CVE-2026-12261
Improper Access Control in nltk/nltk
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.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- High
- Privileges needed
- None
- User interaction
- Required
What an attacker can affect
- Confidentiality
- None
- Integrity
- High
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
5.3
Improper Access Control in nltk/nltk
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.
- Record
CVE-2026-12261- CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Weakness
- CWE-284
- Updated
- 2026-11-02
EUVDEUVD-2026-54393
5.3
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.
- Record
EUVD-2026-54393- CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Updated
- 2026-11-02
GitHub AdvisoriesGHSA-ffj6-66c4-86gw
5.3
NLTK downloader allows cross-package resource and model poisoning
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.
- Record
GHSA-ffj6-66c4-86gw- CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Weakness
- CWE-284
- Updated
- 2026-11-02
NVD
6.5
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.
- Record
CVE-2026-12261- CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N - CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Weakness
- CWE-284
- Updated
- 2026-11-02
OSV
5.3
Improper Access Control in nltk/nltk
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.
- Record
CVE-2026-12261- CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Weakness
- CWE-284
- Updated
- 2026-11-02
OSVDEBIAN-CVE-2026-12261
6.5
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.
- Record
DEBIAN-CVE-2026-12261- CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N - Updated
- 2026-11-02
OSVGHSA-ffj6-66c4-86gw
5.3
NLTK downloader allows cross-package resource and model poisoning
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.
- Record
GHSA-ffj6-66c4-86gw- CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Weakness
- CWE-284
- Updated
- 2026-11-02
OSVPYSEC-2026-3954
6.5
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.
- Record
PYSEC-2026-3954- CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N - Updated
- 2026-11-02
OSVUBUNTU-CVE-2026-12261
6.5
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.
- Record
UBUNTU-CVE-2026-12261- CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N - CVSS 3.0
- 5.3 Medium
CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N - Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
nltkPyPI |
before 3.10.0 | 3.10.0 |
nltkDebian:12 |
All versions, plus listed versions 3.10.0-13.10.1-13.10.3-13.8-13.8.1-1 Show 4 moreShow less3.9.1-13.9.1-23.9.2-13.9.3-1 |
None known |
nltkDebian:13 |
All versions, plus listed versions 3.10.0-13.10.1-13.10.3-13.9.1-23.9.2-13.9.3-1 |
None known |
nltkDebian:14 |
All versions, plus listed versions 3.10.0-13.10.1-13.10.3-13.9.1-23.9.2-13.9.3-1 |
None known |
nltkPyPI |
before 3.10.0, plus listed versions 0.80.90.9.30.9.40.9.5 Show 45 moreShow less0.9.60.9.70.9.80.9.92.0.12.0.1rc12.0.1rc2-git2.0.1rc32.0.1rc42.0.22.0.32.0.42.0.52.0b42.0b52.0b62.0b72.0b82.0b93.0.03.0.0b13.0.0b23.0.13.0.23.0.33.0.43.0.53.13.23.2.13.2.23.2.33.2.43.2.53.33.43.4.13.4.23.4.33.4.43.4.53.53.5b13.63.6.1 |
3.10.0 |
nltkUbuntu:Pro:14.04:LTS |
All versions, plus listed versions 2.0~b9-0ubuntu42.0~b9-0ubuntu4.1~esm22.0~b9-0ubuntu4.1~esm42.0~b9-0ubuntu4.1~esm52.0~b9-0ubuntu4.1~esm6 |
None known |
nltkUbuntu:Pro:16.04:LTS |
All versions, plus listed versions 3.0.4-13.0.5-13.1-13.1-1ubuntu0.13.1-1ubuntu0.1+esm13.1-1ubuntu0.1+esm23.1-1ubuntu0.1+esm33.1-1ubuntu0.1+esm4 |
None known |
nltkUbuntu:Pro:18.04:LTS |
All versions, plus listed versions 3.2.4-13.2.5-13.2.5-1ubuntu0.13.2.5-1ubuntu0.1+esm13.2.5-1ubuntu0.1+esm23.2.5-1ubuntu0.1+esm33.2.5-1ubuntu0.1+esm4 |
None known |
nltkUbuntu:Pro:20.04:LTS |
All versions, plus listed versions 3.4.5-13.4.5-23.4.5-2ubuntu0.1~esm13.4.5-2ubuntu0.1~esm23.4.5-2ubuntu0.1~esm33.4.5-2ubuntu0.1~esm4 |
None known |
nltkUbuntu:Pro:22.04:LTS |
All versions, plus listed versions 3.5-13.6.5-13.6.7-13.7-13.7-1ubuntu0.1~esm13.7-1ubuntu0.1~esm2 |
None known |
nltkUbuntu:Pro:24.04:LTS |
All versions, plus listed versions 3.8.1-13.8.1-1ubuntu0.1~esm13.8.1-1ubuntu0.1~esm2 |
None known |
nltkUbuntu:26.10 |
All versions, plus listed versions 3.9.2-13.9.3-13.10.0-13.10.1-13.10.3-1 |
None known |
nltkUbuntu:Pro:26.04:LTS |
All versions, plus listed versions 3.9.1-23.9.1-2build13.9.2-13.9.2-1ubuntu0.1~esm13.9.2-1ubuntu0.1~esm2 |
None known |
History
| Recorded | Changed | Sources |
|---|---|---|
| first indexed | CVE List, EUVD, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2026-12261 is a medium vulnerability (CVSS 5.3) in Nltk, published August 7, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.21%, the 11th percentile. Fixed in nltk 3.10.0.
References
huntr.com/bounties/8b8c381e-08a8-4e4f-bb46-a320c96a364f
nvd.nist.gov/vuln/detail/CVE-2026-12261
github.com/nltk/nltk/pull/3625
github.com/nltk/nltk/commit/7a5740af89feeb739193db766e11e85dfe78339c
github.com/nltk/nltk/releases/tag/v3.10.0
github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3954.yaml
Left out here: 15 versions. The JSON record has them.