# CatBoost Duplicate feature_id Silent Feature Shadowing

**Status:** Preparing for Huntr submission
**Package:** [`catboost`](https://pypi.org/project/catboost/) (PyPI) — Yandex's gradient boosting library
**File / function:** `catboost/core.py`, multiple methods using `self.feature_names_.index(feature)` (lines ~3910, 3981, 4168)
**Class:** CWE-706 (Use of Incorrectly-Resolved Name) + CWE-20
**Severity:** Medium-High — silent model-integrity issue, not a crash.

## Summary

Several public CatBoost methods resolve a feature given by name via:

```python
feature_idx = self.feature_names_.index(feature)
```

`self.feature_names_` is populated directly from a loaded model file (tested here via CatBoost's JSON model format — a real, documented, supported export/import format). There is no check anywhere in the loading path that `feature_id` values are unique across the model's declared float features.

Python's `list.index()` always returns the **first** matching element. A model file can declare two float features with the same `feature_id` but different real data (different learned split-point borders) — any name-based lookup silently resolves to the first one, and the second feature's real, distinct data becomes permanently unreachable via any name-based API, with zero error or warning.

## Proof of Concept

`poc_catboost_duplicate_feature_id.py`:

1. Trains a small, legitimate 2-feature CatBoost model (`age`, `income`) and exports it to JSON.
2. Patches the second feature's declared `feature_id` from `"income"` to `"age"`, keeping its real, distinct border data unchanged.
3. Loads the patched file via the library's real, public, documented API: `CatBoostClassifier().load_model(path, format="json")`.

```bash
pip install catboost numpy
python poc_catboost_duplicate_feature_id.py
```

### Result

```
=== Loading via the real public API: CatBoostClassifier().load_model(path, format='json') ===
  loaded OK, no error. feature_names_ = ['age', 'age']

  feature_names_.index('age') resolves to position 0
  Position 1's real, distinct border data ([0.76..., 0.89...]) is now unreachable via name-based lookup -- 'age' always resolves to position 0.

CONFIRMED: the second feature's real data is silently shadowed.
```

## Impact

A crafted or corrupted CatBoost model file can declare two structurally distinct, independently-trained features under the same name. Any downstream code that asks for statistics, borders, or feature-dependence plots by name silently receives the first feature's data instead — no error, no warning, no structural signal that something is wrong. This can be used to hide or misrepresent a model's actual behavior on a specific real-world input feature.

## Suggested fix

Validate that `feature_id` values are unique across `features_info.float_features` (and equivalent categorical/text feature sections) during model loading, and raise a clear error on a duplicate.

## Relationship to other findings

Same broad pattern family — duplicate identifier leading to silent "first wins" resolution so real data becomes unreachable — as separately-reported findings this session in `gguf-py` (tensor offset aliasing), `ollama/ollama`'s Go GGUF parser (duplicate tensor names), and `sklearn-pmml-model` (duplicate VectorInstance id) — all first-wins, all missing the same class of uniqueness validation. This is the first confirmed instance of the pattern in a gradient-boosting model format.

## Disclosure

Please do not use this PoC against production systems you do not own or have explicit permission to test.