Is This vulnerability exploited in the wild?

No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

How severe is This vulnerability?

High severity, with a CVSS score of 7.5, published 27 August 2026.

Is there a fix for This vulnerability?

A patch was published on 2026-08-27: Fixed in version 3.10.3 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is This vulnerability attacked?

It is attacked over the network, in a software library.

Check whether you run nltk (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

NLTK PorterStemmer quadratic-time DoS via long runs of 'y'

Severity: high · CVSS 7.5 · Published 2026-08-27

NLTK is a widely-used Python library for natural language processing, with the PorterStemmer being a common component for text analysis in search, indexing, and machine learning pipelines. A flaw in the stemmer causes it to consume excessive CPU when processing specially crafted text inputs containing many repeated 'y' characters, allowing attackers to disrupt service availability by submitting malicious tokens that lock up the server.

Technologies: nltk (PyPI). Vendors: PyPI.

Quick answers

Is This vulnerability exploited in the wild?: No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
How severe is This vulnerability?: High severity, with a CVSS score of 7.5, published 27 August 2026.
Which products and versions are affected?: nltk (PyPI) <= 3.10.2.
Is there a fix for This vulnerability?: A patch was published on 2026-08-27: Fixed in version 3.10.3 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is This vulnerability attacked?: It is attacked over the network, in a software library.
Am I affected?: Check whether you run nltk (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability is an inefficient algorithmic complexity issue (CWE-407) in the PorterStemmer.stem() method, specifically in the _is_consonant() helper function. The _is_consonant() function iteratively walks backward over the entire run of trailing 'y' characters on every invocation, and _measure() calls _is_consonant() for each stem position, resulting in O(n²) behavior. An attacker can craft a 20–50 KB untrusted token consisting of a long run of 'y' characters followed by a suffix like 'ness' to trigger _measure() on the long stem, pinning a CPU core for seconds to minutes. The fix involves memoizing character consonant/vowel status in a single left-to-right O(n) pass rather than re-walking the run on each call. This is a network-reachable vulnerability requiring no authentication or user interaction, as PorterStemmer.stem() is a public API routinely applied to untrusted text. Patched in version 3.10.3.

Affected products

NLTK NLTK <= 3.10.2

Timeline

2026-08-27: disclosed: Advisory published by GitHub (GHSA-ww6m-cw3f-q94g)
2026-08-27: patched: Fixed in version 3.10.3
2026-09-02: other: Duplicate advisory GHSA-8x48-8g7j-rqxp withdrawn

References

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