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BREW-SAFETY-CVE-2026-81722 (PYSEC-2026-3738)

Vulnerability from osv_homebrew – Published: 2026-09-02 09:53 – Updated: 2026-09-17 17:35 – Source website
VLAI
Summary
NLTK: Quadratic-time DoS in PorterStemmer via long runs of 'y'
Details

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).

Severity
6.9 (Medium)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "nltk",
        "resource_purl": "pkg:pypi/nltk@3.10.3",
        "upstream_fixed_in": "3.10.3"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "safety",
        "purl": "pkg:brew/safety"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.3.1"
            },
            {
              "fixed": "3.8.1_2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/nltk@3.10.3",
        "name": "nltk",
        "resource": "nltk",
        "strategy": "registry",
        "subject_version": "3.10.3"
      }
    ]
  },
  "details": "`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 \u0027y\u0027, letting a single ~20-50 KB token pin a CPU core (CWE-407).\n\n## Root cause\n\n`_is_consonant(word, i)` was made *iterative* (commit for #3633, GHSA/CWE-674) to fix an earlier unbounded-recursion `RecursionError` on `\u0027y\u0027*10000`. The iterative form walks *backward* over the whole run of \u0027y\u0027s on every call:\n\n```python\nwhile i \u003e 0 and word[i] == \u0027y\u0027:\n    negate = not negate\n    i -= 1\n```\n\n`_measure()` then calls `_is_consonant(stem, i)` once for **every** position `i` of the stem. For a run of n \u0027y\u0027s 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.\n\n## Proof of concept\n\nMeasured (Python 3.13): `stem(\u0027y\u0027*5000 + \u0027ness\u0027)` = 2.6s, `stem(\u0027y\u0027*10000 + \u0027ness\u0027)` = 11.3s (2x input -\u003e ~4.3x time = quadratic), `stem(\u0027y\u0027*20000 + \u0027ness\u0027)` \u003e 20s. A pure run of \u0027y\u0027 with no matching suffix is fast because the stemmer rules that call `_measure` do not fire; a real suffix such as \u0027ness\u0027 triggers `_measure` on the long stem.\n\n```python\nfrom nltk.stem import PorterStemmer\nPorterStemmer().stem(\u0027y\u0027 * 20000 + \u0027ness\u0027)   # \u003e20s of CPU\n```\n\n## Impact\n\nStemming is routinely applied to untrusted text (search, indexing, NLP pipelines). A single unbroken ~20-50 KB token of \u0027y\u0027 characters (no whitespace, so it survives tokenization) causes multi-second-to-minutes CPU consumption per request. No confidentiality/integrity impact; single-process availability only.\n\n## Fix direction\n\nClassify each character\u0027s consonant/vowel status in a single left-to-right O(n) pass (memoise the \u0027y\u0027 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).",
  "id": "BREW-safety-CVE-2026-81722",
  "modified": "2026-09-17T17:35:56Z",
  "published": "2026-09-02T09:53:14Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81722"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/commit/7808692d451b962711005d954859bb83aabcf8fa"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/nltk/nltk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3738.yaml"
    },
    {
      "type": "WEB",
      "url": "https://www.vulncheck.com/advisories/nltk-porterstemmer-before-3.10.3-quadratic-time-dos"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "NLTK: Quadratic-time DoS in PorterStemmer via long runs of \u0027y\u0027",
  "upstream": [
    "PYSEC-2026-3738",
    "CVE-2026-81722",
    "GHSA-ww6m-cw3f-q94g"
  ]
}



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