Detect Change Default File Association in Elastic Security
Adversaries may establish persistence by changing the default file association for a file extension. When a file is opened, the default program associated with the file extension is executed. Adversaries can exploit this by modifying the registry key that defines the default handler for a given file extension (e.g., .txt, .js, .hta) to point to a malicious executable, causing their payload to execute whenever a user opens a file with that extension.
MITRE ATT&CK
- Tactic
- Privilege Escalation Persistence
- Technique
- T1546 Event Triggered Execution
- Sub-technique
- T1546.001 Change Default File Association
- Canonical reference
- https://attack.mitre.org/techniques/T1546/001/
Elastic Detection Query
registry where event.type in ("creation", "change") and registry.path : ("*\\SOFTWARE\\Classes\\*\\shell\\open\\command*", "*\\HKEY_CLASSES_ROOT\\*\\shell\\open\\command*") and registry.data.strings : ("*powershell*", "*cmd.exe*", "*wscript*", "*cscript*", "*mshta*", "*rundll32*", "*regsvr32*", "*certutil*", "*bitsadmin*", "*AppData*", "*\\Temp\\*", "*ProgramData*") and not registry.data.strings : ("C:\\Windows\\system32\\*", "C:\\Program Files\\*", "C:\\Program Files (x86)\\*")Detects modification of file association handlers under HKCR or HKLM\SOFTWARE\Classes targeting the shell\open\command key with suspicious executables or paths, indicating T1546.001 persistence via file association hijacking.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate software installers that register new file type handlers (e.g., IDEs, media players, archive tools) during installation may modify shell\open\command keys.
- System administrators using scripting tools like PowerShell to programmatically associate file extensions with approved internal tooling.
- Software repair or update processes that re-register file handlers, particularly for productivity suites like Microsoft Office or LibreOffice.
Other platforms for T1546.001
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 1Change .txt File Association to cmd.exe
Expected signal: Sysmon Event ID 13 (Registry Value Set): TargetObject contains HKCU\Software\Classes\.txt\shell\open\command, Details contains 'cmd.exe'. Security Event ID 4657 if object access auditing is enabled. Process creation for reg.exe with the add command.
- Test 2Change .js File Association to PowerShell
Expected signal: Sysmon Event ID 13: TargetObject HKCU\Software\Classes\.js\shell\open\command, Details contains powershell.exe. Process creation for reg.exe.
- Test 3Establish Persistence via .hta File Association
Expected signal: Sysmon Event ID 13: TargetObject HKCU\Software\Classes\.hta\shell\open\command, Details contains mshta.exe. The vbscript: protocol in the handler is a high-fidelity indicator of malicious intent.
References (5)
- https://attack.mitre.org/techniques/T1546/001/
- https://docs.microsoft.com/en-us/windows/win32/shell/fa-intro
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1546.001/T1546.001.md
- https://pentestlab.blog/2020/01/13/persistence-change-default-file-association/
- https://www.hexacorn.com/blog/2018/03/15/beyond-good-ol-run-key-part-75/
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