Vulnerability-Lookup

Name	purl
nltk	pkg:pypi/nltk

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

Vulnerability from osv_homebrew – Published: 2026-09-02 08:43 – Updated: 2026-09-17 18:47 – Source website

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

URL

Type

	https://github.com/nltk/nltk/security/advisories/…	WEB
	https://nvd.nist.gov/vuln/detail/CVE-2026-81722	ADVISORY
	https://github.com/nltk/nltk/commit/7808692d451b9…	WEB
	https://github.com/nltk/nltk	PACKAGE
	https://github.com/nltk/nltk/releases/tag/v3.10.3	WEB
	https://github.com/pypa/advisory-database/tree/ma…	WEB
	https://www.vulncheck.com/advisories/nltk-porters…	WEB

JSON

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": "acronym",
        "purl": "pkg:brew/acronym"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.0"
            },
            {
              "fixed": "2.0.0_5"
            }
          ],
          "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-acronym-CVE-2026-81722",
  "modified": "2026-09-17T18:47:55Z",
  "published": "2026-09-02T08:43:54Z",
  "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"
  ]
}

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

Vulnerability from osv_homebrew – Published: 2026-09-02 09:04 – Updated: 2026-09-17 19:32 – Source website

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

URL

Type

	https://github.com/nltk/nltk/security/advisories/…	WEB
	https://nvd.nist.gov/vuln/detail/CVE-2026-81722	ADVISORY
	https://github.com/nltk/nltk/commit/7808692d451b9…	WEB
	https://github.com/nltk/nltk	PACKAGE
	https://github.com/nltk/nltk/releases/tag/v3.10.3	WEB
	https://github.com/pypa/advisory-database/tree/ma…	WEB
	https://www.vulncheck.com/advisories/nltk-porters…	WEB

JSON

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": "gptline",
        "purl": "pkg:brew/gptline"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.8"
            },
            {
              "fixed": "1.0.8_23"
            }
          ],
          "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-gptline-CVE-2026-81722",
  "modified": "2026-09-17T19:32:01Z",
  "published": "2026-09-02T09:04:32Z",
  "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"
  ]
}

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

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

URL

Type

	https://github.com/nltk/nltk/security/advisories/…	WEB
	https://nvd.nist.gov/vuln/detail/CVE-2026-81722	ADVISORY
	https://github.com/nltk/nltk/commit/7808692d451b9…	WEB
	https://github.com/nltk/nltk	PACKAGE
	https://github.com/nltk/nltk/releases/tag/v3.10.3	WEB
	https://github.com/pypa/advisory-database/tree/ma…	WEB
	https://www.vulncheck.com/advisories/nltk-porters…	WEB

JSON

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"
  ]
}

CVE-2026-81722 (GCVE-0-2026-81722)

Vulnerability from cvelistv5 – Published: 2026-08-27 14:51 – Updated: 2026-08-27 17:49

Title

nltk PorterStemmer before 3.10.3 Quadratic-time DoS

Summary

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.

Severity

8.7 (High)


                        
                          CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

7.5 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-27 17:49 UTC

CWE

CWE-407 - Inefficient Algorithmic Complexity

Assigner

VulnCheck mitre root

References

2 references

URL	Tags
https://github.com/nltk/nltk/security/advisories/…	vendor-advisory
https://www.vulncheck.com/advisories/nltk-porters…	third-party-advisory

Impacted products

1 product

Vendor	Product	Version
nltk	nltk	Affected: 0 , < 3.10.3 (semver) Unaffected: 3.10.3 (semver) cpe:2.3:a:nltk:nltk::::::::

Date Public

2026-08-12 00:00

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-81722",
                "options": [
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                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-08-27T17:49:47.104860Z",
                "version": "2.0.3"
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            "tags": [
              "exploit"
            ],
            "url": "https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
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          "defaultStatus": "unaffected",
          "packageURL": "pkg:pypi/nltk",
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          "versions": [
            {
              "lessThan": "3.10.3",
              "status": "affected",
              "version": "0",
              "versionType": "semver"
            },
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              "status": "unaffected",
              "version": "3.10.3",
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      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
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                  "criteria": "cpe:2.3:a:nltk:nltk:*:*:*:*:*:*:*:*",
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      ],
      "datePublic": "2026-08-12T00:00:00.000Z",
      "descriptions": [
        {
          "lang": "en",
          "value": "nltk PorterStemmer in versions \u003c= 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 \u0027y\u0027 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 \u0027y\u0027 followed by a matching suffix (e.g., \u0027ness\u0027) can pin a CPU core for seconds to minutes, causing availability impact."
        }
      ],
      "metrics": [
        {
          "cvssV4_0": {
            "Automatable": "NOT_DEFINED",
            "Recovery": "NOT_DEFINED",
            "Safety": "NOT_DEFINED",
            "attackComplexity": "LOW",
            "attackRequirements": "NONE",
            "attackVector": "NETWORK",
            "baseScore": 8.7,
            "baseSeverity": "HIGH",
            "exploitMaturity": "NOT_DEFINED",
            "privilegesRequired": "NONE",
            "providerUrgency": "NOT_DEFINED",
            "subAvailabilityImpact": "NONE",
            "subConfidentialityImpact": "NONE",
            "subIntegrityImpact": "NONE",
            "userInteraction": "NONE",
            "valueDensity": "NOT_DEFINED",
            "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
            "version": "4.0",
            "vulnAvailabilityImpact": "HIGH",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "NONE",
            "vulnerabilityResponseEffort": "NOT_DEFINED"
          },
          "format": "CVSS"
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            "attackComplexity": "LOW",
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            "confidentialityImpact": "NONE",
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            "privilegesRequired": "NONE",
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            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
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        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-27T14:51:15.857Z",
        "orgId": "83251b91-4cc7-4094-a5c7-464a1b83ea10",
        "shortName": "VulnCheck"
      },
      "references": [
        {
          "name": "GitHub Security Advisory (GHSA-ww6m-cw3f-q94g)",
          "tags": [
            "vendor-advisory"
          ],
          "url": "https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g"
        },
        {
          "name": "VulnCheck Advisory: nltk PorterStemmer before 3.10.3 Quadratic-time DoS",
          "tags": [
            "third-party-advisory"
          ],
          "url": "https://www.vulncheck.com/advisories/nltk-porterstemmer-before-3.10.3-quadratic-time-dos"
        }
      ],
      "title": "nltk PorterStemmer before 3.10.3 Quadratic-time DoS",
      "x_generator": {
        "engine": "vulncheck-endgame"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "83251b91-4cc7-4094-a5c7-464a1b83ea10",
    "assignerShortName": "VulnCheck",
    "cveId": "CVE-2026-81722",
    "datePublished": "2026-08-27T14:51:15.857Z",
    "dateReserved": "2026-08-27T11:13:14.064Z",
    "dateUpdated": "2026-08-27T17:49:54.623Z",
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}

GHSA-WW6M-CW3F-Q94G

Vulnerability from github – Published: 2026-09-02 14:36 – Updated: 2026-09-02 14:36

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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.10.2"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "nltk"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.10.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-81722"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-02T14:36:15Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "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": "GHSA-ww6m-cw3f-q94g",
  "modified": "2026-09-02T14:36:15Z",
  "published": "2026-09-02T14:36:15Z",
  "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": "1.4.0",
  "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"
}

PYSEC-2026-3738

Tags

Sightings

Nomenclature

Related by attack behaviour

Action not permitted

PYSEC-2026-3738

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

Root cause

Proof of concept

Impact

Fix direction

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

Root cause

Proof of concept

Impact

Fix direction

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

Root cause

Proof of concept

Impact

Fix direction

CVE-2026-81722 (GCVE-0-2026-81722)

GHSA-WW6M-CW3F-Q94G

Root cause

Proof of concept

Impact

Fix direction

Tags

Sightings

Nomenclature

Related by attack behaviour