GHSA-rhp5-r9x4-f5g2 NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution

• PyPI nltk: < 3.10.0 → fixed in 3.10.0

Summary The NLTK library's `TransitionParser.parse()` method deserializes model files using `pickle_load()` with the default `restricted=False` parameter, allowing arbitrary Python code execution when loading a malicious model file. The library provides a `RestrictedUnpickler` class for safe deserialization, but it is never used by production code paths, leaving the vulnerability unpatched. Root Cause File: `nltk/parse/transitionparser.py` (lines 542-557) The `parse()` method calls `pickle_load(f)` without `restricted=True`, routing through `WarningUnpickler` which inherits from `pickle.Unpickler` and does NOT override `find_class()`. This allows arbitrary class/function resolution during unpickling, enabling RCE via standard pickle gadgets (e.g., `os.system`, `subprocess.Popen`). Vulnerability chain in `nltk/picklesec.py`: ```python def pickle_load(file, *, context=None, restricted=False): if restricted: return RestrictedUnpickler(file).load() # Safe: blocks all globals return WarningUnpickler(file, context=context).load() # VULNERABLE PATH ``` `WarningUnpickler` only emits a warning but does NOT block unsafe class loading — it calls `super().load()` which is standard `pickle.Unpickler.load()`. Why this is not by design: • NLTK intentionally created `RestrictedUnpickler` to block unsafe deserialization • The `restricted=True` parameter exists in the API but is never used by any production code path • All call sites use the default `restricted=False`: `transitionparser.py:557`, `parse/chartparser_app.py:816`, `parse/chartparser_app.py:2273`, `parse/chartparser_app.py:2311` Attack Surface Entry point: `TransitionParser().parse(depgraphs, modelFile)` receives a filesystem path with no validation. Exploitation path: 1. Attacker places a malicious pickle file at a known or attacker-controlled location 2. Victim calls `parser.parse(sentences, "/path/to/malicious_model.pkl")` 3. `pickle_load()` deserializes the file with `restricted=False` (default) 4. Standard pickle gadget chain executes arbitrary Python code with victim's privileges Impact: Remote code execution with the privileges of the user running the NLTK-dependent application. Affects researchers, data scientists, and automated ML pipelines using NLTK for parsing tasks. Steps to Reproduce Environment • NLTK version: 3.8.1+ (all versions with `transitionparser.py`) • Python 3.6+ • No special dependencies required Reproduction 1. Create a malicious pickle file that uses `__reduce__` to execute a system command during deserialization. 2. Call `TransitionParser().parse([], '/path/to/malicious_model.pkl')`. 3. The `pickle_load(f)` call at `transitionparser.py:557` uses `restricted=False` by default, routing through `WarningUnpickler`, which does not override `find_class()` and permits full class resolution — executing the embedded gadget. 4. Arbitrary code executes with the victim's privileges. Proof That the Fix Works Changing line 557 in `transitionparser.py` from: ```python model = pickle_load(f) ``` to: ```python model = pickle_load(f, restricted=True) ``` causes `RestrictedUnpickler` to raise an `UnpicklingError` and block execution, confirming the safe path prevents the attack. Working PoC ```python import pickle import os from nltk.parse.transitionparser import TransitionParser Create malicious pickle with RCE payload class Exploit: def __reduce__(self): return (os.system, ('touch /tmp/nltk_poc_triggered',)) with open('/tmp/malicious_model.pkl', 'wb') as f: pickle.dump(Exploit(), f) Trigger the vulnerable code path (requires algorithm argument in ≤ 3.9.4) parser = TransitionParser('arc-standard') # or 'arc-eager' parser.parse([], '/tmp/malicious_model.pkl') # loads and unpickles unsafely Exploit succeeds: file /tmp/nltk_poc_triggered is created ``` On NLTK ≥ 3.10.0 (patched), the same code fails with: ``` _pickle.UnpicklingError: global 'posix.system' is not in the pickle allowlist ``` This proves the vulnerability exists in versions ≤ 3.9.4 and is fixed in 3.10.0+. Recommended Fix Change all call sites to use `restricted=True`: | File | Line | Before | After | |------|------|--------|-------| | `nltk/parse/transitionparser.py` | 557 | `pickle_load(f)` | `pickle_load(f, restricted=True)` | | `nltk/parse/chartparser_app.py` | 816 | `pickle_load(model_data_file)` | `pickle_load(model_data_file, restricted=True)` | | `nltk/parse/chartparser_app.py` | 2273 | `pickle_load(file)` | `pickle_load(file, restricted=True)` | | `nltk/parse/chartparser_app.py` | 2311 | `pickle_load(fp)` | `pickle_load(fp, restricted=True)` | Note: This fix may affect loading older sklearn models. A more robust approach would implement a module allowlist in `RestrictedUnpickler.find_class()`.