Understanding Privilege Escalation

Ansible can use existing privilege escalation systems to allow a user to execute tasks as another.

• Become
• Directives
  • Connection variables
  • Command line options
  • For those from Pre 1.9 , sudo and su still work!
  • Limitations
    • Becoming an Unprivileged User
    • Connection Plugin Support
    • Only one method may be enabled per host
    • Can’t limit escalation to certain commands
• Become and Networks
  • network_cli and become
  • Setting enable mode for all tasks
    • Passwords for enable mode
  • authorize and auth_pass
• Become and Windows
  • Administrative Rights
  • Local Service Accounts
  • Accounts without a Password
  • Become Flags
  • Limitations

Become

Ansible allows you to ‘become’ another user, different from the user that logged into the machine (remote user). This is done using existing privilege escalation tools such as sudo, su, pfexec, doas, pbrun, dzdo, ksu, runas and others.

Directives

These can be set from play to task level, but are overridden by connection variables as they can be host specific.

become

set to yes to activate privilege escalation.

become_user

set to user with desired privileges — the user you become, NOT the user you login as. Does NOT imply become: yes, to allow it to be set at host level.

become_method

(at play or task level) overrides the default method set in ansible.cfg, set to sudo/su/pbrun/pfexec/doas/dzdo/ksu/runas

become_flags

(at play or task level) permit the use of specific flags for the tasks or role. One common use is to change the user to nobody when the shell is set to no login. Added in Ansible 2.2.

For example, to manage a system service (which requires root privileges) when connected as a non-root user (this takes advantage of the fact that the default value of become_user is root):

- name: Ensure the httpd service is running
  service:
    name: httpd
    state: started
  become: yes

To run a command as the apache user:

- name: Run a command as the apache user
  command: somecommand
  become: yes
  become_user: apache

To do something as the nobody user when the shell is nologin:

- name: Run a command as nobody
  command: somecommand
  become: yes
  become_method: su
  become_user: nobody
  become_flags: '-s /bin/sh'

Connection variables

Each allows you to set an option per group and/or host, these are normally defined in inventory but can be used as normal variables.

ansible_become

equivalent of the become directive, decides if privilege escalation is used or not.

ansible_become_method

which privilege escalation method should be used

ansible_become_user

set the user you become through privilege escalation; does not imply ansible_become: yes

ansible_become_pass

set the privilege escalation password. See Using Vault in playbooks for details on how to avoid having secrets in plain text

For example, if you want to run all tasks as root on a server named webserver, but you can only connect as the manager user, you could use an inventory entry like this:

webserver ansible_user=manager ansible_become=yes

Command line options

--ask-become-pass, -K
	ask for privilege escalation password; does not imply become will be used. Note that this password will be used for all hosts.
--become, -b	run operations with become (no password implied)
--become-method=BECOME_METHOD
	privilege escalation method to use (default=sudo), valid choices: [ sudo | su | pbrun | pfexec | doas | dzdo | ksu | runas ]
--become-user=BECOME_USER
	run operations as this user (default=root), does not imply –become/-b

For those from Pre 1.9 , sudo and su still work!

For those using old playbooks will not need to be changed, even though they are deprecated, sudo and su directives, variables and options will continue to work. It is recommended to move to become as they may be retired at one point. You cannot mix directives on the same object (become and sudo) though, Ansible will complain if you try to.

Become will default to using the old sudo/su configs and variables if they exist, but will override them if you specify any of the new ones.

Limitations

Although privilege escalation is mostly intuitive, there are a few limitations on how it works. Users should be aware of these to avoid surprises.

Becoming an Unprivileged User

Ansible 2.0.x and below has a limitation with regards to becoming an unprivileged user that can be a security risk if users are not aware of it. Ansible modules are executed on the remote machine by first substituting the parameters into the module file, then copying the file to the remote machine, and finally executing it there.

Everything is fine if the module file is executed without using become, when the become_user is root, or when the connection to the remote machine is made as root. In these cases the module file is created with permissions that only allow reading by the user and root.

The problem occurs when the become_user is an unprivileged user. Ansible 2.0.x and below make the module file world readable in this case, as the module file is written as the user that Ansible connects as, but the file needs to be readable by the user Ansible is set to become.

If any of the parameters passed to the module are sensitive in nature, then those pieces of data are located in a world readable module file for the duration of the Ansible module execution. Once the module is done executing, Ansible will delete the temporary file. If you trust the client machines then there’s no problem here. If you do not trust the client machines then this is a potential danger.

Ways to resolve this include:

• Use pipelining. When pipelining is enabled, Ansible doesn’t save the module to a temporary file on the client. Instead it pipes the module to the remote python interpreter’s stdin. Pipelining does not work for python modules involving file transfer (for example: copy, fetch, template), or for non-python modules.
• (Available in Ansible 2.1) Install POSIX.1e filesystem acl support on the managed host. If the temporary directory on the remote host is mounted with POSIX acls enabled and the setfacl tool is in the remote PATH then Ansible will use POSIX acls to share the module file with the second unprivileged user instead of having to make the file readable by everyone.
• Don’t perform an action on the remote machine by becoming an unprivileged user. Temporary files are protected by UNIX file permissions when you become root or do not use become. In Ansible 2.1 and above, UNIX file permissions are also secure if you make the connection to the managed machine as root and then use become to an unprivileged account.

In addition to the additional means of doing this securely, Ansible 2.1 also makes it harder to unknowingly do this insecurely. Whereas in Ansible 2.0.x and below, Ansible will silently allow the insecure behaviour if it was unable to find another way to share the files with the unprivileged user, in Ansible 2.1 and above Ansible defaults to issuing an error if it can’t do this securely. If you can’t make any of the changes above to resolve the problem, and you decide that the machine you’re running on is secure enough for the modules you want to run there to be world readable, you can turn on allow_world_readable_tmpfiles in the ansible.cfg file. Setting allow_world_readable_tmpfiles will change this from an error into a warning and allow the task to run as it did prior to 2.1.

Connection Plugin Support

Privilege escalation methods must also be supported by the connection plugin used. Most connection plugins will warn if they do not support become. Some will just ignore it as they always run as root (jail, chroot, etc).

Only one method may be enabled per host

Methods cannot be chained. You cannot use sudo /bin/su - to become a user, you need to have privileges to run the command as that user in sudo or be able to su directly to it (the same for pbrun, pfexec or other supported methods).

Can’t limit escalation to certain commands

Privilege escalation permissions have to be general. Ansible does not always use a specific command to do something but runs modules (code) from a temporary file name which changes every time. If you have ‘/sbin/service’ or ‘/bin/chmod’ as the allowed commands this will fail with ansible as those paths won’t match with the temporary file that ansible creates to run the module.

Become and Networks

network_cli and become

Ansible 2.5 added support for become to be used to enter enable mode (Privileged EXEC mode) on network devices that support it. This replaces the previous authorize and auth_pass options in provider.

You must set the host connection type to connection: network_cli to use become for privilege escalation on network devices. Ansible 2.5.3 supports become for privilege escalation on eos, ios, and nxos.

You can use escalated privileges on only the specific tasks that need them, on an entire play, or on all plays. Adding become: yes and become_method: enable instructs Ansible to enter enable mode before executing the task, play, or playbook.

If you see this error message, the task that generated it requires enable mode to succeed:

Invalid input (privileged mode required)

To set enable mode for a specific task, add become at the task level:

- name: Gather facts (eos)
  eos_facts:
    gather_subset:
      - "!hardware"
  become: yes
  become_method: enable

To set enable mode for all tasks in a single play, add become at the play level:

- hosts: eos-switches
  become: yes
  become_method: enable
  tasks:
    - name: Gather facts (eos)
      eos_facts:
        gather_subset:
          - "!hardware"

Setting enable mode for all tasks

Often you wish for all tasks in all plays to run using privilege mode, that is best achieved by using group_vars:

group_vars/eos.yml

ansible_connection: network_cli
ansible_network_os: eos
ansible_user: myuser
ansible_become: yes
ansible_become_method: enable

Passwords for enable mode

If you need a password to enter enable mode, you can specify it in one of two ways:

• providing the --ask-become-pass command line option
• setting the ansible_become_pass connection variable

authorize and auth_pass

For HTTPS connections that cannot use connection: network_cli, you can enter enable mode using the module options authorize and auth_pass:

- hosts: eos-switches
  ansible_connection: local
  tasks:
    - name: Gather facts (eos)
      eos_facts:
        gather_subset:
          - "!hardware"
      provider:
        authorize: yes
        auth_pass: " {{ secret_auth_pass }}"

Note that over time more platforms and connections will support become. As this happens, the use of authorize and of provider dictionaries will be deprecated. Check the Platform Options and Network modules documentation for details.

Become and Windows

Since Ansible 2.3, become can be used on Windows hosts through the runas method. Become on Windows uses the same inventory setup and invocation arguments as become on a non-Windows host, so the setup and variable names are the same as what is defined in this document.

While become can be used to assume the identity of another user, there are other uses for it with Windows hosts. One important use is to bypass some of the limitations that are imposed when running on WinRM, such as constrained network delegation or accessing forbidden system calls like the WUA API. You can use become with the same user as ansible_user to bypass these limitations and run commands that are not normally accessible in a WinRM session.

Administrative Rights

Many tasks in Windows require administrative privileges to complete. When using the runas become method, Ansible will attempt to run the module with the full privileges that are available to the remote user. If it fails to elevate the user token, it will continue to use the limited token during execution.

Before Ansible 2.5, a token was only able to be elevated when UAC was disabled or the remote user had the SeTcbPrivilege assigned. This restriction has been lifted in Ansible 2.5 and a user that is a member of the BUILTIN\Administrators group should have an elevated token during the module execution.

To determine the type of token that Ansible was able to get, run the following task and check the output:

- win_whoami:
  become: yes

Under the GROUP INFORMATION section, the Mandatory Label entry determines whether the user has Administrative rights. Here are the labels that can be returned and what they mean:

• Medium: Ansible failed to get an elevated token and ran under a limited token. Only a subset of the privileges assigned to user are available during the module execution and the user does not have administrative rights.
• High: An elevated token was used and all the privileges assigned to the user are available during the module execution.
• System: The NT AUTHORITY\System account is used and has the highest level of privileges available.

The output will also show the list of privileges that have been granted to the user. When State==Disabled, the privileges have not been enabled but can be if required. In most scenarios these privileges are automatically enabled when required.

If running on a version of Ansible that is older than 2.5 or the normal runas escalation process fails, an elevated token can be retrieved by:

• Set the become_user to System which has full control over the operating system.

• Grant SeTcbPrivilege to the user Ansible connects with on WinRM. SeTcbPrivilege is a high-level privilege that grants full control over the operating system. No user is given this privilege by default, and care should be taken if you grant this privilege to a user or group. For more information on this privilege, please see Act as part of the operating system. You can use the below task to set this privilege on a Windows host:

  - name: grant the ansible user the SeTcbPrivilege right
    win_user_right:
      name: SeTcbPrivilege
      users: '{{ansible_user}}'
      action: add
  

• Turn UAC off on the host and reboot before trying to become the user. UAC is a security protocol that is designed to run accounts with the least privilege principle. You can turn UAC off by running the following tasks:

  - name: turn UAC off
    win_regedit:
      path: HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system
      name: EnableLUA
      data: 0
      type: dword
      state: present
    register: uac_result
  
  - name: reboot after disabling UAC
    win_reboot:
    when: uac_result is changed
  

Local Service Accounts

Prior to Ansible version 2.5, become only worked with a local or domain user account. Local service accounts like System or NetworkService could not be used as become_user in these older versions. This restriction has been lifted since the 2.5 release of Ansible. The three service accounts that can be set under become_user are:

• System
• NetworkService
• LocalService

Because local service accounts do not have passwords, the ansible_become_password parameter is not required and is ignored if specified.

Accounts without a Password

Ansible can be used to become an account that does not have a password (like the Guest account). To become an account without a password, set up the variables like normal but either do not define ansible_become_pass or set ansible_become_pass: ''.

Before become can work on an account like this, the local policy Accounts: Limit local account use of blank passwords to console logon only must be disabled. This can either be done through a Group Policy Object (GPO) or with this Ansible task:

- name: allow blank password on become
  win_regedit:
    path: HKLM:\SYSTEM\CurrentControlSet\Control\Lsa
    name: LimitBlankPasswordUse
    data: 0
    type: dword
    state: present

Become Flags

Ansible 2.5 adds the become_flags parameter to the runas become method. This parameter can be set using the become_flags task directive or set in Ansible’s configuration using ansible_become_flags. The two valid values that are initially supported for this parameter are logon_type and logon_flags.

The key logon_type sets the type of logon operation to perform. The value can be set to one of the following:

• interactive: The default logon type. The process will be run under a context that is the same as when running a process locally. This bypasses all WinRM restrictions and is the recommended method to use.
• batch: Runs the process under a batch context that is similar to a scheduled task with a password set. This should bypass most WinRM restrictions and is useful if the become_user is not allowed to log on interactively.
• new_credentials: Runs under the same credentials as the calling user, but outbound connections are run under the context of the become_user and become_password, similar to runas.exe /netonly. The logon_flags flag should also be set to netcredentials_only. Use this flag if the process needs to access a network resource (like an SMB share) using a different set of credentials.
• network: Runs the process under a network context without any cached credentials. This results in the same type of logon session as running a normal WinRM process without credential delegation, and operates under the same restrictions.
• network_cleartext: Like the network logon type, but instead caches the credentials so it can access network resources. This is the same type of logon session as running a normal WinRM process with credential delegation.

For more information, see dwLogonType.

The logon_flags key specifies how Windows will log the user on when creating the new process. The value can be set to none or multiple of the following:

• with_profile: The default logon flag set. The process will load the user’s profile in the HKEY_USERS registry key to HKEY_CURRENT_USER.
• netcredentials_only: The process will use the same token as the caller but will use the become_user and become_password when accessing a remote resource. This is useful in inter-domain scenarios where there is no trust relationship, and should be used with the new_credentials logon_type.

By default logon_flags=with_profile is set, if the profile should not be loaded set logon_flags= or if the profile should be loaded with netcredentials_only, set logon_flags=with_profile,netcredentials_only.

For more information, see dwLogonFlags.

Here are some examples of how to use become_flags with Windows tasks:

- name: copy a file from a fileshare with custom credentials
  win_copy:
    src: \\server\share\data\file.txt
    dest: C:\temp\file.txt
    remote_src: yex
  vars:
    ansible_become: yes
    ansible_become_method: runas
    ansible_become_user: DOMAIN\user
    ansible_become_pass: Password01
    ansible_become_flags: logon_type=new_credentials logon_flags=netcredentials_only

- name: run a command under a batch logon
  win_whoami:
  become: yes
  become_flags: logon_type=batch

- name: run a command and not load the user profile
  win_whomai:
  become: yes
  become_flags: logon_flags=

Limitations

Be aware of the following limitations with become on Windows:

• Running a task with async and become on Windows Server 2008, 2008 R2 and Windows 7 does not work.
• By default, the become user logs on with an interactive session, so it must have the right to do so on the Windows host. If it does not inherit the SeAllowLogOnLocally privilege or inherits the SeDenyLogOnLocally privilege, the become process will fail. Either add the privilege or set the logon_type flag to change the logon type used.
• Prior to Ansible version 2.3, become only worked when ansible_winrm_transport was either basic or credssp. This restriction has been lifted since the 2.4 release of Ansible for all hosts except Windows Server 2008 (non R2 version).