CVE-2026-126298 NLTK before 3.10.4 Quadratic CPU Denial of Service via Tree.chomsky_normal_form

8.7 highEPSS 0.6%Fix available

High (8.7) in nltk nltk. EPSS 0.6% estimated probability of exploitation activity in the next 30 days. Fixed in nltk 3.10.4.

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Severity
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Affected
Versions
Fix and patched versions
Packages (OSV)
Attack patterns (CAPEC)
References
Your exposure

NLTK before 3.10.4 contains a denial of service vulnerability in Tree.chomsky_normal_form that allows remote attackers to exhaust CPU through quadratic-time right-factor binarization using list.pop(0). Attackers can supply a wide flat treebank tree parsed by Tree.fromstring to force tens of seconds of single-threaded CPU consumption, blocking the processing thread.

Severity

CVSS 4.0

8.7 high

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

CVSS 3.1

7.5 high

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS: 0.6% estimated probability of exploitation activity in the next 30 days; 48th percentile. Scored 2026-11-13. A forecast for this CVE across the internet, not for your own systems.
CISA's assessment (SSVC): Exploitation: none · Automatable: yes · Technical impact: partial, assessed 2026-11-13. From CISA's Vulnrichment in the CVE List: whether it is exploited, whether exploiting it can be automated, and how much control it gives.
CWE: CWE-400 Uncontrolled Resource Consumption
Published: 2026-11-13 10:17 UTC
Modified: 2026-11-13 23:17 UTC
Assigned by: VulnCheck (CNA)
NVD analysis: Awaiting analysis
Record from: the NVD, synced 2026-11-13 23:44 UTC

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Affected

nltk:nltk

Versions

From the CNA's record in the CVE List.

Product	Affected
nltk	before 3.10.4

Fix and patched versions

nltk (PyPI): fixed in 3.10.4

Packages (OSV)

Advisory	Ecosystem	Package	Affected	Fixed in
GHSA-r53h-rw34-8h97	PyPI	nltk	<3.10.4	3.10.4

Attack patterns (CAPEC)

CAPEC-147 XML Ping of the Death Likelihood: Low · Severity: Medium
CAPEC-227 Sustained Client Engagement
CAPEC-492 Regular Expression Exponential Blowup

The attack patterns MITRE's CAPEC relates to this record's weaknesses, as MITRE names them. Credits.

References

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