Improper Input Validation in nltk/nltk

{
  "id": "9903a5f19b7ec0139077cfb63f4bd3b51b599762bced52f230fba00240ea91bd",
  "url": "https://osv.dev/vulnerability/CVE-2026-12259",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Improper Input Validation in nltk/nltk",
    "summary": "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.",
    "evidence": "osv:CVE-2026-12259",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-12259",
      "aliases": [
        "CVE-2026-12259",
        "CVE-2026-63310",
        "GHSA-5wp5-5229-5g6q",
        "GHSA-gf32-cmjh-8m9v",
        "PYSEC-2026-3729",
        "PYSEC-2026-4318"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/nltk/nltk",
              "type": "GIT",
              "events": [
                {
                  "introduced": "ad9c96ba00a16923ffe917eacf63f1707bfa2d08"
                },
                {
                  "last_affected": "ad9c96ba00a16923ffe917eacf63f1707bfa2d08"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:nltk:nltk:3.9.4:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "3.9.4"
                      },
                      {
                        "last_affected": "3.9.4"
                      }
                    ],
                    "source": "CPE_STRING"
                  }
                ]
              }
            }
          ],
          "versions": [
            "3.9.4"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-12259.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.339380773Z",
      "provider": "osv",
      "references": [
        "https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/12xxx/CVE-2026-12259.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-12259"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-494"
        ],
        "cna_assigner": "@huntr_ai",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/12xxx/CVE-2026-12259.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-12259",
        "evidence": "/id"
      },
      {
        "value": "CVE-2026-63310",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 508140,
  "published_at": "2026-08-03T07:09:35.262+00:00"
}