NLTK Vulnerable to REDoS

HIGH 7.5CVE-2021-3828 · GHSA-2ww3-fxvq-293j · PYSEC-2021-356 · Published 27/09/2021 · updated 17/06/2026

In short

CVE-2021-3828 is a HIGH (CVSS 7.5) vulnerability affecting nltk, nltk (nltk).
Chance of exploitation in the next 30 days (EPSS): 1.6%, higher than 75.7% of all vulnerabilities.
Fix: update to 3.6.4 or later.

Package	Affected versions	Patched versions
nltk PyPI	`< 3.6.4`	`3.6.4`
nltk nltk	`≤ 3.6.3`	No fix yet

The nltk package is vulnerable to ReDoS (regular expression denial of service). An attacker that is able to provide as an input to the [_read_comparison_block()(https://github.com/nltk/nltk/blob/23f4b1c4b4006b0cb3ec278e801029557cec4e82/nltk/corpus/reader/comparative_sents.py#L259) function in the file nltk/corpus/reader/comparative_sents.py may cause an application to consume an excessive amount of CPU.

NVD description

nltk is vulnerable to Inefficient Regular Expression Complexity

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Package

Affected versions

Patched versions

PyPI

< 3.6.4

3.6.4

≤ 3.6.3

No fix yet

Description

NVD description

nltk is vulnerable to Inefficient Regular Expression Complexity

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