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Public proof of conceptFix availableAwaiting NVD analysis

CVE-2026-126303

NLTK before 3.10.4 Denial of Service via SteppingRecursiveDescentParser.parse()

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 an uncontrolled resource consumption vulnerability in SteppingRecursiveDescentParser.parse() that lacks any max_time deadline or step limit. Attackers influencing the grammar can supply an ambiguous self-embedding grammar and short token input to force unbounded top-down search, pinning CPU cores and exhausting worker pools.

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 AdvisoriesadvisoryHIGH2026-11-13

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-4hvg-fv5c-9936 GHSA-8fx7-8jr8-84rv

Frequently asked questions

What is CVE-2026-126303?

NLTK before 3.10.4 contains an uncontrolled resource consumption vulnerability in SteppingRecursiveDescentParser.parse() that lacks any max_time deadline or step limit. Attackers influencing the grammar can supply an ambiguous self-embedding grammar and short token input to force unbounded top-down search, pinning CPU cores and exhausting worker pools.

Is CVE-2026-126303 actively exploited?

A proof-of-concept exploit exists for CVE-2026-126303, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-126303?

CVE-2026-126303 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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