Abuse Elevation Control Mechanism: Sudo and Sudo Caching

Adversaries may perform sudo caching and/or use the suoders file to elevate privileges. Adversaries may do this to execute commands as other users or spawn processes with higher privileges.

Within Linux and MacOS systems, sudo (sometimes referred to as "superuser do") allows users to perform commands from terminals with elevated privileges and to control who can perform these commands on the system. The sudo command "allows a system administrator to delegate authority to give certain users (or groups of users) the ability to run some (or all) commands as root or another user while providing an audit trail of the commands and their arguments."[1] Since sudo was made for the system administrator, it has some useful configuration features such as a timestamp_timeout, which is the amount of time in minutes between instances of sudo before it will re-prompt for a password. This is because sudo has the ability to cache credentials for a period of time. Sudo creates (or touches) a file at /var/db/sudo with a timestamp of when sudo was last run to determine this timeout. Additionally, there is a tty_tickets variable that treats each new tty (terminal session) in isolation. This means that, for example, the sudo timeout of one tty will not affect another tty (you will have to type the password again).

The sudoers file, /etc/sudoers, describes which users can run which commands and from which terminals. This also describes which commands users can run as other users or groups. This provides the principle of least privilege such that users are running in their lowest possible permissions for most of the time and only elevate to other users or permissions as needed, typically by prompting for a password. However, the sudoers file can also specify when to not prompt users for passwords with a line like user1 ALL=(ALL) NOPASSWD: ALL [2]. Elevated privileges are required to edit this file though.

Adversaries can also abuse poor configurations of these mechanisms to escalate privileges without needing the user's password. For example, /var/db/sudo's timestamp can be monitored to see if it falls within the timestamp_timeout range. If it does, then malware can execute sudo commands without needing to supply the user's password. Additional, if tty_tickets is disabled, adversaries can do this from any tty for that user.

In the wild, malware has disabled tty_tickets to potentially make scripting easier by issuing echo \'Defaults !tty_tickets\' >> /etc/sudoers [3]. In order for this change to be reflected, the malware also issued killall Terminal. As of macOS Sierra, the sudoers file has tty_tickets enabled by default.

ID: T1548.003
Sub-technique of:  T1548
Tactics: Privilege Escalation, Defense Evasion
Platforms: Linux, macOS
Permissions Required: User
Effective Permissions: root
Data Sources: File monitoring, Process command-line parameters
Version: 1.0
Created: 30 January 2020
Last Modified: 27 March 2020

Procedure Examples

Name Description
Proton

Proton modifies the tty_tickets line in the sudoers file.[4]

Mitigations

Mitigation Description
Operating System Configuration

Ensuring that the tty_tickets setting is enabled will prevent this leakage across tty sessions.

Privileged Account Management

By requiring a password, even if an adversary can get terminal access, they must know the password to run anything in the sudoers file. Setting the timestamp_timeout to 0 will require the user to input their password every time sudo is executed.

Restrict File and Directory Permissions

The sudoers file should be strictly edited such that passwords are always required and that users can't spawn risky processes as users with higher privilege.

Detection

On Linux, auditd can alert every time a user's actual ID and effective ID are different (this is what happens when you sudo). This technique is abusing normal functionality in macOS and Linux systems, but sudo has the ability to log all input and output based on the LOG_INPUT and LOG_OUTPUT directives in the /etc/sudoers file.

References