CVE-2026-126297
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NLTK ConllCorpusReader Quadratic-Time DoS via Span Rescans

Publication date: 2026-11-13

Last updated on: 2026-11-13

Assigner: VulnCheck

Description
NLTK before 3.10.4 contains a quadratic-time denial of service vulnerability in ConllCorpusReader._get_srl_instances that allows remote attackers to exhaust CPU through nested span rescans. Attackers can supply a crafted CoNLL SRL corpus with many predicates and span columns, placing the head span last, to pin a core for minutes.

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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-2026-126297
EUVD
EUVD-2026-125224

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
nltk nltk to 3.10.4 (exc)

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-400 The product does not properly control the allocation and maintenance of a limited resource.

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Executive Summary

This is a high-severity CPU denial-of-service vulnerability in NLTK versions before 3.10.4. It affects the ConllCorpusReader._get_srl_instances method, which has quadratic-time complexity. Attackers can exploit this by submitting a crafted CoNLL SRL corpus with many predicates and span columns, placing the head span last. This forces the system to repeatedly rescan spans, exhausting CPU resources and pinning a core for extended periods.

Detection Guidance

Detecting this vulnerability requires checking the installed NLTK version. Run: pip show nltk. If the version is below 3.10.4, the system is vulnerable. Monitor CPU usage during corpus processing, especially with CoNLL SRL data, for sustained high utilization.

Impact Analysis

This vulnerability can cause severe performance degradation or complete unavailability of services relying on NLTK for processing CoNLL-formatted data. A single crafted request could consume excessive CPU, leading to slow response times or system crashes. It requires no authentication or user interaction, making it exploitable remotely.

Mitigation Strategies

Upgrade NLTK to version 3.10.4 or later immediately. Use pip install --upgrade nltk. Avoid processing untrusted CoNLL SRL corpora until patched. Implement input validation for corpus files to limit predicates and span columns.

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