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This vulnerability resides in the `UbuntuCorpusTrainer.extract()` method of the ChatterBot Python library. The core issue is a Time-of-Check to Time-of-Use (TOCTOU) flaw combined with the use of a predictable file path.
The trainer, when initialized, constructs a predictable output directory path within the user’s home folder: ~/ubuntu_data/ubuntu_dialogs. During the extraction process, the code first checks if this path exists using os.path.exists(). If it does not, it creates the directory with os.makedirs(). However, if a local attacker pre-creates a symbolic link (symlink) at this exact predictable path pointing to an arbitrary location on the filesystem, the `os.path.exists()` check will follow the symlink and return True. This causes the `os.makedirs()` step to be skipped entirely.
Subsequently, the code calls `tar.extractall(path=self.data_path)` to extract the archive. Because `self.data_path` is now a symlink, the extraction process writes the contents of the tar archive through the symlink, effectively placing files in the attacker-chosen directory.
The library employs a `safe_extract` function to mitigate zip-slip path traversal vulnerabilities by validating the names of files within the archive. However, this defense is ineffective in this scenario. The `safe_extract` function resolves the provided `path` argument to an absolute path using os.path.abspath(). When `self.data_path` is a symlink, this resolution follows the link to the attacker’s target directory. The subsequent path validation checks are then performed relative to this resolved, attacker-controlled base path, rendering them useless. The vulnerability allows an attacker with local access to write arbitrary files to any location on the system where the user running the ChatterBot process has write permissions.
DailyCVE Form
Platform: ……. ChatterBot (Python library)
Version: …….. 1.2.13 (and likely prior)
Vulnerability :…… Symlink TOCTOU Arbitrary File Write
Severity: ……. Medium (Local, Privilege Escalation)
date: ………. 2026-06-19
Prediction: …… 2026-07-19 (Estimated)
What Undercode Say: Analytics
The vulnerability is triggered by a specific sequence of filesystem operations. The following commands and code snippets illustrate the flaw.
1. Predictable Path Construction
The vulnerable code in `trainers.py` constructs the output path as follows:
home_directory = os.path.expanduser('~')
self.data_directory = kwargs.get(
'ubuntu_corpus_data_directory',
os.path.join(home_directory, 'ubuntu_data')
)
self.data_path = os.path.join(
self.data_directory, 'ubuntu_dialogs'
)
2. The Check-Then-Act (TOCTOU) Pattern
The flawed logic in the `extract()` method:
def extract(self, file_path: str): if not os.path.exists(self.data_path): os.makedirs(self.data_path) ... later ... safe_extract(tar, path=self.data_path, ...)
3. Ineffective `safe_extract` Validation
The `safe_extract` function’s path validation is bypassed because it resolves the symlink:
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):
raise Exception('Attempted Path Traversal in Tar File')
tar.extractall(path, members, numeric_owner=numeric_owner)
Exploit
A local attacker can exploit this by creating a symlink at the predictable path before the ChatterBot trainer is executed.
1. Create the symlink pointing to the attacker's target directory ln -s /path/to/attacker/target ~/ubuntu_data/ubuntu_dialogs
When the `UbuntuCorpusTrainer` is subsequently run, the extraction will write the contents of the `ubuntu_dialogs.tgz` archive to `/path/to/attacker/target` instead of the intended `~/ubuntu_data/ubuntu_dialogs` directory.
A full Proof of Concept (PoC) is available in the vulnerability report, which demonstrates writing arbitrary files, such as a `config.py` containing malicious code, to the attacker’s chosen location.
Protection
To protect against this vulnerability, the application must validate that the output directory is not a symlink before proceeding with extraction. The following fix is recommended:
def extract(self, file_path: str):
Refuse to operate if the output path is a symlink
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)
... rest of extraction ...
Additionally, as a general security practice, it is recommended to use a temporary directory with a non-predictable name for extracting untrusted archives.
Impact
- Arbitrary File Write: An attacker with local access can write files to any location on the filesystem that the ChatterBot process has permissions to write to.
- Privilege Escalation: This can lead to privilege escalation if the attacker can overwrite critical system files, configuration files, or user scripts (e.g.,
.bashrc,~/.ssh/authorized_keys). - Code Execution: By writing to a location that is later executed (e.g., a Python script in `sys.path` or a cron job), an attacker can achieve arbitrary code execution. The PoC demonstrates writing a `config.py` file that executes system commands.
- Data Integrity Loss: Overwriting or corrupting files can lead to denial of service or data loss.
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Sources:
Reported By: github.com
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