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Medium, CVSS 6.9

NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input

CVE-2026-81724GHSA-cw6x-m8jw-qmrh

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (nltk 3.10.3), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI nltk

    Affected
    <= 3.10.2
    Fixed in
    3.10.3

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-81724: 0.46% estimated exploitation activity in the next 30 days; percentile 39.62 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary nltk.featstruct.FeatStructReader (used by FeatStruct(str) and by FeatureGrammar.fromstring()) parses feature-structure strings such as [a=1] with a recursive-descent parser that has no nesting-depth limit. A small, trivially-crafted input (~700 bytes) with deeply nested brackets drives the parser past Python's recursion limit and raises an unhandled RecursionError instead of the library's normal, catchable ValueError/LogicalExpressionException. Any application that parses user-supplied feature-structure or feature-grammar text (e.g. NLP teaching tools, grammar "playgrounds", unification-grammar-based NLU pipelines) can be crashed by an unauthenticated input with no special privileges. This is a Denial of Service issue (CWE-674, Uncontrolled Recursion), not a memory-safety or code-execution issue. This appears to be the same bug class as two issues already fixed elsewhere in the codebase — nltk/jsontags.py (JSONTaggedDecoder.decodeobj, guarded by MAXDECODEDEPTH = 200) and…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-cw6x-m8jw-qmrh

Agent brief

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# Patch Next advisory brief: CVE-2026-81724

Advisory: NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input
IDs: CVE-2026-81724, GHSA-cw6x-m8jw-qmrh
Record: https://patchnext.com/advisories/cve-2026-81724 (revision 1, 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 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected <= 3.10.2; fixed in 3.10.3

## 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-cw6x-m8jw-qmrh (GitHub-reviewed advisory GHSA-cw6x-m8jw-qmrh lists affected ranges: PyPI nltk >=0 <=3.10.2)

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