{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "cpe": "cpe:2.3:a:nltk:nltk:3.9.4:*:*:*:*:*:*:*",
            "extracted_events": [
              {
                "introduced": "3.9.4"
              },
              {
                "last_affected": "3.9.4"
              }
            ],
            "source": "CPE_STRING"
          },
          "events": [
            {
              "introduced": "ad9c96ba00a16923ffe917eacf63f1707bfa2d08"
            },
            {
              "last_affected": "ad9c96ba00a16923ffe917eacf63f1707bfa2d08"
            }
          ],
          "repo": "https://github.com/nltk/nltk",
          "type": "GIT"
        }
      ],
      "versions": [
        "3.9.4"
      ]
    }
  ],
  "database_specific": {
    "cna_assigner": "@huntr_ai",
    "cwe_ids": [
      "CWE-494"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/12xxx/CVE-2026-12259.json"
  },
  "details": "In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for `info.url` through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.",
  "id": "CVE-2026-12259",
  "modified": "2026-10-17T02:30:30.476922866Z",
  "published": "2026-08-03T07:09:35.262Z",
  "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"
    }
  ],
  "schema_version": "1.9.0",
  "severity": [
    {
      "score": "CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Improper Input Validation in nltk/nltk"
}