CVE-2026-126299 NLTK before 3.10.4 Quadratic-Time Denial of Service via CCG Lexicon fromstring

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 the CCG lexicon parser fromstring that allows remote attackers to exhaust CPU by supplying wide inputs. Attackers can submit lexicon text with long flat bracket bodies or operator chains, triggering quadratic string re-slicing in matchBrackets and augParseCategory that blocks worker threads.

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: PoC · 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 21:17 UTC
Assigned by: VulnCheck (CNA)
NVD analysis: Awaiting analysis
Record from: the NVD, synced 2026-11-13 21:52 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-89p3-fcch-88ph	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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