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.

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - 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. (CVE-2021-43854)

The remote Ubuntu 14.04 LTS / 16.04 LTS / 18.04 LTS / 20.04 LTS host has packages installed that are affected by multiple vulnerabilities as referenced in the USN-7365-1 advisory.

    It was discovered that NLTK contained a regex that is susceptible to catastrophic backtracking. An     attacker could possibly use this issue to cause a denial of service. (CVE-2021-3842, CVE-2021-43854)

Tenable has extracted the preceding description block directly from the Ubuntu security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - 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. (CVE-2021-43854)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Type	Severity
431710	SCA: security update for nltk (GHSA-f8m6-h2c7-8h9x)	Tenable Cloud Security	SCA Checks	Local	high
431710	SCA: security update for nltk (GHSA-f8m6-h2c7-8h9x)	Tenable Self-Hosted Container Security	SCA Checks	Local	high
233286	Ubuntu 14.04 LTS / 16.04 LTS / 18.04 LTS / 20.04 LTS : NLTK vulnerabilities (USN-7365-1)	Nessus	Ubuntu Local Security Checks	Local	high
224202	Linux Distros Unpatched Vulnerability : CVE-2021-43854	Nessus	Misc.	Local	high

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CVE-2021-43854

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