Detect Change Default File Association in Microsoft Sentinel
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/
KQL Detection Query
DeviceRegistryEvents
| where Timestamp > ago(24h)
| where RegistryKey has_any (
"SOFTWARE\\Classes\\",
"HKEY_CLASSES_ROOT\\"
)
| where RegistryKey matches regex @"\\shell\\open\\command"
| where ActionType in ("RegistryValueSet", "RegistryKeyCreated")
| where RegistryValueData !has_any (
"C:\\Windows\\system32\\",
"C:\\Program Files\\",
"C:\\Program Files (x86)\\"
)
| extend Extension = extract(@"Classes\\(\.\w+)\\", 1, RegistryKey)
| extend SuspiciousHandler = RegistryValueData has_any (
"powershell", "cmd.exe", "wscript", "cscript", "mshta",
"rundll32", "regsvr32", "certutil", "bitsadmin", "AppData", "Temp", "ProgramData"
)
| project Timestamp, DeviceName, AccountName, ActionType, RegistryKey,
RegistryValueName, RegistryValueData, Extension, SuspiciousHandler,
InitiatingProcessFileName, InitiatingProcessCommandLine
| sort by Timestamp descDetects modifications to Windows file association registry keys under HKCU\SOFTWARE\Classes and HKCR that point to suspicious executables. Focuses on shell\open\command subkeys being changed to non-standard locations (outside System32 and Program Files). Flags handlers containing scripting engines, LOLBins, or writable user paths.
Data Sources
Required Tables
False Positives & Tuning
- Software installations that legitimately register file handlers (e.g., installing a new browser changes .html, .htm associations)
- Development tools registering custom file extensions for project files (Visual Studio, JetBrains IDEs)
- PDF readers, media players, and archiving utilities that change handler associations during install or on first run
- Group Policy-driven file association changes pushed by IT during software deployments
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/
Unlock Pro Content
Get the full detection package for T1546.001 including response playbook, investigation guide, and atomic red team tests.