#VU105952 Inefficient regular expression complexity in Natural Language Toolkit - CVE-2021-43854

Vulnerability identifier: #VU105952

Vulnerability risk: Medium

CVSSv4.0: 6.6 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/U:Green]

CVE-ID: CVE-2021-43854

CWE-ID: CWE-1333

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Natural Language Toolkit
Web applications / Modules and components for CMS

Vendor: nltk.org

Description

The vulnerability allows a remote attacker to perform a denial of service (DoS) attack.

The vulnerability exists due to insufficient input validation when processing untrusted input with a regular expressions. A remote attacker can pass specially crafted data to the application and perform regular expression denial of service (ReDos) attack.

Mitigation
Install update from vendor's website.

Vulnerable software versions

Natural Language Toolkit: 2.0.1 - 3.6.5

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

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.