Secure persistence with skops

Warning

This feature is heavily under development, which means the API is unstable and there might be security issues at the moment. Therefore, use caution when loading files from sources you don’t trust.

Skops offers a way to save and load sklearn models without using pickle. The pickle module is not secure, but with skops, you can [more] securely save and load models without using pickle.

Pickle is the standard serialization format for sklearn and for Python in general (cloudpickle and joblib use the same format). One of the main advantages of pickle is that it can be used for almost all Python objects but this flexibility also makes it inherently insecure. This is because loading certain types of objects requires the ability to run arbitrary code, which can be misused for malicious purposes. For example, an attacker can use it to steal secrets from your machine or install a virus. As the Python docs say:

Warning

The pickle module is not secure. Only unpickle data you trust. It is possible to construct malicious pickle data which will execute arbitrary code during unpickling. Never unpickle data that could have come from an untrusted source, or that could have been tampered with.

In contrast to pickle, the skops.io.dump() and skops.io.load() functions have a more limited scope, while preventing users from running arbitrary code or loading unknown and malicious objects.

When loading a file, skops.io.load()/skops.io.loads() will traverse the input, check for known and unknown types, and will only construct those objects if they are trusted, either by default or by the user.

Note

You can try out converting your existing pickle files to the skops format using this Space on Hugging Face Hub: pickle-to-skops.

Usage

The code snippet below illustrates how to use skops.io.dump() and skops.io.load(). Note that one needs XGBoost installed to run this:

from xgboost.sklearn import XGBClassifier
from sklearn.model_selection import GridSearchCV, train_test_split
from sklearn.datasets import load_iris
from skops.io import dump, load

X, y = load_iris(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.4)
param_grid = {"tree_method": ["exact", "approx", "hist"]}
clf = GridSearchCV(XGBClassifier(), param_grid=param_grid).fit(X_train, y_train)
print(clf.score(X_test, y_test))
0.9666666666666667
dump(clf, "my-model.skops")
# ...
loaded = load("my-model.skops", trusted=True)
print(loaded.score(X_test, y_test))
0.9666666666666667

# in memory
from skops.io import dumps, loads
serialized = dumps(clf)
loaded = loads(serialized, trusted=True)

Note that you should only load files with trusted=True if you trust the source. Otherwise you can get a list of untrusted types present in the dump using skops.io.get_untrusted_types():

from skops.io import get_untrusted_types
unknown_types = get_untrusted_types(file="my-model.skops")
print(unknown_types)
['numpy.float64', 'numpy.int64', 'sklearn.metrics._scorer._passthrough_scorer',
'xgboost.core.Booster', 'xgboost.sklearn.XGBClassifier']

Note that everything in the above list is safe to load. We already have many types included as trusted by default, and some of the above values might be added to that list in the future.

Once you check the list and you validate that everything in the list is safe, you can load the file with trusted=unknown_types:

loaded = load("my-model.skops", trusted=unknown_types)

At the moment, we support the vast majority of sklearn estimators. This includes complex use cases such as sklearn.pipeline.Pipeline, sklearn.model_selection.GridSearchCV, classes using objects defined in Cython such as sklearn.tree.DecisionTreeClassifier, and more. If you discover an sklearn estimator that does not work, please open an issue on the skops GitHub page and let us know.

At the moment, skops cannot persist arbitrary Python code. This means if you have custom functions (say, a custom function to be used with sklearn.preprocessing.FunctionTransformer), it will not work. However, most numpy and scipy functions should work. Therefore, you can save objects having references to functions such as numpy.sqrt.

Command Line Interface

Skops has a command line interface to convert scikit-learn models persisted with Pickle to Skops files.

To convert a file from the command line, use the skops convert entrypoint.

Below is an example call to convert a file my_model.pkl to my_model.skops:

skops convert my_model.pkl

To convert multiple files, you can use bash commands to iterate the above call. For example, to convert all .pkl flies in the current directory:

for FILE in *.pkl; do skops convert FILE; done

Further help for the different supported options can be found by calling skops convert --help in a terminal.

Supported libraries

Skops intends to support all of scikit-learn, that is, not only its estimators, but also other classes like cross validation splitters. Furthermore, most types from numpy and scipy should be supported, such as (sparse) arrays, dtypes, random generators, and ufuncs.

Apart from this core, we plan to support machine learning libraries commonly used be the community. So far, we have tested the following libraries:

If you run into a problem using any of the mentioned libraries, this could mean there is a bug in skops. Please open an issue on our issue tracker (but please check first if a corresponding issue already exists).

In terms of security, we do not audit these libraries for security issues. Therefore, you should only load a skops file containing a model of any of those libraries if you trust them to be secure. It’s not a perfect solution, but it’s still better than trusting pickle files, which anyone can tamper with easily.

Roadmap

There needs to be more testing to harden the loader and make sure we don’t run arbitrary code when it’s not intended. However, the safety mechanisms already in place should prevent most cases of abuse.

At the moment, persisting and loading arbitrary C extension types is not possible, unless a python object wraps around them and handles persistence and loading via __getstate__ and __setstate__. We plan to develop an API which would help third party libraries to make their C extension types skops compatible.

You can check on our “issue tracker which features are planned for the near future.