Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2026-66393: 0.52% estimated exploitation activity in the next 30 days; percentile 44.30 (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 JSONTaggedDecoder.decodeobj() in nltk/jsontags.py calls itself recursively without any depth limit. A deeply nested JSON structure exceeding sys.getrecursionlimit() (default: 1000) will raise an unhandled RecursionError, crashing the Python process. Affected code File: nltk/jsontags.py, lines 47–52 Proof of Concept Impact Any code path that passes externally-supplied JSON to JSONTaggedDecoder is vulnerable to denial of service. The severity depends on whether such a path exists in the calling code (e.g. nltk/data.py). Suggested Fix Add a depth parameter with a hard limit:

Published by the source

18 Mar 2026, 20:17 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rf74-v2fm-23pw

2. Revision 2 · amended · 25 Oct 2026

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

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-rf74-v2fm-23pw

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# Patch Next advisory brief: CVE-2026-66393

Advisory: Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS
IDs: CVE-2026-66393, GHSA-rf74-v2fm-23pw
Record: https://patchnext.com/advisories/cve-2026-66393 (revision 3, 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: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.9.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.9.3; fixed in 3.9.4

## 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://osv.dev/vulnerability/GHSA-rf74-v2fm-23pw (OSV record GHSA-rf74-v2fm-23pw lists affected ranges: PyPI nltk >=0 <3.9.4 (fixed))
- Verified: https://github.com/advisories/GHSA-rf74-v2fm-23pw (GitHub-reviewed advisory GHSA-rf74-v2fm-23pw lists affected ranges: PyPI nltk >=0 <=3.9.3)

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