Vulnerability-Lookup

CVE-2026-12259 (GCVE-0-2026-12259)

Vulnerability from cvelistv5 – Published: 2026-08-03 07:09 – Updated: 2026-08-03 14:59

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.

Severity

5.3 (Medium)


                        
                          CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-03 14:55 UTC

CWE

CWE-494 - Download of Code Without Integrity Check

Assigner

@huntr_ai mitre root

References

1 reference

URL	Tags
https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c…

Impacted products

1 product

Vendor	Product	Version	CPE status
nltk	nltk/nltk	Affected: unspecified , ≤ latest (custom)	guessed

Show details on NVD website

JSON

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FKIE_CVE-2026-12259

Vulnerability from fkie_nvd - Published: 2026-08-03 08:17 - Updated: 2026-08-31 19:35

Severity

5.3 (Medium) - CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

Summary

References

	URL	Tags
	security@huntr.dev	https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "nltk/nltk",
          "vendor": "nltk",
          "versions": [
            {
              "lessThanOrEqual": "latest",
              "status": "affected",
              "version": "unspecified",
              "versionType": "custom"
            }
          ]
        }
      ],
      "source": "security@huntr.dev"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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",
  "lastModified": "2026-08-31T19:35:51.890",
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          "vectorString": "CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N",
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        "impactScore": 3.6,
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        "type": "Secondary"
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            }
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          "timestamp": "2026-08-03T14:55:19.698104Z",
          "version": "2.0.3"
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  "references": [
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}

GHSA-5WP5-5229-5G6Q

Vulnerability from github – Published: 2026-09-08 15:27 – Updated: 2026-09-08 15:27

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.

The download flow at lines 789-825: 1. File is downloaded to a temp path via HTTP 2. os.replace(tmp_filepath, filepath) moves it to the final location (line 799) 3. Extraction begins via _unzip_iter() (line 825)

Between 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.

Attack vectors: 1. MITM during HTTP download (NLTK downloads from http:// by default on some mirrors) 2. Race condition on shared filesystems (attacker replaces file between os.replace and _unzip_iter) 3. DNS poisoning redirecting to attacker-controlled server

PoC:

import nltk
import unittest.mock
import zipfile
import io
import os

# Create a malicious zip that will be "downloaded"
malicious_zip = io.BytesIO()
with zipfile.ZipFile(malicious_zip, 'w') as zf:
    zf.writestr('punkt_tab/tokenizers/punkt_tab/english.pickle', 
                b'MALICIOUS PAYLOAD - attacker controlled content')

# Patch urllib to return our malicious zip
with unittest.mock.patch('urllib.request.urlopen') as mock_urlopen:
    mock_response = unittest.mock.MagicMock()
    mock_response.read.return_value = malicious_zip.getvalue()
    mock_response.headers = {'Content-Length': str(len(malicious_zip.getvalue()))}
    mock_urlopen.return_value = mock_response

    # Download proceeds, no integrity check catches the swap
    # nltk.download('punkt_tab')  # Would install attacker payload

This 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.

Suggested 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

5.3 (Medium)


                  
                    CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.9.2"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "nltk"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.9.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-12259"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-494"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-08T15:27:55Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "NLTK\u0027s 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, \u0027w\u0027) as zf:\n    zf.writestr(\u0027punkt_tab/tokenizers/punkt_tab/english.pickle\u0027, \n                b\u0027MALICIOUS PAYLOAD - attacker controlled content\u0027)\n\n# Patch urllib to return our malicious zip\nwith unittest.mock.patch(\u0027urllib.request.urlopen\u0027) as mock_urlopen:\n    mock_response = unittest.mock.MagicMock()\n    mock_response.read.return_value = malicious_zip.getvalue()\n    mock_response.headers = {\u0027Content-Length\u0027: 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(\u0027punkt_tab\u0027)  # 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.",
  "id": "GHSA-5wp5-5229-5g6q",
  "modified": "2026-09-08T15:27:55Z",
  "published": "2026-09-08T15:27:55Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection"
}

GHSA-PV39-QRFQ-G8GC

Vulnerability from github – Published: 2026-08-03 09:32 – Updated: 2026-09-01 20:30

Withdrawn 2026-09-01

Summary

Duplicate Advisory: NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection

Details

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-5wp5-5229-5g6q. This link is maintained to preserve external references.

Original Description

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.

Severity

5.3 (Medium)


                  
                    CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "nltk"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "3.9.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-494"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-01T20:30:34Z",
    "nvd_published_at": "2026-08-03T08:17:17Z",
    "severity": "MODERATE"
  },
  "details": "## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of\u00a0GHSA-5wp5-5229-5g6q. This link is maintained to preserve external references.\n\n## Original Description\nIn 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": "GHSA-pv39-qrfq-g8gc",
  "modified": "2026-09-01T20:30:34Z",
  "published": "2026-08-03T09:32:37Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-12259"
    },
    {
      "type": "WEB",
      "url": "https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d"
    }
  ],
  "schema_version": "1.4.0",
  "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": "Duplicate Advisory: NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection",
  "withdrawn": "2026-09-01T20:30:34Z"
}

ubuntu-cve-2026-12259

Vulnerability from osv_ubuntu

Published

2026-08-03 08:17

Modified

2026-08-06 00:12

Summary

Details

Severity

5.3 (Medium)


                    
                      CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-12259	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-12259	REPORT
	https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c…	REPORT

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-nltk",
            "binary_version": "2.0~b9-0ubuntu4.1~esm6"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:14.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@2.0~b9-0ubuntu4.1~esm6?arch=source\u0026distro=esm-infra-legacy/trusty"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.0~b9-0ubuntu4",
        "2.0~b9-0ubuntu4.1~esm2",
        "2.0~b9-0ubuntu4.1~esm4",
        "2.0~b9-0ubuntu4.1~esm5",
        "2.0~b9-0ubuntu4.1~esm6"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-nltk",
            "binary_version": "3.1-1ubuntu0.1+esm4"
          },
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.1-1ubuntu0.1+esm4"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:16.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.1-1ubuntu0.1+esm4?arch=source\u0026distro=esm-apps-legacy/xenial"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.0.4-1",
        "3.0.5-1",
        "3.1-1",
        "3.1-1ubuntu0.1",
        "3.1-1ubuntu0.1+esm1",
        "3.1-1ubuntu0.1+esm2",
        "3.1-1ubuntu0.1+esm3",
        "3.1-1ubuntu0.1+esm4"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-nltk",
            "binary_version": "3.2.5-1ubuntu0.1+esm4"
          },
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.2.5-1ubuntu0.1+esm4"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:18.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.2.5-1ubuntu0.1+esm4?arch=source\u0026distro=esm-apps/bionic"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.2.4-1",
        "3.2.5-1",
        "3.2.5-1ubuntu0.1",
        "3.2.5-1ubuntu0.1+esm1",
        "3.2.5-1ubuntu0.1+esm2",
        "3.2.5-1ubuntu0.1+esm3",
        "3.2.5-1ubuntu0.1+esm4"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.4.5-2ubuntu0.1~esm4"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:20.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.4.5-2ubuntu0.1~esm4?arch=source\u0026distro=esm-apps/focal"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.4.5-1",
        "3.4.5-2",
        "3.4.5-2ubuntu0.1~esm1",
        "3.4.5-2ubuntu0.1~esm2",
        "3.4.5-2ubuntu0.1~esm3",
        "3.4.5-2ubuntu0.1~esm4"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.7-1ubuntu0.1~esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:22.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.7-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/jammy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.5-1",
        "3.6.5-1",
        "3.6.7-1",
        "3.7-1",
        "3.7-1ubuntu0.1~esm1",
        "3.7-1ubuntu0.1~esm2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.8.1-1ubuntu0.1~esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:24.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.8.1-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/noble"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.8.1-1",
        "3.8.1-1ubuntu0.1~esm1",
        "3.8.1-1ubuntu0.1~esm2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.9.2-1ubuntu0.1~esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:26.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.9.2-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/resolute"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.9.1-2",
        "3.9.1-2build1",
        "3.9.2-1",
        "3.9.2-1ubuntu0.1~esm1",
        "3.9.2-1ubuntu0.1~esm2"
      ]
    }
  ],
  "aliases": [],
  "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": "UBUNTU-CVE-2026-12259",
  "modified": "2026-08-06T00:12:15Z",
  "published": "2026-08-03T08:17:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-12259"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-12259"
    },
    {
      "type": "REPORT",
      "url": "https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d"
    }
  ],
  "related": [],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-12259"
  ]
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-12259 (GCVE-0-2026-12259)

FKIE_CVE-2026-12259

GHSA-5WP5-5229-5G6Q

GHSA-PV39-QRFQ-G8GC

Duplicate Advisory

Original Description

ubuntu-cve-2026-12259

Vulnerability from osv_ubuntu

Tags

Sightings

Nomenclature

Related by attack behaviour