TI-2026-56372 Aliases CVE-2026-81724 GHSA-cw6x-m8jw-qmrh

NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion

HIGH 70CWE-674Published 2026-08-27 · Updated 2026-08-27

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Attack requirements: None
Impact type: Denial of service
Confidentiality · integrity · availability: None · None · Low
Fix status: Partial Red Hat: 4 fixed, 5 without a fix

Attack profile from CVSS 4.0 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-674.

Summary

NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV

EPSS 0.00456 39th percentileScored 2026-11-07

Public exploit Yes 1 exploit referenceTagged by NVD

Fix Partial nltk (nltk): 3.10.3nltk (PyPI): 3.10.3Red Hat: 5 advisoriesRed Hat: 4 fixed, 5 without a fix

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: Poc
Automatable: Yes
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	before 3.10.3	3.10.3	Patch available	CNA
—	nltk (PyPI)	before 3.10.3	3.10.3	Patch available	GHSA-cw6x-m8jw-qmrh

nltk Vendornltk Affected before 3.10.3 Fixed in 3.10.3 Patch available CNA
nltk (PyPI) Vendor— Affected before 3.10.3 Fixed in 3.10.3 Patch available GHSA-cw6x-m8jw-qmrh

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:nltk:nltk::::::::	before 3.10.3	Vulnerable	nltk

cpe:2.3:a:nltk:nltk:*:*:*:*:*:*:*:* Versions before 3.10.3 Vulnerable ·nltk

Remediation

Fix · CNA nltk (nltk) Fixed in 3.10.3
Fix · GHSA-cw6x-m8jw-qmrh nltk (PyPI) Fixed in 3.10.3 Advisory →

Red Hat 5 advisories

RHSA-2026:87410 Released 2026-10-21 · Red Hat rating Critical
RHOAI 2.25.12 - Red Hat OpenShift AI
- Red Hat OpenShift AI 2.25 registry.redhat.io/rhoai/odh-built-in-detector-rhel9, registry.redhat.io/rhoai/odh-caikit-nlp-rhel9, registry.redhat.io/rhoai/odh-codeflare-operator-rhel9 +47
RHSA-2026:86891 Released 2026-10-21 · Red Hat rating Critical
RHOAI 3.5.2 - Red Hat OpenShift AI
- Red Hat OpenShift AI 3.5 registry.redhat.io/rhoai/odh-automl-rhel9, registry.redhat.io/rhoai/odh-built-in-detector-rhel9, registry.redhat.io/rhoai/odh-cli-rhel9 +47
RHSA-2026:84734 Released 2026-10-19 · Red Hat rating Critical
RHOAI 3.4.5 - Red Hat OpenShift AI
- Red Hat OpenShift AI 3.4 registry.redhat.io/rhoai/odh-ai-gateway-payload-processing-rhel9, registry.redhat.io/rhoai/odh-automl-rhel9, registry.redhat.io/rhoai/odh-autorag-rhel9 +47

Show 2 more Red Hat advisories

RHSA-2026:73987 Released 2026-09-30 · Red Hat rating Critical
RHOAI 3.3.7 - Red Hat OpenShift AI
- Red Hat OpenShift AI 3.3 registry.redhat.io/rhoai/odh-built-in-detector-rhel9, registry.redhat.io/rhoai/odh-dashboard-rhel9, registry.redhat.io/rhoai/odh-data-science-pipelines-argo-argoexec-rhel9 +47
RHSA-2026:69539 Released 2026-09-21 · Red Hat rating Important
RHOAI 3.5.1 - Red Hat OpenShift AI
- Red Hat OpenShift AI 3.5 registry.redhat.io/rhoai/odh-cli-rhel9, registry.redhat.io/rhoai/odh-mlserver-cuda-rhel9, registry.redhat.io/rhoai/odh-mlserver-rhel9 +28

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

Release · 2026-08-27
nltk 3.10.3
5 records Highest priority Priority HIGH 2 HIGH, 2 MEDIUM, 1 LOW
Other records, highest priority first
- TI-2026-56370 nltk PorterStemmer before 3.10.3 Quadratic-time DoSPriority HIGH 77
- TI-2026-56375 NLTK before 3.10.3 Hardlink File Overwrite via downloaderPriority MEDIUM 64
- TI-2026-56371 NLTK before 3.10.3 Quadratic CPU Exhaustion via XMLCorpusViewPriority MEDIUM 61
Show all 5 records →

Vendor assessments

Red Hat

Red Hat rating: Important. Red Hat's page for this CVE →

Product	Status	Plan or fix
Exploit Intelligence	Affected	No fix planned: Will not fix
Lightspeed Core	Affected	No fix yet
OpenShift Lightspeed	Affected	No fix yet
Red Hat Ansible Automation Platform 2	Affected	No fix planned: Will not fix
Red Hat OpenShift AI (RHOAI)	Affected	No fix yet
Red Hat OpenShift AI 2.25	Fixed	RHSA-2026:87410
Red Hat OpenShift AI 3.3	Fixed	RHSA-2026:73987
Red Hat OpenShift AI 3.4	Fixed	RHSA-2026:84734
Red Hat OpenShift AI 3.5	Fixed	RHSA-2026:69539, RHSA-2026:86891

Not affected: 5 products

OpenShift Lightspeed
Red Hat OpenShift AI 2.25
Red Hat OpenShift AI 3.3
Red Hat OpenShift AI 3.4
Red Hat OpenShift AI 3.5

Timeline

2026-10-02 Threatist record created from CVE-2026-81724
2026-10-03 Alias added: GHSA-cw6x-m8jw-qmrh
2026-10-07 Fix available: RHSA-2026:73987 · Red Hat
2026-10-20 Fix available: RHSA-2026:84734 · Red Hat
2026-10-20 Fix available: Red Hat OpenShift AI 3.4 · Red Hat
2026-10-22 Fix available: RHSA-2026:86891 · Red Hat
2026-10-22 Fix available: RHSA-2026:87410 · Red Hat
2026-10-23 Fix available: Red Hat OpenShift AI 2.25 · Red Hat

References

Exploit 1

GitHub Security Advisory (GHSA-cw6x-m8jw-qmrh)

Advisory 1

https://nvd.nist.gov/vuln/detail/CVE-2026-81724

Other 4

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

See all 41 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences