|Windows File and Directory Permissions Modification
|Linux and Mac File and Directory Permissions Modification
Adversaries may modify file or directory permissions/attributes to evade access control lists (ACLs) and access protected files. File and directory permissions are commonly managed by ACLs configured by the file or directory owner, or users with the appropriate permissions. File and directory ACL implementations vary by platform, but generally explicitly designate which users or groups can perform which actions (read, write, execute, etc.).
Windows implements file and directory ACLs as Discretionary Access Control Lists (DACLs). Similar to a standard ACL, DACLs identifies the accounts that are allowed or denied access to a securable object. When an attempt is made to access a securable object, the system checks the access control entries in the DACL in order. If a matching entry is found, access to the object is granted. Otherwise, access is denied.
Adversaries can interact with the DACLs using built-in Windows commands, such as
attrib, which can grant adversaries higher permissions on specific files and folders. Further, PowerShell provides cmdlets that can be used to retrieve or modify file and directory DACLs. Specific file and directory modifications may be a required step for many techniques, such as establishing Persistence via Accessibility Features, Boot or Logon Initialization Scripts, or tainting/hijacking other instrumental binary/configuration files via Hijack Execution Flow.
|Privileged Account Management
Ensure critical system files as well as those known to be abused by adversaries have restrictive permissions and are owned by an appropriately privileged account, especially if access is not required by users nor will inhibit system functionality.
|Restrict File and Directory Permissions
Applying more restrictive permissions to files and directories could prevent adversaries from modifying the access control lists.
|Active Directory Object Modification
Monitor for changes made to DACLs and file/directory ownership. Many of the commands used to modify DACLs and file/directory ownership are built-in system utilities and may generate a high false positive alert rate, so compare against baseline knowledge for how systems are typically used and correlate modification events with other indications of malicious activity where possible.
Monitor for executed commands and arguments for PowerShell cmdlets that can be used to retrieve or modify file and directory DACLs.
Consider enabling file/directory permission change auditing on folders containing key binary/configuration files. For example, Windows Security Log events (Event ID 4670) are created when DACLs are modified.
Adversaries sometimes modify object access rights at the operating system level. There are varying motivations behind this action - they may not want some files/objects to be changed on systems for persistence reasons and therefore provide admin only rights; also, they may want files to be accessible with lower levels of permissions.
Windows environment logs can be noisy, so we take the following into consideration:
Linux environment logs can be more noisy than the Windows-specific implementation, although Linux does not generate logs for system triggered activities like in Windows. In addition, it may be necessary to whitelist cron jobs that regularly run and execute chmod.
Analytic 1 : Access Permission Modification for Windows
Analytic 2 - Access Permission Modification for Linux
Monitor for newly constructed processes and/or command-lines that can interact with the DACLs using built-in Windows commands, such as icacls, cacls, takeown, and attrib, which can grant adversaries higher permissions on specific files and folders.