---
id: "CVE-2026-126301"
page: "https://infrastack.ai/live/CVE-2026-126301"
kind: "vulnerability"
aliases: ["CVE-2026-126301"]
title: "NLTK: long vowel tokens cause CPU exhaustion in syllable tokenizer"
severity: "medium"
cvss: 6.9
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["nltk", "package", "python"]
not_affected_if: ["tokenizer-unused", "trusted-input-only"]
published_at: "2026-11-13T10:17:35+00:00"
first_seen_at: "2026-11-13T10:47:14+00:00"
updated_at: "2026-11-13T10:54:47+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-126301", "first_seen_at": "2026-11-13T10:47:14+00:00"}]
exploit_sources: []
---

# NLTK: long vowel tokens cause CPU exhaustion in syllable tokenizer

Link for people: https://infrastack.ai/live/CVE-2026-126301 (this .md is for agents)

NLTK's LegalitySyllableTokenizer.tokenize runs a quadratic onset reversal loop on long all-vowel tokens. A remote attacker who supplies such a token can exhaust CPU and deny service to syllabification code.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed NLTK version with `pip show nltk`
- Look for calls to `LegalitySyllableTokenizer` in your code that take untrusted input

## Fix

- NLTK: move to `3.10.4` or later (from the product range: fixed before 3.10.4)

## Timeline

- 2026-11-13T10:47:14+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-126301

## Documents

- [advisory] Vendor advisory: github.com https://github.com/nltk/nltk/security/advisories/GHSA-gpwc-27cw-rh9r
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/nltk-from-3.6-before-3.10.4-quadratic-cpu-exhaustion-via-legalitysyllabletokenizer-tokenize

## From the sources

NLTK from 3.6 before 3.10.4 contains an algorithmic complexity vulnerability in LegalitySyllableTokenizer.tokenize that allows remote attackers to exhaust CPU by supplying long all-vowel tokens. Attackers can submit a token such as a long run of vowels, causing the unguarded onset reversal loop to perform quadratic work and deny service to syllabification components.

---

_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_