Resource Exhaustion Affecting nltk package, versions <3.6.7-1


Severity

Recommended
0.0
high
0
10

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Threat Intelligence

EPSS
2.67% (84th percentile)

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  • Snyk IDSNYK-ECHOLATEST-NLTK-18321528
  • published27 Jul 2026
  • disclosed23 Dec 2021

Introduced: 23 Dec 2021

CVE-2021-43854  (opens in a new tab)
CWE-400  (opens in a new tab)

How to fix?

Upgrade Echo:latest nltk to version 3.6.7-1 or higher.

NVD Description

Note: Versions mentioned in the description apply only to the upstream nltk package and not the nltk package as distributed by Echo. See How to fix? for Echo:latest relevant fixed versions and status.

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. Versions prior to 3.6.5 are vulnerable to regular expression denial of service (ReDoS) attacks. The vulnerability is present in PunktSentenceTokenizer, sent_tokenize and word_tokenize. Any users of this class, or these two functions, are vulnerable to the ReDoS attack. In short, a specifically crafted long input to any of these vulnerable functions will cause them to take a significant amount of execution time. If your program relies on any of the vulnerable functions for tokenizing unpredictable user input, then we would strongly recommend upgrading to a version of NLTK without the vulnerability. For users unable to upgrade the execution time can be bounded by limiting the maximum length of an input to any of the vulnerable functions. Our recommendation is to implement such a limit.

CVSS Base Scores

version 3.1