NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.3 · CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:L/VA:L/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-81726: 0.34% estimated exploitation activity in the next 30 days; percentile 27.45 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Several model-artifact APIs still treat caller-controlled model paths as ordinary filenames even when NLTK path security is enforced. The same outside-root paths are rejected by guarded helpers, but these public read and write flows still use raw file APIs.

Details - Vulnerability type: File sandbox bypass - Affected component: `TransitionParser.train`, `TransitionParser.parse`, `AveragedPerceptron.save`, `AveragedPerceptron.load`, `PerceptronTagger.save_to_json`, `save_maxent_params` - Affected versions: Published `3.9.4` and current source `v3.10.0-rc2` both reproduced. - Patched versions: Not yet patched - Root cause: Model import and export helpers use built-in `open()` on caller-controlled paths instead of pathsec-aware helpers. `TransitionParser.train()` writes outside allowed roots, `TransitionParser.parse()` reads outside allowed roots, `AveragedPerceptron` bypasses the sandbox in both directions, and adjacent read-side helpers in the same…

Published by the source

2 Sep 2026, 14:35 UTC

Last modified by the source

2 Nov 2026, 22:15 UTC

Source last checked

3 Nov 2026, 06:10 UTC

1. Revision 3 · amended · 3 Nov 2026

   Amended from osv record GHSA-8mgp-746c-j5xp

2. Revision 2 · amended · 2 Nov 2026

   Amended from ghsa record GHSA-8mgp-746c-j5xp

3. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-8mgp-746c-j5xp

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# Patch Next advisory brief: CVE-2026-81726

Advisory: NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots
IDs: CVE-2026-81726, GHSA-8mgp-746c-j5xp
Record: https://patchnext.com/advisories/cve-2026-81726 (revision 3, source checked 3 Nov 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected < 3.10.4; fixed in 3.10.4

## Suggested inspection (read-only)
- `python -m pip show nltk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nltk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-8mgp-746c-j5xp (GitHub-reviewed advisory GHSA-8mgp-746c-j5xp lists affected ranges: PyPI nltk >=0 <=3.10.3)
- Verified: https://osv.dev/vulnerability/GHSA-8mgp-746c-j5xp (OSV record GHSA-8mgp-746c-j5xp lists affected ranges: PyPI nltk >=0 <3.10.4 (fixed))

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