MLflow: MLFLOW_ALLOW_PICKLE_DESERIALIZATION=False safety control bypassed by mlflow.statsmodels flavor…

Summary

MLflow introduced MLFLOW_ALLOW_PICKLE_DESERIALIZATION as a security control to prevent unsafe pickle.load execution during model loading, in response to CVE-2024-37052 through CVE-2024-37060. When set to False, operators expect all pickle deserialization to be blocked. The most recent related fix (#21188) patched a bypass in the pyfunc flavor.

However, the mlflow.statsmodels flavor completely omits this guard. An attacker who places a crafted MLmodel artifact into any accessible artifact store can trigger arbitrary code execution on any process that calls mlflow.pyfunc.load_model() against the malicious model — even when MLFLOW_ALLOW_PICKLE_DESERIALIZATION=False.

This is a security control bypass. The operator believes pickle RCE is mitigated; the statsmodels flavor silently ignores the control.

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Root Cause

mlflow.pyfunc.load_model() dispatches to flavor _load_pyfunc implementations via:

# mlflow/pyfunc/__init__.py L1170-1172
model_impl = importlib.import_module(conf[MAIN])._load_pyfunc(data_path)

The guarded pattern (from mlflow/sklearn/__init__.py L526-533, the reference implementation) is:

if (
    not MLFLOW_ALLOW_PICKLE_DESERIALIZATION.get()
    and not is_in_databricks_runtime()
    and not is_in_databricks_model_serving_environment()
):
    raise MlflowException("Deserializing model using pickle is disallowed...")

mlflow/statsmodels/__init__.py has no such check:

# L307-320 — no guard anywhere in this file
def _load_model(path):
    import statsmodels.iolib.api as smio
    return smio.load_pickle(path)   # calls pickle.load() directly

def _load_pyfunc(path):
    return _StatsmodelsModelWrapper(_load_model(path))

statsmodels.iolib.api.load_pickle is a thin wrapper around pickle.load. Its own docstring warns: *"Never unpickle data received from an untrusted or unauthenticated source."*

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Trigger

An attacker crafts an MLmodel YAML that specifies mlflow.statsmodels as the loader module:

flavors:
  python_function:
    loader_module: mlflow.statsmodels
    data: model.pkl
  statsmodels:
    data: model.pkl
    statsmodels_version: 0.14.0

With a malicious model.pkl placed alongside it in the artifact store, any call to:

os.environ["MLFLOW_ALLOW_PICKLE_DESERIALIZATION"] = "False"
mlflow.pyfunc.load_model("models:/MaliciousModel/1")

...deserializes the pickle file with no guard check, executing arbitrary code with the privileges of the calling process.

On default MLflow deployments (no --app-name basic-auth), authentication is disabled, so artifact upload requires no credentials.

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Affected Code

- mlflow/statsmodels/__init__.py L307-310: _load_model — calls smio.load_pickle without checking MLFLOW_ALLOW_PICKLE_DESERIALIZATION - mlflow/statsmodels/__init__.py L313-320: _load_pyfunc — dispatches to _load_model without checking the control

Permalink (commit 0b0c576c): - https://github.com/mlflow/mlflow/blob/0b0c576c642b5b0d9496c829809c7d097403bc9f/mlflow/statsmodels/__init__.py#L307-L310 - https://github.com/mlflow/mlflow/blob/0b0c576c642b5b0d9496c829809c7d097403bc9f/mlflow/statsmodels/__init__.py#L313-L320

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Recommended Fix

Add the missing guard to mlflow/statsmodels/__init__.py:

from mlflow.environment_variables import MLFLOW_ALLOW_PICKLE_DESERIALIZATION
from mlflow.utils.databricks_utils import (
    is_in_databricks_model_serving_environment,
    is_in_databricks_runtime,
)

def _load_model(path):
    if (
        not MLFLOW_ALLOW_PICKLE_DESERIALIZATION.get()
        and not is_in_databricks_runtime()
        and not is_in_databricks_model_serving_environment()
    ):
        raise MlflowException(
            "Deserializing model using pickle is disallowed, but this statsmodels "
            "model requires pickle deserialization. Set environment variable "
            "'MLFLOW_ALLOW_PICKLE_DESERIALIZATION' to 'true' to allow this."
        )
    import statsmodels.iolib.api as smio
    return smio.load_pickle(path)