Understanding privilege escalation: become

Ansible uses existing privilege escalation systems to execute tasks with root privileges or with another user’s permissions. Because this feature allows you to ‘become’ another user, different from the user that logged into the machine (remote user), we call it become. The become keyword leverages existing privilege escalation tools like sudo, su, pfexec, doas, pbrun, dzdo, ksu, runas, machinectl and others.

Using become

You can control the use of become with play or task directives, connection variables, or at the command line. If you set privilege escalation properties in multiple ways, review the general precedence rules to understand which settings will be used.

A full list of all become plugins that are included in Ansible can be found in the Plugin List.

Become directives

You can set the directives that control become at the play or task level. You can override these by setting connection variables, which often differ from one host to another. These variables and directives are independent. For example, setting become_user does not set become.

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. Default value is root.

become_method

(at play or task level) overrides the default method set in ansible.cfg, set to use any of the Become Plugins.

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 nologin. Added in Ansible 2.2.

For example, to manage a system service (which requires root privileges) when connected as a non-root user, you can use the default value of become_user (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'

To specify a password for sudo, run ansible-playbook with --ask-become-pass (-K for short). If you run a playbook utilizing become and the playbook seems to hang, most likely it is stuck at the privilege escalation prompt. Stop it with CTRL-c, then execute the playbook with -K and the appropriate password.

Become connection variables

You can define different become options for each managed node or group. You can define these variables in inventory or use them as normal variables.

ansible_become

overrides 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_password

set the privilege escalation password. See Using encrypted variables and files for details on how to avoid having secrets in plain text

ansible_common_remote_group

determines if Ansible should try to chgrp its temporary files to a group if setfacl and chown both fail. See Risks of becoming an unprivileged user for more information. Added in version 2.10.

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

Note

The variables defined above are generic for all become plugins but plugin specific ones can also be set instead. Please see the documentation for each plugin for a list of all options the plugin has and how they can be defined. A full list of become plugins in Ansible can be found at Become Plugins.

Become 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 | machinectl ]

--become-user=BECOME_USER

run operations as this user (default=root), does not imply –become/-b

Risks and limitations of become

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

Risks of becoming an unprivileged user

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 Ansible creates the module file with permissions that only allow reading by the user and root, or only allow reading by the unprivileged user being switched to.

However, when both the connection user and the become_user are unprivileged, the module file is written as the user that Ansible connects as (the remote_user), but the file needs to be readable by the user Ansible is set to become. The details of how Ansible solves this can vary based on platform. However, on POSIX systems, Ansible solves this problem in the following way:

First, if setfacl is installed and available in the remote PATH, and the temporary directory on the remote host is mounted with POSIX.1e filesystem ACL support, Ansible will use POSIX ACLs to share the module file with the second unprivileged user.

Next, if POSIX ACLs are not available or setfacl could not be run, Ansible will attempt to change ownership of the module file using chown for systems which support doing so as an unprivileged user.

New in Ansible 2.11, at this point, Ansible will try chmod +a which is a macOS-specific way of setting ACLs on files.

New in Ansible 2.10, if all of the above fails, Ansible will then check the value of the configuration setting ansible_common_remote_group. Many systems will allow a given user to change the group ownership of a file to a group the user is in. As a result, if the second unprivileged user (the become_user) has a UNIX group in common with the user Ansible is connected as (the remote_user), and if ansible_common_remote_group is defined to be that group, Ansible can try to change the group ownership of the module file to that group by using chgrp, thereby likely making it readable to the become_user.

At this point, if ansible_common_remote_group was defined and a chgrp was attempted and returned successfully, Ansible assumes (but, importantly, does not check) that the new group ownership is enough and does not fall back further. That is, Ansible does not check that the become_user does in fact share a group with the remote_user; so long as the command exits successfully, Ansible considers the result successful and does not proceed to check allow_world_readable_tmpfiles per below.

If ansible_common_remote_group is not set and the chown above it failed, or if ansible_common_remote_group is set but the chgrp (or following group-permissions chmod) returned a non-successful exit code, Ansible will lastly check the value of allow_world_readable_tmpfiles. If this is set, Ansible will place the module file in a world-readable temporary directory, with world-readable permissions to allow the become_user (and incidentally any other user on the system) to read the contents of the file. If any of the parameters passed to the module are sensitive in nature, and you do not trust the remote machines, then this is a potential security risk.

Once the module is done executing, Ansible deletes the temporary file.

Several ways exist to avoid the above logic flow entirely:

  • Use pipelining. When pipelining is enabled, Ansible does not 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.

  • Avoid 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 access an unprivileged account.

Warning

Although the Solaris ZFS filesystem has filesystem ACLs, the ACLs are not POSIX.1e filesystem acls (they are NFSv4 ACLs instead). Ansible cannot use these ACLs to manage its temp file permissions so you may have to resort to allow_world_readable_tmpfiles if the remote machines use ZFS.

Changed in version 2.1.

Ansible makes it hard to unknowingly use become insecurely. Starting in Ansible 2.1, Ansible defaults to issuing an error if it cannot execute securely with become. If you cannot use pipelining or POSIX ACLs, must connect as an unprivileged user, must use become to execute as a different unprivileged user, and decide that your managed nodes are 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.

Changed in version 2.10.

Ansible 2.10 introduces the above-mentioned ansible_common_remote_group fallback. As mentioned above, if enabled, it is used when remote_user and become_user are both unprivileged users. Refer to the text above for details on when this fallback happens.

Warning

As mentioned above, if ansible_common_remote_group and allow_world_readable_tmpfiles are both enabled, it is unlikely that the world-readable fallback will ever trigger, and yet Ansible might still be unable to access the module file. This is because after the group ownership change is successful, Ansible does not fall back any further, and also does not do any check to ensure that the become_user is actually a member of the “common group”. This is a design decision made by the fact that doing such a check would require another round-trip connection to the remote machine, which is a time-expensive operation. Ansible does, however, emit a warning in this case.

Not supported by all connection plugins

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, and so on).

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).

Privilege escalation must be general

You cannot limit privilege escalation permissions to certain commands. 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. If you have security rules that constrain your sudo/pbrun/doas environment to running specific command paths only, use Ansible from a special account that does not have this constraint, or use AWX or the Red Hat Ansible Automation Platform to manage indirect access to SSH credentials.

May not access environment variables populated by pamd_systemd

For most Linux distributions using systemd as their init, the default methods used by become do not open a new “session”, in the sense of systemd. Because the pam_systemd module will not fully initialize a new session, you might have surprises compared to a normal session opened through ssh: some environment variables set by pam_systemd, most notably XDG_RUNTIME_DIR, are not populated for the new user and instead inherited or just emptied.

This might cause trouble when trying to invoke systemd commands that depend on XDG_RUNTIME_DIR to access the bus:

$ echo $XDG_RUNTIME_DIR

$ systemctl --user status
Failed to connect to bus: Permission denied

To force become to open a new systemd session that goes through pam_systemd, you can use become_method: machinectl.

For more information, see this systemd issue.

Become and network automation

As of version 2.6, Ansible supports become for privilege escalation (entering enable mode or privileged EXEC mode) on all Ansible-maintained network platforms that support enable mode. Using become replaces the authorize and auth_pass options in a provider dictionary.

You must set the connection type to either connection: ansible.netcommon.network_cli or connection: ansible.netcommon.httpapi to use become for privilege escalation on network devices. Check the Platform Options documentation for details.

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 where those parameters are set.

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)
  arista.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)
      arista.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: ansible.netcommon.network_cli
ansible_network_os: arista.eos.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_password connection variable

Warning

As a reminder passwords should never be stored in plain text. For information on encrypting your passwords and other secrets with Ansible Vault, see Encrypting content with Ansible Vault.

authorize and auth_pass

Ansible still supports enable mode with connection: local for legacy network playbooks. To enter enable mode with connection: local, use 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 }}"

We recommend updating your playbooks to use become for network-device enable mode consistently. The use of authorize and of provider dictionaries will be deprecated in future. 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.

A user must have the SeDebugPrivilege to run a become process with elevated privileges. This privilege is assigned to Administrators by default. If the debug privilege is not available, the become process will run with a limited set of privileges and groups.

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

- Check my user name
  ansible.windows.win_whoami:
  become: yes

The output will look something similar to the below:

ok: [windows] => {
    "account": {
        "account_name": "vagrant-domain",
        "domain_name": "DOMAIN",
        "sid": "S-1-5-21-3088887838-4058132883-1884671576-1105",
        "type": "User"
    },
    "authentication_package": "Kerberos",
    "changed": false,
    "dns_domain_name": "DOMAIN.LOCAL",
    "groups": [
        {
            "account_name": "Administrators",
            "attributes": [
                "Mandatory",
                "Enabled by default",
                "Enabled",
                "Owner"
            ],
            "domain_name": "BUILTIN",
            "sid": "S-1-5-32-544",
            "type": "Alias"
        },
        {
            "account_name": "INTERACTIVE",
            "attributes": [
                "Mandatory",
                "Enabled by default",
                "Enabled"
            ],
            "domain_name": "NT AUTHORITY",
            "sid": "S-1-5-4",
            "type": "WellKnownGroup"
        },
    ],
    "impersonation_level": "SecurityAnonymous",
    "label": {
        "account_name": "High Mandatory Level",
        "domain_name": "Mandatory Label",
        "sid": "S-1-16-12288",
        "type": "Label"
    },
    "login_domain": "DOMAIN",
    "login_time": "2018-11-18T20:35:01.9696884+00:00",
    "logon_id": 114196830,
    "logon_server": "DC01",
    "logon_type": "Interactive",
    "privileges": {
        "SeBackupPrivilege": "disabled",
        "SeChangeNotifyPrivilege": "enabled-by-default",
        "SeCreateGlobalPrivilege": "enabled-by-default",
        "SeCreatePagefilePrivilege": "disabled",
        "SeCreateSymbolicLinkPrivilege": "disabled",
        "SeDebugPrivilege": "enabled",
        "SeDelegateSessionUserImpersonatePrivilege": "disabled",
        "SeImpersonatePrivilege": "enabled-by-default",
        "SeIncreaseBasePriorityPrivilege": "disabled",
        "SeIncreaseQuotaPrivilege": "disabled",
        "SeIncreaseWorkingSetPrivilege": "disabled",
        "SeLoadDriverPrivilege": "disabled",
        "SeManageVolumePrivilege": "disabled",
        "SeProfileSingleProcessPrivilege": "disabled",
        "SeRemoteShutdownPrivilege": "disabled",
        "SeRestorePrivilege": "disabled",
        "SeSecurityPrivilege": "disabled",
        "SeShutdownPrivilege": "disabled",
        "SeSystemEnvironmentPrivilege": "disabled",
        "SeSystemProfilePrivilege": "disabled",
        "SeSystemtimePrivilege": "disabled",
        "SeTakeOwnershipPrivilege": "disabled",
        "SeTimeZonePrivilege": "disabled",
        "SeUndockPrivilege": "disabled"
    },
    "rights": [
        "SeNetworkLogonRight",
        "SeBatchLogonRight",
        "SeInteractiveLogonRight",
        "SeRemoteInteractiveLogonRight"
    ],
    "token_type": "TokenPrimary",
    "upn": "[email protected]",
    "user_flags": []
}

Under the label key, the account_name entry determines whether the user has Administrative rights. Here are the labels that can be returned and what they represent:

  • 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 the privilege value is disabled, the privilege is assigned to the logon token but has not been enabled. 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
  ansible.windows.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

Note

Granting the SeTcbPrivilege or turning UAC off can cause Windows security vulnerabilities and care should be given if these steps are taken.

Local service accounts

Prior to Ansible version 2.5, become only worked on Windows 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.

Become without setting a password

As of Ansible 2.8, become can be used to become a Windows local or domain account without requiring a password for that account. For this method to work, the following requirements must be met:

  • The connection user has the SeDebugPrivilege privilege assigned

  • The connection user is part of the BUILTIN\Administrators group

  • The become_user has either the SeBatchLogonRight or SeNetworkLogonRight user right

Using become without a password is achieved in one of two different methods:

  • Duplicating an existing logon session’s token if the account is already logged on

  • Using S4U to generate a logon token that is valid on the remote host only

In the first scenario, the become process is spawned from another logon of that user account. This could be an existing RDP logon, console logon, but this is not guaranteed to occur all the time. This is similar to the Run only when user is logged on option for a Scheduled Task.

In the case where another logon of the become account does not exist, S4U is used to create a new logon and run the module through that. This is similar to the Run whether user is logged on or not with the Do not store password option for a Scheduled Task. In this scenario, the become process will not be able to access any network resources like a normal WinRM process.

To make a distinction between using become with no password and becoming an account that has no password make sure to keep ansible_become_password as undefined or set ansible_become_password:.

Note

Because there are no guarantees an existing token will exist for a user when Ansible runs, there’s a high change the become process will only have access to local resources. Use become with a password if the task needs to access network resources

Accounts without a password

Warning

As a general security best practice, you should avoid allowing accounts without passwords.

Ansible can be used to become a Windows 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 set ansible_become_password: ''.

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
  ansible.windows.win_regedit:
    path: HKLM:\SYSTEM\CurrentControlSet\Control\Lsa
    name: LimitBlankPasswordUse
    data: 0
    type: dword
    state: present

Note

This is only for accounts that do not have a password. You still need to set the account’s password under ansible_become_password if the become_user has a password.

Become flags for Windows

Ansible 2.5 added 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.

Note

These flags should only be set when becoming a normal user account, not a local service account like LocalSystem.

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
  ansible.windows.win_copy:
    src: \\server\share\data\file.txt
    dest: C:\temp\file.txt
    remote_src: yes
  vars:
    ansible_become: yes
    ansible_become_method: runas
    ansible_become_user: DOMAIN\user
    ansible_become_password: Password01
    ansible_become_flags: logon_type=new_credentials logon_flags=netcredentials_only

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

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

Limitations of become on Windows

  • Running a task with async and become on Windows Server 2008, 2008 R2 and Windows 7 only works when using Ansible 2.7 or newer.

  • 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).

  • The Secondary Logon service seclogon must be running to use ansible_become_method: runas

See also

Mailing List

Questions? Help? Ideas? Stop by the list on Google Groups

irc.libera.chat

#ansible IRC chat channel