File and Directory Permissions Modification: Linux and Mac File and Directory Permissions Modification

ID Name
T1222.001 Windows File and Directory Permissions Modification
T1222.002 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.[1][2] 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.).

Most Linux and Linux-based platforms provide a standard set of permission groups (user, group, and other) and a standard set of permissions (read, write, and execute) that are applied to each group. While nuances of each platform’s permissions implementation may vary, most of the platforms provide two primary commands used to manipulate file and directory ACLs: chown (short for change owner), and chmod (short for change mode).

Adversarial may use these commands to make themselves the owner of files and directories or change the mode if current permissions allow it. They could subsequently lock others out of the file. Specific file and directory modifications may be a required step for many techniques, such as establishing Persistence via .bash_profile and .bashrc or tainting/hijacking other instrumental binary/configuration files via Hijack Execution Flow.

ID: T1222.002
Sub-technique of:  T1222
Tactic: Defense Evasion
Platforms: Linux, macOS
Permissions Required: User, root
Data Sources: File monitoring, Process command-line parameters, Process monitoring
Version: 1.0
Created: 04 February 2020
Last Modified: 29 March 2020

Procedure Examples

Name Description
APT32

APT32's macOS backdoor changes the permission of the file it wants to execute to 755.[4]

OSX/Shlayer

OSX/Shlayer can use the chmod utility to set a .app file as executable.[3]

Mitigations

Mitigation Description
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.

Detection

Monitor and investigate attempts to modify ACLs and file/directory ownership. Many of the commands used to modify ACLs 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.

Consider enabling file/directory permission change auditing on folders containing key binary/configuration files.

References