Improper Access Control in nltk/nltk
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
- Attack complexity
- Low
- Impact type
- Security bypass
- Confidentiality · integrity · availability
- None · High · None
- Fix status
- Partial Red Hat: 5 without a fix
Attack profile from CVSS 3.1 · NVD. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-284.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- Poc
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| nltk | nltk/nltk | up to latest | — | No fix listed | CNA |
| — | nltk (PyPI) | before 3.10.0 | 3.10.0 | Patch available | GHSA-ffj6-66c4-86gw |
- nltk/nltk Vendornltk Affected up to latest Fixed in — No fix listed CNA
- nltk (PyPI) Vendor— Affected before 3.10.0 Fixed in 3.10.0 Patch available GHSA-ffj6-66c4-86gw
Remediation
- Fix · GHSA-ffj6-66c4-86gw nltk (PyPI) Fixed in 3.10.0 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Exploit Intelligence | Affected | No fix planned: Out of support scope |
| Lightspeed Core | Affected | No fix yet: Fix deferred |
| OpenShift Lightspeed | Affected | No fix yet: Fix deferred |
| Red Hat Ansible Automation Platform 2 | Affected | No fix planned: Out of support scope |
| Red Hat OpenShift AI (RHOAI) | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2026-12261
- 2026-10-03 Alias added: GHSA-ffj6-66c4-86gw
References
Other 6
- https://huntr.com/bounties/8b8c381e-08a8-4e4f-bb46-a320c96a364f
- https://github.com/nltk/nltk/pull/3625
- https://github.com/nltk/nltk/commit/7a5740af89feeb739193db766e11e85dfe78339c
- https://github.com/nltk/nltk
- https://github.com/nltk/nltk/releases/tag/v3.10.0
- https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3954.yaml
More Nltk vulnerabilities
- TI-2026-00629 Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading CVSS 3.0CRITICAL 10.0
- TI-2025-12800 Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution CVSS 3.0CRITICAL 10.0
- TI-2026-55173 NLTK before 3.10.3 JVM Argument Injection via Per-Call Options CVSS 4.0CRITICAL 9.3
- TI-2026-55155 NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization CVSS 4.0CRITICAL 9.3
- TI-2026-56370 nltk PorterStemmer before 3.10.3 Quadratic-time DoS CVSS 4.0HIGH 8.7
- TI-2026-55172 NLTK 3.10.2 Path Traversal via corpus-reader constructors CVSS 4.0HIGH 8.8
Sources
- CVE-2026-12261 · CVE Program
- CVE-2026-12261 · NVD
- FIRST EPSS
- GHSA-ffj6-66c4-86gw · GitHub Advisory Database · CC BY 4.0
- CVE-2026-12261 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences