Detect Double File Extension in CrowdStrike LogScale
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/
LogScale Detection Query
#event_simpleName=FileOpenInfo OR #event_simpleName=NewExecutableWritten
| TargetFileName = /(?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)$/
| regex("(?i)(?<second_ext>\.[a-z0-9]{2,5})\.(?<final_ext>[a-z0-9]{2,5})$", field=TargetFileName, as=[second_ext, final_ext])
| table([_timstamp, ComputerName, UserName, TargetFileName, second_ext, final_ext, ImageFileName, CommandLine])
| sort(field=@timestamp, order=desc)CrowdStrike Falcon LogScale (CQL) query detecting file creation and executable write events where the TargetFileName matches the double extension masquerading pattern. Uses regex to extract and display the decoy and true extensions for analyst triage.
Data Sources
Required Tables
False Positives & Tuning
- CrowdStrike Falcon itself or other endpoint security tools that write quarantine copies of files with compound extensions appended to the original filename
- Software development or CI/CD pipelines running on endpoints that create build artifacts with version strings or type qualifiers embedded before the final extension
- End users who intentionally save documents with compound names for organizational purposes, and whose naming convention happens to end in an executable extension
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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