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Introduction

Warning:

Poor implementations of this system may cause root-privilege vulnerabilities and compromised servers. Read the Security requirements section of this document before you use this system.

To run a function with escalated privileges, call a function through the Call method or use the send_cpwrapd_request pluggable wrapper. The system manages privilege escalation, and ensures that the user can only run the permitted code.

Notes:

  • This set of documents applies to cPanel & WHM version 11.38 and later. For information about the legacy method of API privilege escalation (for cPanel & WHM version 11.36 and earlier), read our Legacy API Privilege Escalation documentation.
  • cPanel, Inc. introduced the Call method for Perl admin modules in cPanel & WHM version 54 in order to simplify construction of modules. We recommend that you write admin modules with this method when possible. For earlier versions and other programming languages, use the Standard Method.
  • To query all of the SETUID and SETGID values on a server, run the following command:

    find / -type f \( -perm -04000 -o -perm -02000 \) \-exec ls -lg {} \;

Note:

cPanel, Inc. introduced this functionality in cPanel & WHM version 54 for admin modules in Perl.

Basic Usage

As of cPanel & WHM version 54, cPanel, Inc. recommends that you develop admin modules as subclasses of the cPanel-provided Cpanel::AdminBin::Script::Call base class. This class, and the Cpanel::AdminBin::Call module that accompanies it, contain privilege escalation logic so that admin modules are simpler to write and call.

To make a TheModule module in the TheNameSpace namespace, perform the following steps:

  1. Create the /usr/local/cpanel/bin/admin/TheNameSpace directory.
  2. Run the following command to create the configuration file:

    echo mode=full > /usr/local/cpanel/bin/admin/TheNamespace/TheModule.conf
  3. Create your new module in the /usr/local/cpanel/bin/admin/TheNamespace/TheModule file.

  4. Set that file to be executable with the following command:

    chmod 0700 /usr/local/cpanel/bin/admin/TheNamespace/TheModule

Example

The following example admin module uses the Call method:

#!/usr/local/cpanel/3rdparty/bin/perl

package TheNamespace::TheModule;

use strict;

use parent 'Cpanel::AdminBin::Script::Call';

__PACKAGE__->run() if !caller;

sub _actions { return qw( DO_GOOD ) }

sub DO_GOOD {
 my ($self, $arg1, $arg2) = @_;
 return "I did good with “$arg1” and “$arg2->[0]”.";
}

1;

Class methods

The run(OPTS) method runs the script after it checks whether the first element in the value of the @ARGV array is --bincheck. If --bincheck exists in the @ARGV array, the system prints the BinCheck ok message with a trailing newline and the process performs an exit(). This is normally how Call-type admin modules invoke themselves when run from a script.

  • The OPTS object is an optional list of arguments that can include the alarm argument, an integer that represents a timeout value for the script. This option defaults to 350 seconds.
  • The following example includes the run() method with the alarm argument:

    __PACKAGE__->run( alarm => 600 ); #allow 10 minutes rather than the default 350 seconds 

The new(OPTS) method is identical to the run() method, but it does not check whether the first element in the value of the @ARGV array is the --bincheck flag.

  • This method helps you test your module.
  • For example, the following example includes the new() method:

    TheTestedModule->new();

DO_GOOD

To call the DO_GOOD function from unprivileged code, your code should resemble the following example:

my $resp = Cpanel::AdminBin::Call::call('TheNamespace', 'TheModule', 'DO_GOOD', 'first arg', ['second arg']); 

The above example passes the first arg and second arg parameters to the DO_GOOD method in the admin module.

  • This example returns one value, but you may also return a list.
  • Input parameters and return values may be scalars, array references, or hash references.
  • This structure does not support scalar references, references that use the bless() function, and self-referential data structures as inputs or outputs.

The code calls the admin function with the same context (void, scalar, or list) as the Cpanel::AdminBin::Call::call() method itself. The system handles mismatched returns (for example, an array in scalar context as a return value) as Perl handles them. The admin module traps exceptions and then re-throws them without a stack trace in the code that calls them.

For a full example, read our Guide to API Privilege Escalation - Call Your Application documentation.

Note:

The $self parameter in the example contains a reference to the instance of your admin module and class. Your class will inherit several useful methods such as get_caller_username() from the Cpanel::AdminBin::Script::Call module.

For more information, read our Guide to API Privilege Escalation - Object Methods documentation.

Basic Usage

To use privilege escalation in your custom code, perform the following steps:

  1. Create the following files:

    Notes:

    • Download a simple example file: Simple.tar.gz.
    • Download an advanced example file: Advanced.tar.gz.
    • Never use this any of these example files on a production system.
  2. Store these files in a new namespace in the  /usr/local/cpanel/bin/admin/  directory. 
    • The namespace and the directory name must be identical.
    • Do not create your AdminBin application in the Cpanel namespace.
  3. Use the Cpanel::AdminBin::send_cpwrapd_request method to call the application as the authenticated user.

Security requirements

Whenever you use this system, do not manipulate files or directories that a user owns as the root user or execute any actions on unvalidated input.

Warning:

You must adhere to the following security practices:

  • Only use this system to execute code that must run as the root user.
  • Thoroughly validate any input that passes through this system.
  • Sanitize the admin script's environment.

To sanitize the admin script's environment, set environment variables to limit the paths from which the script loads libraries. 

  • This action sanitizes the @INC array and ensures that users cannot load arbitrary libraries.
  • The following example adds the /usr/local/cpanel/ directory to the environment, and then removes entries that do not match standard Perl library paths:

    BEGIN {
        unshift @INC, '/usr/local/cpanel';
        @INC = grep( !/(^\.|\.\.|\/\.+)/, @INC );
    }