NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-78683: 0.51% estimated exploitation activity in the next 30 days; percentile 43.60 (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 The NLTK library's TransitionParser.parse() method deserializes model files using pickleload() with the default restricted=False parameter, allowing arbitrary Python code execution when loading a malicious model file. The library provides a RestrictedUnpickler class for safe deserialization, but it is never used by production code paths, leaving the vulnerability unpatched. Root Cause File: nltk/parse/transitionparser.py (lines 542-557) The parse() method calls pickleload(f) without restricted=True, routing through WarningUnpickler which inherits from pickle.Unpickler and does NOT override findclass(). This allows arbitrary class/function resolution during unpickling, enabling RCE via standard pickle gadgets (e.g., os.system, subprocess.Popen). Vulnerability chain in nltk/picklesec.py: WarningUnpickler only emits a warning but does NOT block unsafe class loading — it calls super().load() which is standard pickle.Unpickler.load(). Why this is not by design: - NLTK…

Published by the source

8 Sep 2026, 16:41 UTC

Last modified by the source

8 Sep 2026, 16:45 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rhp5-r9x4-f5g2

2. Revision 3 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-rhp5-r9x4-f5g2

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-rhp5-r9x4-f5g2

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# Patch Next advisory brief: CVE-2026-78683

Advisory: NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution
IDs: CVE-2026-78683, GHSA-rhp5-r9x4-f5g2
Record: https://patchnext.com/advisories/cve-2026-78683 (revision 4, source checked 26 Oct 2026, 13:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.9.4; fixed in 3.10.0

## 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-rhp5-r9x4-f5g2 (GitHub-reviewed advisory GHSA-rhp5-r9x4-f5g2 lists affected ranges: PyPI nltk >=0 <=3.9.4)
- Verified: https://osv.dev/vulnerability/GHSA-rhp5-r9x4-f5g2 (OSV record GHSA-rhp5-r9x4-f5g2 lists affected ranges: PyPI nltk >=0 <3.10.0 (fixed))

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