CVE-2026-12261
Received Received - Intake

Cross-Package Resource Poisoning in NLTK Downloader

Publication date: 2026-08-07

Last updated on: 2026-08-07

Assigner: huntr.dev

Description
A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and model poisoning. The downloader extracts package archives into shared namespaces such as `corpora/` and `taggers/` instead of package-isolated roots, and validates package integrity only after the archive has been written and extracted. This design flaw enables one package to overwrite another package's trusted resources within the same namespace, making the changes immediately active through ordinary NLTK APIs. This issue persists across fresh interpreter restarts and can affect downstream workflows, including machine learning pipelines and reproducibility-sensitive environments.

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Meta Information

Published
2026-08-07
Last Modified
2026-08-07
Generated
2026-08-07
AI Q&A
2026-08-07
EPSS Evaluated
N/A
NVD
CVE-2026-12261
EUVD
EUVD-2026-54393

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
nltk nltk to 3.9.4 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-284 The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor.

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Executive Summary

This vulnerability in nltk.downloader allows one NLTK package to overwrite trusted resources of another package within shared namespaces like corpora/ or taggers/. The downloader extracts archives without isolating them, validating integrity only after extraction. This means malicious or compromised packages can inject harmful data that becomes active immediately through NLTK APIs.

Impact Analysis

If you use NLTK for NLP tasks, this flaw could allow an attacker to replace legitimate language models or datasets with malicious versions. This might corrupt your machine learning pipelines, produce incorrect results, or introduce backdoors into your workflows. The impact persists even after restarting your interpreter.

Compliance Impact

This vulnerability could violate compliance requirements for data integrity and security, such as GDPR's accuracy principle or HIPAA's integrity safeguards. If corrupted NLTK resources produce incorrect outputs in healthcare or legal NLP applications, it may lead to regulatory violations, compromised decision-making, or unauthorized data exposure.

Mitigation Strategies

Upgrade NLTK to a version higher than 3.9.4 to address the vulnerability. Avoid using nltk.downloader for package installation and manually verify package integrity before use.

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