CVE-2021-43854: NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Lang...

Description

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

2.67%

83.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-400

Affected Software

Vendor	Product	Versions
Nltk	Nltk	< 3.6.5

References

https://github.com/nltk/nltk/commit/1405aad979c6b8080dbbc8e0858f89b2e3690341Patch, Third Party Advisory
https://github.com/nltk/nltk/issues/2866Exploit, Issue Tracking, Patch, Third Party Advisory
https://github.com/nltk/nltk/pull/2869Exploit, Patch, Third Party Advisory
https://github.com/nltk/nltk/security/advisories/GHSA-f8m6-h2c7-8h9xExploit, Patch, Third Party Advisory
https://github.com/nltk/nltk/commit/1405aad979c6b8080dbbc8e0858f89b2e3690341Patch, Third Party Advisory
https://github.com/nltk/nltk/issues/2866Exploit, Issue Tracking, Patch, Third Party Advisory
https://github.com/nltk/nltk/pull/2869Exploit, Patch, Third Party Advisory
https://github.com/nltk/nltk/security/advisories/GHSA-f8m6-h2c7-8h9xExploit, Patch, Third Party Advisory

Timeline

Published: Dec 23, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-43854?

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.

How severe is CVE-2021-43854?

CVE-2021-43854 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 2.67% probability of exploitation in the next 30 days.

How do I fix CVE-2021-43854?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.