Detect Space after Filename in Splunk
Adversaries can hide a program's true filetype by changing the extension of a file. With certain file types (specifically this does not work with .app extensions), appending a space to the end of a filename will change how the file is processed by the operating system. For example, if a Mach-O executable file called evil.bin is renamed to evil.txt (space at end), when double clicked by a user, the true file type is determined by the OS and handled appropriately and the binary will be executed. This technique primarily targets macOS and Linux systems.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1036 Masquerading
- Sub-technique
- T1036.006 Space after Filename
- Canonical reference
- https://attack.mitre.org/techniques/T1036/006/
SPL Detection Query
index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" (EventCode=11 OR EventCode=15)
| where match(TargetFilename, "\.(txt|pdf|doc|docx|jpg|jpeg|png|gif|mp4|mp3|csv|xls|xlsx|rtf)\s+$")
| table _time, host, User, Image, TargetFilename, Hashes
| sort - _timeDetects file creation events (Sysmon Event ID 11) and alternate data stream creation (Event ID 15) where the target filename contains a trailing space after a benign extension. This is a macOS/Linux-focused technique but file creation may be logged on Windows hosts when files are transferred or synced.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Users accidentally adding trailing spaces when renaming files
- File synchronization tools preserving trailing spaces from other operating systems
- Automated file processing systems generating files with improperly trimmed names
Other platforms for T1036.006
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 Executable with Trailing Space (macOS)
Expected signal: Process creation event for '/tmp/report.txt ' (with trailing space). File creation event showing the trailing space in the filename.
- Test 2Create Disguised Binary with Trailing Space (Linux)
Expected signal: Auditd SYSCALL execve event for '/tmp/photo.jpg '. File creation event for the disguised binary.
- Test 3Create File with Multiple Trailing Spaces (Windows)
Expected signal: Sysmon Event ID 11: FileCreate for the file (Windows may strip the space). PowerShell ScriptBlock Log Event ID 4104.
References (4)
- https://attack.mitre.org/techniques/T1036/006/
- https://arstechnica.com/security/2016/07/after-hiatus-in-the-wild-mac-backdoors-are-suddenly-back/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1036.006/T1036.006.md
- https://www.sentinelone.com/blog/trail-of-windows-mitre-attack-evasion-techniques/
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