GHSA-72r2-7mfr-5xr9 NLTK: FileSystemPathPointer.open() sandbox check is dead code — arbitrary file read via file:// protocol

• PyPI nltk: < 3.10.0 → fixed in 3.10.0

Summary There's a logic bug in `FileSystemPathPointer.open()` inside `nltk/data.py` that makes the sandbox check permanently inert. The guard condition is always `False` — meaning any file the process can read is accessible by passing a `file://` URL to `nltk.data.load()`. --- Details In `nltk/data.py`, `FileSystemPathPointer.open()` was patched at some point with a comment saying "SECURITY PATCH ENFORCING SANDBOX", but the check doesn't work: ```python def open(self, encoding=None): path = os.path.normpath(self._path) # Block raw absolute reads such as "/" "C:\\Windows" etc. if os.path.isabs(path) and path != os.path.normpath(self._path): raise ValueError(f"Direct absolute file access blocked: {path}") stream = open(self._path, "rb") ``` `path` is set to `os.path.normpath(self._path)` on line 1, then compared against `os.path.normpath(self._path)` again in the condition. They are always equal. The `ValueError` never fires. On top of that, `__init__` already calls `os.path.abspath()` before storing `self._path`, so it's normalized before `open()` is even called. Running `normpath` on it again changes nothing. The `stream = open(self._path, "rb")` line is always reached regardless of what path was passed in. --- PoC Tested on Python 3.11, NLTK 3.9.1, Ubuntu 22.04. ```python import nltk from nltk.data import FileSystemPathPointer direct construction ptr = FileSystemPathPointer("/etc/passwd") with ptr.open() as f: print(f.read(300)) via load() using file:// URL data = nltk.data.load("file:///etc/passwd", format="raw") print(data[:300]) ``` Both print file contents. No exception is raised. --- Impact Any app that lets users influence the string passed to `nltk.data.load()` or `nltk.data.find()` is exposed — web APIs, notebook servers, multi-tenant pipelines. An attacker can read any file the process user has access to: `/etc/passwd`, `.env` files, private keys, `~/.aws/credentials`, etc. Suggested Fix File: `nltk/data.py` — `FileSystemPathPointer.open()` (lines 378–390) What's wrong Line 387 compares `normpath(self._path)` against itself — always equal, so the `ValueError` never fires. The check is dead code. `__init__` already calls `abspath()` on construction, so re-running `normpath` inside `open()` changes nothing either. --- Fix Validate against the actual list of permitted data directories instead: ```python def open(self, encoding=None): import nltk.data as _d allowed = [os.path.abspath(p) for p in _d.path if p] if allowed and not any( os.path.commonpath([self._path, r]) == r for r in allowed ): raise ValueError( f"Access outside nltk_data blocked: {self._path!r}" ) stream = open(self._path, "rb") if encoding is not None: stream = SeekableUnicodeStreamReader(stream, encoding) return stream ``` --- Why `commonpath` not `startswith` `startswith` is bypassable by a path that shares a prefix: ``` /tmp/nltk_data_evil".startswith("/tmp/nltk_data") → True ✗ commonpath(["/tmp/nltk_data_evil", "/tmp/nltk_data"]) → "/tmp" ✓ ``` --- Diff ```diff • path = os.path.normpath(self._path) • if os.path.isabs(path) and path != os.path.normpath(self._path): • raise ValueError(f"Direct absolute file access blocked: {path}") - + import nltk.data as _d + allowed = [os.path.abspath(p) for p in _d.path if p] + if allowed and not any( + os.path.commonpath([self._path, r]) == r for r in allowed + ): + raise ValueError(f"Access outside nltk_data blocked: {self._path!r}") stream = open(self._path, "rb") ```