What is CVE-2026-81722?

CVE-2026-81722 is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.

Which versions of nltk are affected by CVE-2026-81722?

nltk (pip) versions <= 3.10.2 is affected.

Is there a fix for CVE-2026-81722?

Yes. CVE-2026-81722 is fixed in 3.10.3. Upgrade to this version or later.

Is CVE-2026-81722 exploitable, and should I be worried?

Whether CVE-2026-81722 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-81722 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

CVE-2026-81722 in nltk

Q: Which versions of nltk are affected by CVE-2026-81722?

nltk (pip) versions <= 3.10.2 is affected.

Q: Is there a fix for CVE-2026-81722?

Yes. CVE-2026-81722 is fixed in 3.10.3. Upgrade to this version or later.

Q: Is CVE-2026-81722 exploitable, and should I be worried?

Whether CVE-2026-81722 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-81722 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-81722?

Upgrade nltk to 3.10.3 or later.

Summary

NLTK: Quadratic-time DoS in PorterStemmer via long runs of 'y'

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

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).

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.

Affected versions

nltk (<= 3.10.2)

Security releases

nltk → 3.10.3 (pip)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade nltk to 3.10.3 or later to resolve this vulnerability.

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

What is CVE-2026-81722? CVE-2026-81722 is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.
Which versions of nltk are affected by CVE-2026-81722? nltk (pip) versions <= 3.10.2 is affected.
Is there a fix for CVE-2026-81722? Yes. CVE-2026-81722 is fixed in 3.10.3. Upgrade to this version or later.
Is CVE-2026-81722 exploitable, and should I be worried? Whether CVE-2026-81722 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-81722 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-81722? Upgrade nltk to 3.10.3 or later.