How severe is CVE-2026-81722?

No CVSS score has been assigned to CVE-2026-81722 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-81722?

CVE-2026-81722 affects the following packages: nltk (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-81722?

Update nltk to 3.10.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-81722 is resolved across your whole dependency graph.

How do I detect CVE-2026-81722 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nltk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-81722 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-81722 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-81722 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-81722?

CVE-2026-81722 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 43th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-81722?

CVE-2026-81722 is classified as CWE-407 (CWE-407). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-81722 published, and has it been updated?

CVE-2026-81722 was published on August 27, 2026 and was last updated on September 4, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-81722 — nltk

Fix: nltk/nltk@7808692

CVE-2026-81722 is a CWE-407 vulnerability in nltk. A fix is available for nltk — see the affected versions and patch details below.

nltk PorterStemmer before 3.10.3 Quadratic-time DoS

Also known asGHSA-ww6m-cw3f-q94gPYSEC-2026-3738

Published

Aug 27, 2026

Updated

Sep 4, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-81722.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs43th percentile — riskier than 43% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐍nltk

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

nltk.stem.PorterStemmer.stem() -- a ubiquitous public API applied to arbitrary, often untrusted, tokens -- runs in O(n^2) time on a token containing a long run of the letter 'y', letting a single ~20-50 KB token pin a CPU core (CWE-407).

Root cause

_is_consonant(word, i) was made iterative (commit for #3633, GHSA/CWE-674) to fix an earlier unbounded-recursion RecursionError on 'y'*10000. The iterative form walks backward over the whole run of 'y's on every call:

while i > 0 and word[i] == 'y':
    negate = not negate
    i -= 1

_measure() then calls _is_consonant(stem, i) once for every position i of the stem. For a run of n 'y's that is sum_{i} O(i) = O(n^2). The recursion fix therefore traded a CWE-674 RecursionError for a CWE-407 quadratic-time DoS.

Proof of concept

Measured (Python 3.13): stem('y'*5000 + 'ness') = 2.6s, stem('y'*10000 + 'ness') = 11.3s (2x input -> ~4.3x time = quadratic), stem('y'*20000 + 'ness') > 20s. A pure run of 'y' with no matching suffix is fast because the stemmer rules that call _measure do not fire; a real suffix such as 'ness' triggers _measure on the long stem.

from nltk.stem import PorterStemmer
PorterStemmer().stem('y' * 20000 + 'ness')   # >20s of CPU

Impact

Stemming is routinely applied to untrusted text (search, indexing, NLP pipelines). A single unbroken ~20-50 KB token of 'y' characters (no whitespace, so it survives tokenization) causes multi-second-to-minutes CPU consumption per request. No confidentiality/integrity impact; single-process availability only.

Fix direction

Classify each character's consonant/vowel status in a single left-to-right O(n) pass (memoise the 'y' run parity) instead of re-walking the run on every _is_consonant call, so _measure and stemming are linear. This is a sibling of the corpus-reader quadratic advisories GHSA-vp2x-qp44-57v7 and GHSA-8mpw-7fpc-4gqj (CWE-407).

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`nltk`	all versions	3.10.3`pip install --upgrade 'nltk==3.10.3'`

Affected Products

1 product · 1 configurations

Application

nltknltk

< 3.10.3

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nltk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nltk to 3.10.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-81722 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

This is denial of service vulnerability in the `nltk PorterStemmer` component, which is utilized across various Red Hat products. Processing a specially crafted, untrusted token can lead to excessive CPU consumption due to inefficient algorithmic complexity, potentially causing a denial of service for applications…

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Source: Red Hat security advisory for CVE-2026-81722 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat OpenShift AI 3.3	`rhoai/odh-llama-stack-core-rhel9:1789121286`	RHSA-2026:73987
Red Hat OpenShift AI 3.5	`rhoai/odh-ta-lmes-job-rhel9:1788935863`	RHSA-2026:69539

Frequently Asked Questions

`nltk.stem.PorterStemmer.stem()` -- a ubiquitous public API applied to arbitrary, often untrusted, tokens -- runs in O(n^2) time on a token containing a long run of the letter 'y', letting a single ~20-50 KB token pin a CPU core (CWE-407). ## Root cause `_is_consonant(word, i)` was made *iterative* (commit for #3633, GHSA/CWE-674) to fix an earlier unbounded-recursion `RecursionError` on `'y'*10000`. The iterative form walks *backward* over the whole run of 'y's on every call: ```python while i > 0 and word[i] == 'y': negate = not negate i -= 1 ``` `_measure()` then calls `_is_cons

O3 Security · Impact-Aware SCA

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