CVE-2026-81722

High

Description

nltk PorterStemmer in versions <= 3.10.2 (fixed in 3.10.3) contains an inefficient-algorithmic-complexity denial of service in PorterStemmer.stem(). The _is_consonant() helper walks backward over the entire run of trailing 'y' characters on every call, and _measure() invokes it for each stem position, causing O(n^2) behavior. A single ~20-50 KB untrusted token consisting of a long run of the letter 'y' followed by a matching suffix (e.g., 'ness') can pin a CPU core for seconds to minutes, causing availability impact.

SeverityHigh

CVSS7.5

Exploit probability—

ProductNltk

PublishedAug 27, 2026

First seenAug 27, 2026

Weakness typeCWE-407CWE-407

How to fix

Update

nltk nltk3.10.3 or later
nltk3.10.3 or later

Vendor advisories

github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g

Affected products

Product	Affected versions	Fixed in
nltk nltk	v < 3.10.3	3.10.3
nltkPyPI	v < 3.10.3	3.10.3

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