311 lines
14 KiB
ReStructuredText
311 lines
14 KiB
ReStructuredText
Become (Privilege Escalation)
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+++++++++++++++++++++++++++++
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Ansible can use existing privilege escalation systems to allow a user to execute tasks as another.
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.. contents:: Topics
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Become
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``````
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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.
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.. note:: Prior to version 1.9, Ansible mostly allowed the use of `sudo` and a limited use of `su` to allow a login/remote user to become a different user and execute tasks and create resources with the second user's permissions. As of Ansible version 1.9, `become` supersedes the old sudo/su, while still being backwards compatible. This new implementation also makes it easier to add other privilege escalation tools, including `pbrun` (Powerbroker), `pfexec`, `dzdo` (Centrify), and others.
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.. note:: Become vars and directives are independent. For example, setting `become_user` does not set `become`.
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Directives
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-----------
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These can be set from play to task level, but are overridden by connection variables as they can be host specific.
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become
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set to 'true'/'yes' to activate privilege escalation.
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become_user
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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.
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become_method
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(at play or task level) overrides the default method set in ansible.cfg, set to `sudo`/`su`/`pbrun`/`pfexec`/`doas`/`dzdo`/`ksu`/`runas`
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become_flags
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(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.
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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``)::
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- name: Ensure the httpd service is running
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service:
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name: httpd
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state: started
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become: true
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To run a command as the ``apache`` user::
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- name: Run a command as the apache user
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command: somecommand
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become: true
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become_user: apache
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To do something as the ``nobody`` user when the shell is nologin::
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- name: Run a command as nobody
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command: somecommand
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become: true
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become_method: su
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become_user: nobody
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become_flags: '-s /bin/sh'
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Connection variables
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--------------------
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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.
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ansible_become
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equivalent of the become directive, decides if privilege escalation is used or not.
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ansible_become_method
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allows to set privilege escalation method
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ansible_become_user
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allows to set the user you become through privilege escalation, does not imply `ansible_become: True`
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ansible_become_pass
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allows you to set the privilege escalation password
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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::
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webserver ansible_user=manager ansible_become=true
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Command line options
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--------------------
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--ask-become-pass, -K
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ask for privilege escalation password, does not imply become will be used
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--become, -b
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run operations with become (no password implied)
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--become-method=BECOME_METHOD
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privilege escalation method to use (default=sudo),
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valid choices: [ sudo | su | pbrun | pfexec | doas | dzdo | ksu | runas ]
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--become-user=BECOME_USER
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run operations as this user (default=root), does not imply --become/-b
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For those from Pre 1.9 , sudo and su still work!
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------------------------------------------------
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For those using old playbooks will not need to be changed, even though they are deprecated, sudo and su directives, variables and options
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will continue to work. It is recommended to move to become as they may be retired at one point.
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You cannot mix directives on the same object (become and sudo) though, Ansible will complain if you try to.
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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.
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Limitations
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-----------
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Although privilege escalation is mostly intuitive, there are a few limitations
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on how it works. Users should be aware of these to avoid surprises.
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Becoming an Unprivileged User
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=============================
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Ansible 2.0.x and below has a limitation with regards to becoming an
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unprivileged user that can be a security risk if users are not aware of it.
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Ansible modules are executed on the remote machine by first substituting the
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parameters into the module file, then copying the file to the remote machine,
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and finally executing it there.
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Everything is fine if the module file is executed without using ``become``,
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when the ``become_user`` is root, or when the connection to the remote machine
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is made as root. In these cases the module file is created with permissions
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that only allow reading by the user and root.
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The problem occurs when the ``become_user`` is an unprivileged user. Ansible
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2.0.x and below make the module file world readable in this case, as the module
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file is written as the user that Ansible connects as, but the file needs to
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be readable by the user Ansible is set to ``become``.
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.. note:: In Ansible 2.1, this window is further narrowed: If the connection
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is made as a privileged user (root), then Ansible 2.1 and above will use
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chown to set the file's owner to the unprivileged user being switched to.
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This means both the user making the connection and the user being switched
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to via ``become`` must be unprivileged in order to trigger this problem.
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If any of the parameters passed to the module are sensitive in nature, then
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those pieces of data are located in a world readable module file for the
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duration of the Ansible module execution. Once the module is done executing,
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Ansible will delete the temporary file. If you trust the client machines then
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there's no problem here. If you do not trust the client machines then this is
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a potential danger.
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Ways to resolve this include:
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* Use :ref:`pipelining`. When pipelining is enabled, Ansible doesn't save the
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module to a temporary file on the client. Instead it pipes the module to
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the remote python interpreter's stdin. Pipelining does not work for
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non-python modules.
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* (Available in Ansible 2.1) Install POSIX.1e filesystem acl support on the
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managed host. If the temporary directory on the remote host is mounted with
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POSIX acls enabled and the :command:`setfacl` tool is in the remote ``PATH``
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then Ansible will use POSIX acls to share the module file with the second
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unprivileged user instead of having to make the file readable by everyone.
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* Don't perform an action on the remote machine by becoming an unprivileged
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user. Temporary files are protected by UNIX file permissions when you
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``become`` root or do not use ``become``. In Ansible 2.1 and above, UNIX
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file permissions are also secure if you make the connection to the managed
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machine as root and then use ``become`` to an unprivileged account.
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.. warning:: Although the Solaris ZFS filesystem has filesystem ACLs, the ACLs
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are not POSIX.1e filesystem acls (they are NFSv4 ACLs instead). Ansible
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cannot use these ACLs to manage its temp file permissions so you may have
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to resort to ``allow_world_readable_tmpfiles`` if the remote machines use ZFS.
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.. versionchanged:: 2.1
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In addition to the additional means of doing this securely, Ansible 2.1 also
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makes it harder to unknowingly do this insecurely. Whereas in Ansible 2.0.x
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and below, Ansible will silently allow the insecure behaviour if it was unable
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to find another way to share the files with the unprivileged user, in Ansible
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2.1 and above Ansible defaults to issuing an error if it can't do this
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securely. If you can't make any of the changes above to resolve the problem,
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and you decide that the machine you're running on is secure enough for the
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modules you want to run there to be world readable, you can turn on
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``allow_world_readable_tmpfiles`` in the :file:`ansible.cfg` file. Setting
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``allow_world_readable_tmpfiles`` will change this from an error into
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a warning and allow the task to run as it did prior to 2.1.
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Connection Plugin Support
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=========================
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Privilege escalation methods must also be supported by the connection plugin
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used. Most connection plugins will warn if they do not support become. Some
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will just ignore it as they always run as root (jail, chroot, etc).
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Only one method may be enabled per host
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=======================================
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Methods cannot be chained. You cannot use ``sudo /bin/su -`` to become a user,
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you need to have privileges to run the command as that user in sudo or be able
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to su directly to it (the same for pbrun, pfexec or other supported methods).
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Can't limit escalation to certain commands
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==========================================
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Privilege escalation permissions have to be general. Ansible does not always
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use a specific command to do something but runs modules (code) from
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a temporary file name which changes every time. If you have '/sbin/service'
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or '/bin/chmod' as the allowed commands this will fail with ansible as those
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paths won't match with the temporary file that ansible creates to run the
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module.
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Become and Windows
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``````````````````
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Since Ansible 2.3, ``become`` can be used on Windows hosts through the
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``runas`` method. Become on Windows uses the same inventory setup and
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invocation arguments as ``become`` on a non-Windows host, so the setup and
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variable names are the same as what is defined in this document.
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While ``become`` can be used to assume the identity of another user, there are other uses for
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it with Windows hosts. One important use is to bypass some of the
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limitations that are imposed when running on WinRM, such as constrained network
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delegation or accessing forbidden system calls like the WUA API. You can use
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``become`` with the same user as ``ansible_user`` to bypass these limitations
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and run commands that are not normally accessible in a WinRM session.
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.. note:: Prior to Ansible 2.4, become would only work when ``ansible_winrm_transport`` was
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set to either ``basic`` or ``credssp``, but since Ansible 2.4 become now works on
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all transport types.
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Administrative Rights
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---------------------
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Many tasks in Windows require administrative privileges to complete. When using
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the ``runas`` become method, Ansible will automatically run the module with the
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full privileges of the remote user, unless User Account Control (UAC) Admin Approval
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Mode is enabled on the target hosts. When UAC is enabled, Ansible attempts to elevate
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module processes with administrative privileges, but uses a limited token if elevation
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fails.
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There are several ways to use the ``runas`` become method with full privileges
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when UAC is enabled:
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* Set the ``become_user`` to ``System`` which has full control over the
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operating system.
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* Grant ``SeTcbPrivilege`` to the user Ansible connects with on
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WinRM. ``SeTcbPrivilege`` is a high-level privilege that grants
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full control over the operating system. No user is given this privilege by
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default, and care should be taken if you grant this privilege to a user or group.
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For more information on this privilege, please see
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`Act as part of the operating system <https://technet.microsoft.com/en-us/library/dn221957(v=ws.11).aspx>`_.
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You can use the below task to set this privilege on a Windows host::
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- name: grant the ansible user the SeTcbPrivilege right
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win_user_right:
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name: SeTcbPrivilege
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users: '{{ansible_user}}'
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action: add
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* Turn UAC off on the host and reboot before trying to become the user. UAC is
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a security protocol that is designed to run accounts with the
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``least privilege`` principle. You can turn UAC off by running the following
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tasks::
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- name: turn UAC off
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win_regedit:
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path: HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system
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name: EnableLUA
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data: 0
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type: dword
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state: present
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register: uac_result
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- name: reboot after disabling UAC
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win_reboot:
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when: uac_result|changed
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Local Service Accounts
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----------------------
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Prior to Ansible version 2.5, ``become`` only worked with a local or domain
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user account. Local service accounts like ``System`` or ``NetworkService``
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could not be used as ``become_user`` in these older versions. This restriction
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has been lifted since the 2.5 release of Ansible. The three service accounts
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that can be set under ``become_user`` are:
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* System
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* NetworkService
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* LocalService
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Because local service accounts do not have passwords, the
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``ansible_become_password`` parameter is not required and is ignored if
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specified.
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Limitations
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-----------
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Be aware of the following limitations with ``become`` on Windows:
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* Running a task with ``async`` and ``become`` does not work.
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* The become user logs on with an interactive session, so it must have the
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ability to do so on the Windows host. If it does not inherit the
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``SeAllowLogOnLocally`` privilege or inherits the ``SeDenyLogOnLocally``
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privilege, the become process will fail.
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* Prior to Ansible version 2.3, become only worked when ``ansible_winrm_transport`` was
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either ``basic`` or ``credssp``. This restriction has been lifted since the
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2.4 release of Ansible.
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.. seealso::
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`Mailing List <http://groups.google.com/group/ansible-project>`_
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Questions? Help? Ideas? Stop by the list on Google Groups
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`irc.freenode.net <http://irc.freenode.net>`_
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#ansible IRC chat channel
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