CVE-2026-12261

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.

CVSS 3.1

6.5CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

EPSS

0.2% chance of exploitation in the next 30 days; higher than 11% of CVEs (scores of 2026-10-25)

CWE

CWE-284

Published

2026-08-07

Modified

2026-10-23

Record from

the NVD, synced 2026-10-24

Affected

• nltk:nltk

Packages (OSV)

References

• https://huntr.com/bounties/8b8c381e-08a8-4e4f-bb46-a320c96a364f
• https://huntr.com/bounties/8b8c381e-08a8-4e4f-bb46-a320c96a364f

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