poc-cntk-userfunction-rce

PoC — code execution when loading a CNTK v2 (.cntk) model

Loading an untrusted CNTK v2 model with the normal cntk.Function.load() runs code embedded in the file. A CNTK graph can hold a Python UserFunction node, and the node stores the function's module and class as plain strings. On load CNTK's UDF deserializer does exec("from {module} import {class}") on those attacker-controlled strings, so the file alone decides what gets imported and run.

model_inject_linux.cntk needs no external file: the module is byte-patched to builtins and the class string carries a newline-injected payload, so loading it runs os.system('id ...') and drops /tmp/PWNED_cntk.

Files

• model_inject_linux.cntk — self-contained malicious model (stdlib-only, no attacker .py).
• verify.py — loads it and prints the marker.
• model_evil.cntk + evil_udf.py — the planted-module variant (module named in the model is imported from sys.path).
• build_inject_linux.py — how the injected model was built.
• Dockerfile — the exact clean-room image (python:3.6 + cntk 2.7 + OpenMPI 1.10).

Reproduce

CNTK 2.7 is py3.5/3.6 only. In the clean room I confirmed it on a stock wheel:

docker build -t cntk-poc .          # python:3.6-slim + pip install cntk==2.7 + OpenMPI 1.10
docker run --rm -v "$PWD":/poc -w /poc cntk-poc python verify.py
# marker after : True
# uid=0(root) gid=0(root) groups=0(root)
# INJECTED_NO_EXTERNAL_FILE

Or on any py3.6 box: pip install cntk==2.7 then python verify.py.

Why it matters

Any service that loads a user-supplied .cntk runs attacker code as the loading process. It is the default load path, no flag turns it off, and ModelScan does not scan .cntk at all, so the file looks clean.