CVE-2026-126301
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Algorithmic Complexity Flaw in NLTK Tokenizer

Publication date: 2026-11-13

Last updated on: 2026-11-13

Assigner: VulnCheck

Description
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.

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Meta Information

Published
2026-11-13
Last Modified
2026-11-13
Generated
2026-11-13
AI Q&A
2026-11-13
EPSS Evaluated
N/A
NVD
CVE-2026-126301
EUVD
EUVD-2026-125220

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
nltk nltk to 3.10.4 (exc)
nltk nltk to 3.10.3 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-407 An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.

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Executive Summary

CVE-2026-126301 is an algorithmic complexity vulnerability in NLTK versions 3.6 to 3.10.3 affecting the LegalitySyllableTokenizer.tokenize function. It allows remote attackers to cause CPU exhaustion by submitting long tokens composed entirely of vowels. The unguarded onset reversal loop processes these tokens with quadratic O(n^2) complexity, leading to denial of service for syllabification components.

Detection Guidance

Monitor CPU usage spikes when processing text inputs, particularly with long vowel-only tokens. Check NLTK logs for slow tokenization tasks. Use network monitoring to detect unusual traffic patterns targeting text processing endpoints.

Impact Analysis

This vulnerability can impact you by causing denial of service or increased latency in services processing untrusted text. Attackers can send crafted tokens to saturate CPU cores for extended periods. Even moderately sized tokens (e.g., 64,000 characters) can cause significant performance degradation. Memory usage remains linear but CPU consumption becomes quadratic.

Mitigation Strategies

Upgrade NLTK to version 3.10.4 or later if available. Implement input validation to reject tokens longer than 4096 characters. Apply rate limiting to text processing endpoints. Monitor CPU usage and set alerts for abnormal spikes.

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