CVE-2021-43854Public exploitNetwork attack vector

Application hang via quadratic sentence-boundary matching

Published Dec 23, 2021 · Updated Aug 4, 2024

Regular expression denial of service in NLTK before 3.6.5 allows remote attackers to hang applications via crafted tokenizer input. PunktSentenceTokenizer's period-context expression greedily matches non-whitespace text, then repeatedly backtracks while searching for a sentence-ending character. Exposure requires an application to pass unpredictable input to PunktSentenceTokenizer, sent_tokenize, or word_tokenize; processing can occupy the application for hours.

CVSS severity7.5
High
EPSS probability2.67%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in NLTK before 3.6.5 allows remote attackers to hang applications via crafted tokenizer input. PunktSentenceTokenizer's period-context expression greedily matches non-whitespace text, then repeatedly backtracks while searching for a sentence-ending character. Exposure requires an application to pass unpredictable input to PunktSentenceTokenizer, sent_tokenize, or word_tokenize; processing can occupy the application for hours.

The record

CVE
CVE-2021-43854
Published
Dec 23, 2021
Updated
Aug 4, 2024
Vendor
NLTK Project
Product
NLTK
Classifications
CWE-400, CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 23, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 3.6.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

NLTK Project · NLTKversion=< 3.6.5

Published CVSS scores

7.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
CWE-1333Inefficient Regular Expression Complexity
T1499.004Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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