CVE-2021-43854

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    CVE-2021-43854

    Published: 23 Dec 2021Last Modified: 9 Jul 2026

    Summary:

    Details: 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.

    References: https://github.com/nltk/nltk/commit/1405aad979c6b8080dbbc8e0858f89b2e3690341, https://github.com/nltk/nltk/issues/2866, https://github.com/nltk/nltk/pull/2869, https://github.com/nltk/nltk/security/advisories/GHSA-f8m6-h2c7-8h9x

    Affected packages

    Package

    Name:

    Purl:

    Affected ranges

    Type: GIT

    Events:

    Introduced- 0
    Fixed -b42236425456418b64803f099f6ca1fc3d24031f

    Affected versions

    3.6.5
    3.6.2
    3.6.1
    3.6

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    CVE-2021-43854 | CVE-DB