Multiple vulnerabilities in Natural Language Toolkit (nltk)

Published: 2021-12-23 | Updated: 2025-03-24

Risk	Medium
Patch available	YES
Number of vulnerabilities	2
CVE-ID	CVE-2021-43854 CVE-2021-3842
CWE-ID	CWE-1333
Exploitation vector	Network
Public exploit	N/A
Vulnerable software	Natural Language Toolkit Web applications / Modules and components for CMS
Vendor	nltk.org

Security Bulletin

This security bulletin contains information about 2 vulnerabilities.

1) Inefficient regular expression complexity

EUVDB-ID: #VU105952

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 - Inefficient Regular Expression Complexity

Exploit availability: No

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

CPE2.3

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

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.

2) Inefficient regular expression complexity

EUVDB-ID: #VU105951