File/Path Exclusions

What the technique is

Adversaries may attempt to hide their file-based artifacts by writing them to specific folders or file names excluded from antivirus (AV) scanning and other defensive capabilities. AV and other file-based scanners often include exclusions to optimize performance as well as ease installation and legitimate use of applications. These exclusions may be contextual (e.g., scans are only initiated in response to specific triggering events/alerts), but are also often hardcoded strings referencing specific folders and/or files assumed to be trusted and legitimate. Adversaries may abuse these exclusions to hide their file-based artifacts. For example, rather than tampering with tool settings to add a new exclusion (i.e., Disable or Modify Tools), adversaries may drop their file-based payloads in default or otherwise well-known exclusions. Adversaries may also use Security Software Discovery and other Discovery/Reconnaissance activities to both discover and verify existing exclusions in a victim environment.

What MITRE says you would watch

• AN0139

  Creation or modification of files in directories known to be excluded from AV scanning (e.g., C:\Windows\Temp, Exchange server directories, or default AV exclusions). Defender perspective: correlate file creation with execution behavior or anomalous parent processes writing to excluded paths.

• AN0140

  Adversaries writing or moving payloads into directories configured as AV/EDR exclusion paths (e.g., /tmp, /var/lib, or custom directories from auditd exclusion rules). Defender perspective: detect file creation in paths matching known exclusions correlated with unusual parent processes.

• AN0141

  Suspicious file creation or modification in directories ignored by XProtect or AV exclusions (e.g., ~/Library, temporary cache directories). Defender perspective: monitor file events in ignored paths with correlation to execution or persistence activity.