# !pip install -U keras tensorflow huggingface_hub
# Keras needs TensorFlow installed to read "hf://" paths, so the tensorflow backend is selected here;
# "jax" and "torch" also work for computation once TensorFlow is installed.
import os
os.environ["KERAS_BACKEND"] = "tensorflow"
import keras
model = keras.saving.load_model("hf://unicuervo/keras-nested-lambda-modelscan-bypass")
Keras Native nested Lambda ModelScan bypass PoC
This repository contains a benign proof-of-concept .keras model for a Huntr Model File Vulnerability report.
Summary
The file nested_lambda.keras contains a Keras Lambda layer nested inside an inner Functional model.
ModelScan detects a top-level Keras Lambda layer, but this nested variant is reported as clean:
--- Summary ---
No issues found!
However, recursive inspection of config.json shows the hidden Lambda:
[TOP_LEVEL_LAMBDAS] []
[RECURSIVE_LAMBDA_COUNT] 1
[RECURSIVE_LAMBDA] hidden_lambda at $.config.layers[1].config.layers[1]
Keras itself recognizes the nested Lambda as unsafe during default loading and blocks it with safe_mode=True.
When the same model is loaded with safe_mode=False, the nested Lambda is deserialized and changes model behavior. This PoC only performs benign output manipulation and returns 1337.0 for input 5.0.
Files
nested_lambda.keras: benign PoC model file.reproduce.py: local reproduction script.requirements.txt: dependency list.generate_model.py: optional script to regenerate the PoC model.
Reproduction
Install dependencies:
python -m venv .venv
source .venv/bin/activate
python -m pip install -r requirements.txt
Run ModelScan:
modelscan -p nested_lambda.keras
Expected result:
No issues found!
Run the local reproduction:
KERAS_BACKEND=tensorflow python reproduce.py
Expected result:
[TOP_LEVEL_LAMBDAS] []
[RECURSIVE_LAMBDA_COUNT] 1
[RECURSIVE_LAMBDA] hidden_lambda at $.config.layers[1].config.layers[1]
[DEFAULT_LOAD_BLOCKED] ValueError
Requested the deserialization of a `Lambda` layer whose `function` is a Python lambda.
[UNSAFE_LOAD_OK]
[OUTPUT] [[1337.0]]
Security impact
A scanner or model registry relying on ModelScan may incorrectly classify a .keras model as clean even though it contains an unsafe Lambda layer.
This is a scanner bypass / false negative. The PoC does not claim code execution under Keras default safe_mode=True. Keras blocks the nested Lambda by default. The impact is that ModelScan fails to detect the same unsafe construct that Keras later identifies and blocks.
If a downstream consumer trusts the clean ModelScan result and loads the model with safe_mode=False or globally enables unsafe deserialization, the hidden Lambda is deserialized and can manipulate model behavior.
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