CVE-2026-78683: NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization

Summary

The NLTK library's TransitionParser.parse() method deserializes model files using pickleload() 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 pickleload(f) without restricted=True, routing through WarningUnpickler which inherits from pickle.Unpickler and does NOT override findclass(). 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 pickleload(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/chartparserapp.py:816, parse/chartparserapp.py:2273, parse/chartparserapp.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/maliciousmodel.pkl") 3. pickleload() 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/maliciousmodel.pkl').

3. The pickleload(f) call at transitionparser.py:557 uses restricted=False by default, routing through WarningUnpickler, which does not override findclass() 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 = pickleload(f) to: python model = pickleload(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/nltkpoctriggered',))

with open('/tmp/maliciousmodel.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/maliciousmodel.pkl') # loads and unpickles unsafely

Exploit succeeds: file /tmp/nltkpoctriggered 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 | pickleload(f) | pickleload(f, restricted=True) | | nltk/parse/chartparserapp.py | 816 | pickleload(modeldatafile) | pickleload(modeldatafile, restricted=True) | | nltk/parse/chartparserapp.py | 2273 | pickleload(file) | pickleload(file, restricted=True) | | nltk/parse/chartparserapp.py | 2311 | pickleload(fp) | pickleload(fp, restricted=True) |

Note: This fix may affect loading older sklearn models. A more robust approach would implement a module allowlist in RestrictedUnpickler.findclass().