NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization

{
  "id": "5b02b0874da9127fc82fcfe4810900c356abfdd2f0708a15d8edce387253c83a",
  "url": "https://osv.dev/vulnerability/CVE-2026-78683",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization",
    "summary": "NLTK before 3.10.0 (affected versions <=3.9.4) contains an unsafe pickle deserialization vulnerability in the TransitionParser.parse() method (nltk/parse/transitionparser.py). The method calls pickle_load() with the default restricted=False, routing deserialization through WarningUnpickler, which does not override find_class() and therefore permits arbitrary class resolution. When an application loads an attacker-crafted model file, embedded pickle gadget chains execute arbitrary Python code with the privileges of the user running the application. NLTK provides a RestrictedUnpickler for safe deserialization, but it is not used by production code paths. Fixed in 3.10.0.",
    "evidence": "osv:CVE-2026-78683",
    "products": [],
    "severity": "critical",
    "structured": {
      "id": "CVE-2026-78683",
      "aliases": [
        "CVE-2026-78683",
        "GHSA-rhp5-r9x4-f5g2",
        "PYSEC-2026-3734"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/nltk/nltk",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "bd49f9011d7dc8c6a36b3c4ae71f04060c9b3fb9"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "3.10.0"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "cpe": "cpe:2.3:a:nltk:nltk:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "3.10.0"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v3.10.0-rc1",
            "3.9.4",
            "3.9.3",
            "v3.9.2",
            "3.9.2",
            "3.9.1",
            "3.9",
            "3.8.2",
            "3.8.1",
            "3.8",
            "3.7",
            "3.6.7",
            "3.6.6",
            "3.6.5",
            "3.6.2",
            "3.6.1",
            "3.6",
            "3.5",
            "3.5b1",
            "3.4.4",
            "3.4.3",
            "3.4.1",
            "3.4",
            "3.3",
            "3.2.5",
            "3.2.4",
            "3.2.3",
            "3.2.2",
            "3.2.1",
            "3.2",
            "3.1",
            "3.0.5",
            "3.0.4",
            "3.0.3",
            "3.0.2",
            "3.0.0b1",
            "3.0a4",
            "2.0.1rc4",
            "2.0.1rc3",
            "2.0.1rc2",
            "2.0.1rc1"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-78683.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:34.613233307Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/78xxx/CVE-2026-78683.json",
        "https://github.com/nltk/nltk/security/advisories/GHSA-rhp5-r9x4-f5g2",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-78683",
        "https://www.vulncheck.com/advisories/nltk-before-remote-code-execution-via-unsafe-pickle-deserialization"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-502"
        ],
        "cna_assigner": "VulnCheck",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/78xxx/CVE-2026-78683.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-78683",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 501956,
  "published_at": "2026-08-25T01:30:39.451+00:00"
}