GHSA-r6gq-whwq-mvg9 NLTK: Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root

• PyPI nltk: < 3.9.4 → fixed in 3.9.4

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: ```text 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: ```python 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: ```text secret-data ``` Docker re-test output: ```text 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: ```text 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.