Ubuntu Manpage: pkexec - Execute a command as another user

NAME

       pkexec - Execute a command as another user

SYNOPSIS

       pkexec [--version] [--disable-internal-agent] [--help]

       pkexec [--keep-cwd] [--user username] PROGRAM [ARGUMENTS...]

DESCRIPTION

       pkexec allows an authorized user to execute PROGRAM as another user. If PROGRAM is not
       specified, the default shell will be run. If username is not specified, then the program
       will be executed as the administrative super user, root.

RETURN VALUE

       Upon successful completion, the return value is the return value of PROGRAM. If the
       calling process is not authorized or an authorization could not be obtained through
       authentication or an error occured, pkexec exits with a return value of 127. If the
       authorization could not be obtained because the user dismissed the authentication dialog,
       pkexec exits with a return value of 126.

AUTHENTICATION AGENT

       pkexec, like any other polkit application, will use the authentication agent registered
       for the calling process or session. However, if no authentication agent is available, then
       pkexec will register its own textual authentication agent. This behavior can be turned off
       by passing the --disable-internal-agent option.

SECURITY NOTES

Executing a program as another user is a privileged operation. By default the action to
check for (see the section called “ACTION AND AUTHORIZATIONS”) requires administrator
authentication. In addition, the authentication dialog presented to the user will display
the full path to the program to be executed so the user is aware of what will happen.

The environment that PROGRAM will run it, will be set to a minimal known and safe
environment in order to avoid injecting code through LD_LIBRARY_PATH or similar
mechanisms. In addition the PKEXEC_UID environment variable is set to the user id of the
process invoking pkexec. As a result, pkexec will not by default allow you to run X11
applications as another user since the $DISPLAY and $XAUTHORITY environment variables are
not set. These two variables will be retained if the
org.freedesktop.policykit.exec.allow_gui annotation on an action is set to a nonempty
value; this is discouraged, though, and should only be used for legacy programs.

pkexec will run PROGRAM in username's home directory, unless --keep-cwd is used to
override this behavior

Note that pkexec does no validation of the ARGUMENTS passed to PROGRAM. In the normal case
(where administrator authentication is required every time pkexec is used), this is not a
problem since if the user is an administrator he might as well just run pkexec bash to get
root.

However, if an action is used for which the user can retain authorization (or if the user
is implicitly authorized) this could be a security hole. Therefore, as a rule of thumb,
programs for which the default required authorization is changed, should never implicitly
trust user input (e.g. like any other well-written suid program).

ACTION AND AUTHORIZATIONS

       By default, the org.freedesktop.policykit.exec action is used. To use another action, use
       the org.freedesktop.policykit.exec.path annotation on an action with the value set to the
       full path of the program. In addition to specifying the program, the authentication
       message, description, icon and defaults can be specified. If the
       org.freedesktop.policykit.exec.argv1 annotation is present, the action will only be picked
       if the first argument to the program matches the value of the annotation.

       Note that authentication messages may reference variables (see the section called
       “VARIABLES”), for example $(user) will be expanded to the value of the user variable.

WRAPPER USAGE

       To avoid modifying existing software to prefix their command-line invocations with pkexec,
       it's possible to use pkexec in a she-bang wrapper[1] like this:

           #!/usr/bin/pkexec /usr/bin/python

           import os
           import sys

           print "Hello, I'm running as uid %d"%(os.getuid())

           for n in range(len(sys.argv)):
               print "arg[%d]=`%s'"%(n, sys.argv[n])

       If this script is installed into /usr/bin/my-pk-test, then the following annotations

             [...]
             <annotate key="org.freedesktop.policykit.exec.path">/usr/bin/python</annotate>
             <annotate key="org.freedesktop.policykit.exec.argv1">/usr/bin/my-pk-test</annotate>
             [...]

       can be used to select the appropriate polkit action. Be careful to get the latter
       annotation right, otherwise it will match any pkexec invocation of /usr/bin/python
       scripts.

VARIABLES

       The following variables are set by pkexec. They can be used in authorization rules and
       messages shown in authentication dialogs:

       program
           Fully qualified path to the program to be executed. Example: “/bin/cat”

       command_line
           The requested command-line (do not use this for any security checks, it is not
           secure). Example: “cat /srv/xyz/foobar”

       user
           The user name of the user to execute the program as. Example: “davidz”

       user.gecos
           The full name of the user to execute the program as. Example: “David Zeuthen”

       user.display
           A representation of the user to execute the program as that is suitable for display in
           an authentication dialog. Is typically set to a combination of the user name and the
           full name. Example: “David Zeuthen (davidz)”

AUTHOR

       Written by David Zeuthen <davidz@redhat.com> with a lot of help from many others.

BUGS

       Please send bug reports to either the distribution or the polkit-devel mailing list, see
       the link https://gitlab.freedesktop.org/polkit/polkit/-/issues/ on how to subscribe.

NOTES

        1. she-bang wrapper
           http://en.wikipedia.org/wiki/Shebang_(Unix)