NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution

## 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 'po

CVSS 4.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-502

Also known as

CVE-2026-78683, PYSEC-2026-3734

• github.com/nltk/nltk/security/advisories/GHSA-rhp5-r9x4-f5g2
• nvd.nist.gov/vuln/detail/CVE-2026-78683
• github.com/nltk/nltk/pull/3631
• github.com/nltk/nltk/commit/f26b3753038d937b68145daf15e9636f8451053c
• github.com/nltk/nltk
• github.com/nltk/nltk/releases/tag/v3.10.0
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3734.yaml
• www.vulncheck.com/advisories/nltk-before-remote-code-execution-via-unsafe-pickle-deserialization