T1042 · threatengine.sh

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ATT&CK Technique

Change Default File Association

T1042 · persistence

▤ Generate a SIEM detection for T1042 ◈ Deployable detections for T1042 ⚠ CVEs mapped to T1042 ♛ Hunt package for T1042

When a file is opened, the default program used to open the file (also called the file association or handler) is checked. File association selections are stored in the Windows Registry and can be edited by users, administrators, or programs that have Registry access or by administrators using the built-in assoc utility. Applications can modify the file association for a given file extension to call an arbitrary program when a file with the given extension is opened.

System file associations are listed under HKEY_CLASSES_ROOT\.[extension], for example HKEY_CLASSES_ROOT\.txt. The entries point to a handler for that extension located at HKEY_CLASSES_ROOT\[handler]. The various commands are then listed as subkeys underneath the shell key at HKEY_CLASSES_ROOT\[handler]\shell\[action]\command.

For example: HKEY_CLASSES_ROOT\txtfile\shell\open\command HKEY_CLASSES_ROOT\txtfile\shell\print\command * HKEY_CLASSES_ROOT\txtfile\shell\printto\command The values of the keys listed are commands that are executed when the handler opens the file extension. Adversaries can modify these values to continually execute arbitrary commands.

Windows

▸ How to use this page - the detection-engineering loop

Attackers have goals (tactics - “get credentials”, “move laterally”) and techniques are the concrete methods they use to reach them. This page is one method - T1042 - broken into everything you need to catch it.

The loop this page is built for (this is the job):

Understand the behaviour - read the description and the Atomic Tests to see exactly what the attacker does on a host or network.
Find the telemetry - what data source would reveal it (process creation, registry, network flow, auth logs). Detection Coverage shows which surfaces already have a rule and which are blind.
Get or write the detection - adapt ready logic (CAR Analytics, SIEM Detections, Falco, or Sigma via Generate a SIEM detection), or author your own.
Test it - run an Atomic Test in a lab and confirm your rule actually fires. A detection you have not tested is a hope, not coverage.
Deploy and tune - push it, then watch for false positives and adjust.

What each panel is for:

Atomic Testssafely reproduce the technique in a lab to validate that a detection fires. Detection Coveragewhich detection surfaces have a rule for this technique; none is a blind spot to close, or simply not applicable (YARA matches files, not network behaviour). CAR / SIEM / Falcoready-made detection logic (Splunk SPL, Elastic EQL, Sentinel KQL, Falco) you adapt to your own SIEM. Mitigationsreduce exposure so the technique is harder to use at all - prevent, not just detect. Actors / Attributionwho actually uses this, so you prioritise by your own threat model. Attack Path / LOTLwhat attackers do before and after this step, and the legitimate tools they abuse to do it.

Where this fits: you usually arrive here from a CVE (“which techniques does it enable”) and leave with a tested detection deployed. The buttons above jump straight to building one, the deployable rules, the CVEs that use T1042, and a hunt package.

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Detection Coverage

0/9 layers

Coverage across standard detection surfaces. Rows marked none have no rule of that type mapped. Some are real blind spots worth closing; others are simply not applicable to this technique (e.g. YARA matches malware files, not network behaviour).

Behavioral / log (Sigma) none

Analytics (MITRE CAR) none

Runtime / container (Falco) none

File / malware (YARA) none

Network (Suricata/Snort) none

Vuln scan (Nuclei) none

SIEM (Splunk ESCU) none

SIEM (Elastic) none

SIEM (Azure Sentinel) none

External lookups - second-class, for what we don’t hold ourselves

MITRE ATT&CK Atomic Red Team car (MITRE)