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No exploitation reportedFix available

CVE-2021-43854

No exploitation reported by the sources we read.

  • Published2021-12-23
  • Fixed in3.6.6 · 3.6.5
  • WeaknessCWE-400

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2021-12-2318:15CVE publishedNVD

Versions per branch

OSV · NVD
Nltk
BranchAffectedFixed in
3.6< 3.6.63.6.6
3.6< 3.6.53.6.5

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • nltkUbuntu:Pro:20.04:LTSall versions · fixed in 3.4.5-2ubuntu0.1~esm2
  • nltkUbuntu:Pro:18.04:LTSall versions · fixed in 3.2.5-1ubuntu0.1+esm2
  • nltkUbuntu:Pro:16.04:LTSall versions · fixed in 3.1-1ubuntu0.1+esm2
  • nltkUbuntu:Pro:14.04:LTSall versions · fixed in 2.0~b9-0ubuntu4.1~esm4
  • nltkDebian:14all versions · fixed in 3.6.7-1
  • nltkDebian:13all versions · fixed in 3.6.7-1
  • nltkDebian:12all versions · fixed in 3.6.7-1

Full record on NVD

Fix

  1. Update Nltk to 3.6.6 · 3.6.5

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof of concept
Metasploit modules
none listed
Metasploit Framework

Sources

  • CNACVSS 3.17.5known since 2026-10-23
  • NVDCVSS 2.05read 2026-10-23
  • CNAweakness (CWE)CWE-400known since 2026-10-23
  • FIRST EPSSEPSS score2.8 %2026-11-08
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryHIGH2022-01-05

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2021-12-23
Last Modified
2026-06-17
NVD status
Modified
GitHub Advisory
GHSA-f8m6-h2c7-8h9x

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.

Is CVE-2021-43854 actively exploited?

Active exploitation of CVE-2021-43854 has not been confirmed. Its EPSS score was 2.8% on 2026-11-08, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-43854?

CVE-2021-43854 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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