf.exe' in the user's Temp directory. Sysmon Event ID 1: Process Create for cmd.exe with CommandLine containing 'copy' and 'report.pdf.exe'. DeviceFileEvents with ActionType=FileCreated and FileName=report.pdf.exe.
- Test 2Create Double Extension LNK File (Kimsuky/DarkGate Pattern)
Expected signal: Sysmon Event ID 11: FileCreate with TargetFilename ending in 'invoice.pdf.lnk'. Sysmon Event ID 1: Process Create for powershell.exe with CommandLine referencing WScript.Shell and CreateShortcut. DeviceFileEvents with FileName=invoice.pdf.lnk.
- Test 3Create and Execute Double Extension SCR File
Expected signal: Sysmon Event ID 11: FileCreate with TargetFilename 'photo.jpg.scr'. Sysmon Event ID 1: Process Create with Image path ending in 'photo.jpg.scr'. DeviceFileEvents for file creation AND DeviceProcessEvents for process execution, both with the double extension filename.
References (7)
- https://attack.mitre.org/techniques/T1036/007/
- https://socprime.com/blog/rule-of-the-week-possible-malicious-file-double-extension/
- https://www.pcmag.com/encyclopedia/term/double-extension
- https://www.seqrite.com/blog/how-to-avoid-dual-attack-and-vulnerable-files-with-double-extension/
- https://learn.microsoft.com/en-us/defender-endpoint/advanced-hunting-devicefileevents-table
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1036.007/T1036.007.md
- https://www.trellix.com/blogs/research/the-darkgate-menace/
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