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CVE-2026-33236 — NLTK has a Downloader Path Traversal Vulnerability (AFO) - Arbitrary File Overwrite

Affected: nltk, Red Hat OpenShift AI 2.25, Red Hat OpenShift AI 3.3, Red Hat Lightspeed Core, Red Hat OpenShift Lightspeed, Red Hat Ansible Automation Platform 2, Red Hat OpenShift AI (RHOAI)

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, the NLTK downloader does not validate the `subdir` and `id` attributes when processing remote XML index files. Attackers can control a remote XML index server to provide malicious values containing path traversal sequences (such as `../`), which can lead to arbitrary directory creation, arbitrary file creation, and arbitrary file overwrite. Commit 89fe2ec2c6bae6e2e7a46dad65cc34231976ed8a patches the issue.

GHSA-469j-vmhf-r6v7 — NLTK has a Downloader Path Traversal Vulnerability (AFO) - Arbitrary File Overwrite

Affected: PyPI: nltk

## Vulnerability Description

The NLTK downloader does not validate the `subdir` and `id` attributes when processing remote XML index files. Attackers can control a remote XML index server to provide malicious values containing path traversal sequences (such as `../`), which can lead to:

1. **Arbitrary Directory Creation**: Create directories at arbitrary locations in the file system 2. **Arbitrary File Creation**: Create arbitrary files 3. **Arbitrary File Overwrite**: Overwrite critical system files (such as `/etc/passwd`, `~/.ssh/authorized_keys`, etc.)

## Vulnerability Principle

### Key Code Locations

**1. XML Parsing Without Validation** (`nltk/downloader.py:253`) ```python self.filename = os.path.join(subdir, id + ext) ``` - `subdir` and `id` are directly from XML attributes without any validation

**2. Path Construction Without Checks** (`nltk/downloader.py:679`) ```python filepath = os.path.join(download_dir, info.filename) ``` - Directly uses `filename` which may contain path traversal

**3. Unrestricted Directory Creation** (`nltk/downloader.py:687`) ```python os.makedirs(os.path.join(download_dir, info.subdir), exist_ok=True) ``` - Can create arbitrary directories outside the download directory

**4. File Writing Without Protection** (`nltk/downloader.py:695`) ```python with open(filepath, "wb") as outfile: ``` - Can write to arbitrary locations in the file system

### Attack Chain

``` 1. Attacker controls remote XML index server ↓ 2. Provides malicious XM…

PYSEC-2026-2237

Affected: PyPI: nltk