CVE-2026-12261

Name of the Vulnerable Software and Affected Versions nltk versions prior to 3.9.5

Description A flaw in nltk.downloader allows for cross-package resource and model poisoning. The downloader extracts package archives into shared namespaces, such as corpora/ and taggers/, rather than using package-isolated roots. Additionally, package integrity is validated only after the archive has been written and extracted. This design allows one package to overwrite trusted resources of another package within the same namespace, making the modifications immediately active through standard NLTK APIs. These changes persist across interpreter restarts and can impact machine learning pipelines and environments where reproducibility is critical.

Recommendations Update to a version newer than 3.9.4.