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Public proof of conceptFix availableAwaiting NVD analysis

CVE-2026-126299

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

A public proof of concept exists; no exploitation attested.

  • Published2026-11-13
  • Fixed in3.10.4
  • WeaknessCWE-400

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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.

Timeline

dates in UTC
  1. 2026-11-1309:36CVE publishedCNA

Versions per branch

OSV
nltk
BranchAffectedFixed in
3.10< 3.10.43.10.4

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Products declared by the CNA1 product

As VulnCheck, the CNA that assigned this CVE, declared them in the CVE record; versions as written.

  • nltkother versions: not affected
    Affected
    • < 3.10.4fixed in 3.10.4semver

    Not affected = 3.10.4

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-11-13

Fix

  1. Update nltk to 3.10.4

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • CISA-ADPSSVC decisionpoc2026-11-13
  • CNACVSS 4.08.72026-11-13
  • CNACVSS 3.17.52026-11-13
  • NVDCVSS 3.17.5read 2026-11-13
  • CNAweakness (CWE)CWE-4002026-11-13
  • NVDweakness (CWE)CWE-400read 2026-11-13
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryHIGH2026-11-14

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 4.0 vector
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Published
2026-11-13
Last Modified
2026-11-13
NVD status
Awaiting Analysis
GitHub Advisory
GHSA-89p3-fcch-88ph GHSA-jh6r-h4rx-2j3w

Frequently asked questions

What is CVE-2026-126299?

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.

Is CVE-2026-126299 actively exploited?

A proof-of-concept exploit exists for CVE-2026-126299, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-126299?

CVE-2026-126299 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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