CVE-2026-12259: NLTK Missing Post-Download Integrity Verification Allows Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5wp5-5229-5g6q",
  "modified" : "2026-09-08T15:27:55Z",
  "published" : "2026-09-08T15:27:55Z",
  "aliases" : [ "CVE-2026-12259" ],
  "summary" : "NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection",
  "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.9.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.9.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-5wp5-5229-5g6q"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-12259"
  }, {
    "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://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-before-missing-post-download-integrity-verification"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-494" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T15:27:55Z",
    "nvd_published_at" : null
  }
}