skops is a Python library which helps users share and ship their scikit-learn based models. Versions 0.11.0 and below contain an inconsistency in MethodNode, which can be exploited to access unexpected object fields through dot notation. This can be used to achieve arbitrary code execution at load time. While this issue may seem similar to GHSA-m7f4-hrc6-fwg3, it is actually more severe, as it relies on fewer assumptions about trusted types. This is fixed in version 12.0.0.

Affected: PyPI: skops

## Summary

An inconsistency in `MethodNode` can be exploited to access unexpected object fields through dot notation. This can be used to achieve **arbitrary code execution at load time**.

While this issue may seem similar to https://github.com/skops-dev/skops/security/advisories/GHSA-m7f4-hrc6-fwg3, it is actually more severe, as it relies on fewer assumptions about trusted types.

## Details

The `MethodNode` allows access to attributes of existing objects via dot notation. However, there are several critical shortcomings:

* Although the `__class__` and `__module__` fields are checked via `get_untrusted_types` and during the `load` phase (as a concatenated string), **they are not actually used by `MethodNode`**. Instead, the `func` and `obj` entries in the `schema.json` are used to determine behavior. This means that even an apparently harmless `__module__.__class__` pair can lead to access of arbitrary attributes or methods of loaded objects, without any additional checks.

* **Nothing prevents an attacker from chaining multiple `MethodNode` instances** to traverse the object hierarchy and access harmful attributes.

An object can be loaded using the `ObjectNode`, which normally enforces strict checks and allows only trusted or explicitly permitted objects. However, once the object is loaded, dot notation can be used to access any of its attributes or methods. Furthermore, by chaining multiple `MethodNode`s, one can traverse the Python object hierarchy and reach dange…

Affected: PyPI: skops

## Summary

An inconsistency in `MethodNode` can be exploited to access unexpected object fields through dot notation. This can be used to achieve **arbitrary code execution at load time**.

While this issue may seem similar to https://github.com/skops-dev/skops/security/advisories/GHSA-m7f4-hrc6-fwg3, it is actually more severe, as it relies on fewer assumptions about trusted types.

## Details

The `MethodNode` allows access to attributes of existing objects via dot notation. However, there are several critical shortcomings:

* Although the `__class__` and `__module__` fields are checked via `get_untrusted_types` and during the `load` phase (as a concatenated string), **they are not actually used by `MethodNode`**. Instead, the `func` and `obj` entries in the `schema.json` are used to determine behavior. This means that even an apparently harmless `__module__.__class__` pair can lead to access of arbitrary attributes or methods of loaded objects, without any additional checks.

* **Nothing prevents an attacker from chaining multiple `MethodNode` instances** to traverse the object hierarchy and access harmful attributes.

An object can be loaded using the `ObjectNode`, which normally enforces strict checks and allows only trusted or explicitly permitted objects. However, once the object is loaded, dot notation can be used to access any of its attributes or methods. Furthermore, by chaining multiple `MethodNode`s, one can traverse the Python object hierarchy and reach dange…