{"id":"CVE-2026-12259","aliases":["CVE-2026-63310","GHSA-5wp5-5229-5g6q","GHSA-gf32-cmjh-8m9v","PYSEC-2026-3729","PYSEC-2026-4318"],"url":"https://o3.security/vulnerability/CVE-2026-12259","summary":"Improper Input Validation in nltk/nltk","details":"NLTK's package downloader in nltk/downloader.py does not verify file integrity after download and before extraction.\n\nThe download flow at lines 789-825:\n1. File is downloaded to a temp path via HTTP\n2. os.replace(tmp_filepath, filepath) moves it to the final location (line 799)\n3. Extraction begins via _unzip_iter() (line 825)\n\nBetween steps 2 and 3, there is no SHA-256 verification. The checksum logic exists in _pkg_status() (lines 982-1015) but it is only used BEFORE download as a status check (\"is this package already installed and up-to-date?\"). It is never called after download to verify the file that was actually received.\n\nAttack vectors:\n1. MITM during HTTP download (NLTK downloads from http:// by default on some mirrors)\n2. Race condition on shared filesystems (attacker replaces file between os.replace and _unzip_iter)\n3. DNS poisoning redirecting to attacker-controlled server\n\nPoC:\n```python\nimport nltk\nimport unittest.mock\nimport zipfile\nimport io\nimport os\n\n# Create a malicious zip that will be \"downloaded\"\nmalicious_zip = io.BytesIO()\nwith zipfile.ZipFile(malicious_zip, 'w') as zf:\n    zf.writestr('punkt_tab/tokenizers/punkt_tab/english.pickle', \n                b'MALICIOUS PAYLOAD - attacker controlled content')\n\n# Patch urllib to return our malicious zip\nwith unittest.mock.patch('urllib.request.urlopen') as mock_urlopen:\n    mock_response = unittest.mock.MagicMock()\n    mock_response.read.return_value = malicious_zip.getvalue()\n    mock_response.headers = {'Content-Length': str(len(malicious_zip.getvalue()))}\n    mock_urlopen.return_value = mock_response\n    \n    # Download proceeds, no integrity check catches the swap\n    # nltk.download('punkt_tab')  # Would install attacker payload\n```\n\nThis is distinct from CVE-2024-39705 (pickle deserialization via download) and CVE-2025-14009 (zip-slip path traversal). Those address what happens AFTER extraction. This finding addresses the gap BEFORE extraction where integrity is never verified.\n\nSuggested fix: After os.replace() and before _unzip_iter(), compute SHA-256 of the final file and compare against the expected checksum from the package index. Reject and delete the file if the hash does not match.","published":"2026-08-03T07:09:35.262Z","modified":"2026-10-25T02:30:38.536369958Z","cvss":{"score":5.3,"severity":"MEDIUM","vector":"CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"nltk","fixedVersion":"3.9.3"}],"fix":{"url":"https://github.com/nltk/nltk/pull/3449","label":"nltk/nltk#3449"},"references":[{"type":"WEB","url":"https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/12xxx/CVE-2026-12259.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-12259"},{"type":"WEB","url":"https://github.com/nltk/nltk/security/advisories/GHSA-5wp5-5229-5g6q"},{"type":"WEB","url":"https://github.com/nltk/nltk/pull/3449"},{"type":"WEB","url":"https://github.com/nltk/nltk/commit/0e26734a61094b628d93e26dc18dd7302567ac46"},{"type":"PACKAGE","url":"https://github.com/nltk/nltk"},{"type":"WEB","url":"https://github.com/nltk/nltk/releases/tag/3.9.3"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3729.yaml"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/nltk-before-missing-post-download-integrity-verification"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-25T02:30:38.536369958Z"}}