Visualizing clfs.sys Threat Activity in Microsoft Defender with Uncoder AI’s Decision Tree

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May 01, 2025 · 5 min read
Visualizing clfs.sys Threat Activity in Microsoft Defender with Uncoder AI’s Decision Tree

Loading legitimate system drivers from illegitimate or suspicious directories is a known tactic for persistence, evasion, or execution by adversaries. One high-value target in this category is clfs.sys — a legitimate Windows driver tied to the Common Log File System.

To detect this activity, Microsoft Defender for Endpoint supports advanced KQL-based detection logic. But to truly operationalize these queries, analysts need visibility into how they work. That’s exactly what Uncoder AI’s AI-generated Decision Tree delivers.

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Use Case: clfs.sys Loaded From User or Temp Paths

This query identifies when clfs.sys is loaded from non-standard or user-controlled directories, which may include:

  • \Users\Public\

  • \Temporary Internet\

  • \Windows\Temp\

  • Subfolders under \Users\ such as \Pictures\ , \Contacts\ , or \Favourites\

These paths are rarely expected to host or initiate the loading of system-level drivers, making such activity highly suspicious.

Input we used (click to show the text)
DeviceImageLoadEvents | where (FolderPath endswith @'\clfs.sys' and ((InitiatingProcessFolderPath contains @':\Perflogs\' or InitiatingProcessFolderPath contains @':\Users\Public\' or InitiatingProcessFolderPath contains @'\Temporary Internet' or InitiatingProcessFolderPath contains @'\Windows\Temp\') or (InitiatingProcessFolderPath contains @':\Users\' and InitiatingProcessFolderPath contains @'\Favorites\') or (InitiatingProcessFolderPath contains @':\Users\' and InitiatingProcessFolderPath contains @'\Favourites\') or (InitiatingProcessFolderPath contains @':\Users\' and InitiatingProcessFolderPath contains @'\Contacts\') or (InitiatingProcessFolderPath contains @':\Users\' and InitiatingProcessFolderPath contains @'\Pictures\')))

How the Decision Tree Breaks It Down

Uncoder AI automatically transforms this detection logic into a human-readable decision tree structure, showing:

  1. Query Scope:
    It filters from DeviceImageLoadEvents where the folder path ends with clfs.sys.
  2. Folder Path Conditions:
    The decision tree checks whether the initiating process’s folder path contains any unusual user or temporary storage locations.
  3. Branching Conditions:
    Sub-branches clarify which paths are considered:

    • Generic temp or cache folders
    • User-specific locations that imply personal data access
    • Combined conditions like \Users\ + \Favourites\ or \Pictures\
  4. Outcome:
    If any one of these conditions is met, the event is marked as relevant—flagging a potential misuse of clfs.sys from suspicious origin paths.
AI Output (click to show the text)
The Microsoft Defender for Endpoint Query you’ve provided is designed to filter `DeviceImageLoadEvents` based on specific conditions related to the loading of a particular system file (`clfs.sys`) and the paths from which the initiating processes are launched. Here’s a breakdown of the decision tree involved in the condition part of this query:

1. **Initial Condition**: The query starts by looking at all `DeviceImageLoadEvents` where the `FolderPath` ends with `\clfs.sys`. This means it’s specifically interested in events involving the loading of the `clfs.sys` file, which is a system file related to the Common Log File System (CLFS) used by Windows.

2. **Main Conditional Branch**:
– The query then applies an OR condition to filter these events based on the `InitiatingProcessFolderPath`. This means it checks if any of the following conditions are true for the folder path of the process that initiated the loading of `clfs.sys`.

3. **Sub-Conditions for Initiating Process Folder Path**:
– The first set of sub-conditions checks if the `InitiatingProcessFolderPath` contains specific directories that could indicate potentially malicious or unauthorized activity, such as:
– `:\Perflogs\`
– `:\Users\Public\`
– `\Temporary Internet`
– `\Windows\Temp\`
– These paths are often associated with temporary files, public user data, internet cache, and system temporary files, respectively. Loading system files like `clfs.sys` from these contexts could be unusual or indicative of malicious activity.

– The next set of sub-conditions checks if the `InitiatingProcessFolderPath` contains specific user directories that might indicate unauthorized access to personal data:
– `:\Users\` combined with any of the following: `\Favorites\`, `\Favourites\`, `\Contacts\`, or `\Pictures\`.
– These conditions aim to identify scenarios where a process, potentially malicious, is accessing sensitive user data locations while loading system files.

4. **Decision Outcome**:
– If any of these sub-conditions are met (i.e., if the `InitiatingProcessFolderPath` matches any of the specified patterns), then the event is included in the query results.
– Essentially, the query is looking for instances where the loading of `clfs.sys` might be associated with potentially suspicious or unauthorized activity based on the initiating process’s location.

In summary, this Microsoft Defender for Endpoint Query uses a detailed decision tree to identify and flag `DeviceImageLoadEvents` involving the `clfs.sys` file loaded from specific folder paths that could indicate malicious or unusual system activity.

Why This Matters

  •  
  • Elevated Context: Analysts no longer need to decipher layered OR conditions to understand risk coverage.
  • Faster Triage: You can see in seconds whether the rule catches temp path misuse, user folder abuse, or both.
  • Audit-Ready Logic: The tree-style breakdown supports easier documentation and SOC training efforts.

Ultimately, loading clfs.sys from outside its expected system directories may represent driver abuse, living-off-the-land tactics, or malware masquerading as legitimate processes.

From Folder Flags to Threat Defense

Uncoder AI’s Decision Tree takes verbose KQL and turns it into an intuitive, SOC-friendly format. Whether you’re tuning this rule, hunting for anomalies, or explaining detection logic to leadership—this feature makes it actionable.

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