Nltk
CVE-2021-43854
HIGH
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A06:2025 - Insecure Design
ASVS 5.0
V2 - Validation and Business Logic Resource consumption limits
MITRE ATT&CK
1 technique across 1 tactic: Impact The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. |
| Impact | CVSS score of 7.5 (HIGH). Attack characteristics: network attack vector, low complexity, no privileges required, no user interaction not required. Public proof-of-concept exploit code is available. |
| Exploit ScenarioDerived | A remote attacker could cause denial of service by exhausting system resources by sending crafted requests to the affected application. No authentication is required to exploit this vulnerability. |
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Alert meAnalysisAI
NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. Rated high severity (CVSS 7.5), this vulnerability is remotely exploitable, no authentication required, low attack complexity. Public exploit code available.
RemediationAI
A vendor patch is available. Apply the latest security update as soon as possible. Implement rate limiting, set resource quotas, validate input sizes, use timeouts.
Affected Packages
Source: OSV (Open Source Vulnerabilities)
Affects your stack
Affects 1 package in pypi
Checking this CVE against your own stacks and servers is part of the Basic plan and up.
Technical ContextAI · show details
This vulnerability is classified as Uncontrolled Resource Consumption (CWE-400), which allows attackers to cause denial of service by exhausting system resources. 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. Affected products include: Nltk. Version information: prior to 3.6.5.
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Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Primary rating from Vendor (github) · only source for this CVE.
CVSS VectorVendor: github
DescriptionCVE.org
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.
Threat Intelligence Overview
NVD
CVE.org
EPSS
GitHub
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.
Affected Products (CPE)CVE.org
Arbitrary code execution in the Natural Language Toolkit (NLTK) before 3.10.3 allows attackers to bypass the library's p
Arbitrary code execution in NLTK (Natural Language Toolkit) through 3.9.4 allows attackers to run Python code when a vic
Arbitrary code execution in the NLTK (Natural Language Toolkit) Python library affects all versions through its data dow
StreamBackedCorpusView in NLTK before 3.10.0 allows arbitrary local file reads by calling Python's builtins.open() direc
Symlink-based path traversal in NLTK's FramenetCorpusReader before version 3.10.2 permits attackers with write access to
Unbounded recursion in NLTK's JSONTaggedDecoder.decode_obj() (nltk/jsontags.py) allows unauthenticated remote attackers
Path traversal and pickle deserialization protections in NLTK before 3.10.0 are effectively disabled by default - pathse
XML Entity Expansion (Billion Laughs) in NLTK before 3.10.3 allows unauthenticated remote attackers to cause denial of s
Server-side request forgery in NLTK before 3.10.3 allows unauthenticated remote attackers to reach internal HTTP service
Symlink escape in NLTK's CorpusReader.open() allows local attackers to read arbitrary files outside the designated corpu
Path traversal in NLTK's FramenetCorpusReader and NKJPCorpusReader (all versions before 3.10.0) enables unauthenticated
nltk is vulnerable to Inefficient Regular Expression Complexity. Rated high severity (CVSS 7.5), this vulnerability is r
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