BREW-ACRONYM-CVE-2026-81725 (PYSEC-2026-3752)
Vulnerability from osv_homebrew – Published: 2026-09-03 08:45 – Updated: 2026-09-17 18:47 – Source websiteSummary
Pl196xCorpusReader still parses whole TEI blocks with multiple lazy regexes over attacker-controlled text. A malformed file with many opening tags and no matching closing tags forces repeated rescans and produces quadratic CPU growth in public reader APIs.
Details
- Vulnerability type: Regular-expression denial of service
- Affected component:
nltk.corpus.reader.pl196x.TEICorpusView.read_blockandPl196xCorpusReaderpublic methods - Affected versions: Published
3.9.4and current sourcev3.10.0-rc2both reproduced. - Patched versions: Not yet patched
- Root cause: Lazy
.*?whole-block regexes rescan untrusted XML-like blocks from each opening-tag position.
The parser uses regexes for paragraphs, sentences, and word tags across the whole <text> block. When the attacker supplies many unmatched opening tags, each attempt scans toward the end of the block and fails, then restarts from the next opening tag. There is near four-times runtime growth each time the number of malformed <p> tags doubled, through normal public calls such as words() and tagged_words().
PoC
Preconditions - The application parses attacker-influenced PL196X or TEI-like corpus files through public reader APIs.
Steps
1. Create a corpus file with a valid header followed by a <text> block that contains many opening tags and no matching closing tags.
2. Instantiate Pl196xCorpusReader on that corpus.
3. Call words() or tagged_words() and measure elapsed time as the malformed tag count doubles.
4. Observe near quadratic growth instead of near-linear behavior.
Minimal reproducible excerpt
size=1000 0.014s
size=2000 0.057s
size=4000 0.231s
size=8000 0.927s
Impact
A consumer that accepts attacker-influenced corpus files can be forced into heavy CPU use and parser-thread stalling before the application concludes the input contains no valid content.
Remediation
Replace the whole-block lazy-regex parser with a linear parser or bounded tokenizer, and add regression tests that assert near-linear behavior on malformed inputs with many unmatched tags.
{
"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": "### Summary\n\n`Pl196xCorpusReader` still parses whole TEI blocks with multiple lazy regexes over attacker-controlled text. A malformed file with many opening tags and no matching closing tags forces repeated rescans and produces quadratic CPU growth in public reader APIs.\n\n### Details\n\n- **Vulnerability type:** Regular-expression denial of service\n- **Affected component:** `nltk.corpus.reader.pl196x.TEICorpusView.read_block` and `Pl196xCorpusReader` public methods\n- **Affected versions:** Published `3.9.4` and current source `v3.10.0-rc2` both reproduced.\n- **Patched versions:** Not yet patched\n- **Root cause:** Lazy `.*?` whole-block regexes rescan untrusted XML-like blocks from each opening-tag position.\n\nThe parser uses regexes for paragraphs, sentences, and word tags across the whole `\u003ctext\u003e` block. When the attacker supplies many unmatched opening tags, each attempt scans toward the end of the block and fails, then restarts from the next opening tag. There is near four-times runtime growth each time the number of malformed `\u003cp\u003e` tags doubled, through normal public calls such as `words()` and `tagged_words()`.\n\n### PoC\n\n**Preconditions**\n- The application parses attacker-influenced PL196X or TEI-like corpus files through public reader APIs.\n\n**Steps**\n1. Create a corpus file with a valid header followed by a `\u003ctext\u003e` block that contains many opening tags and no matching closing tags.\n2. Instantiate `Pl196xCorpusReader` on that corpus.\n3. Call `words()` or `tagged_words()` and measure elapsed time as the malformed tag count doubles.\n4. Observe near quadratic growth instead of near-linear behavior.\n\n**Minimal reproducible excerpt**\n\n```text\nsize=1000 0.014s\nsize=2000 0.057s\nsize=4000 0.231s\nsize=8000 0.927s\n```\n\n### Impact\n\nA consumer that accepts attacker-influenced corpus files can be forced into heavy CPU use and parser-thread stalling before the application concludes the input contains no valid content.\n\n### Remediation\n\nReplace the whole-block lazy-regex parser with a linear parser or bounded tokenizer, and add regression tests that assert near-linear behavior on malformed inputs with many unmatched tags.",
"id": "BREW-acronym-CVE-2026-81725",
"modified": "2026-09-17T18:47:55Z",
"published": "2026-09-03T08:45:01Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-8mpw-7fpc-4gqj"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81725"
},
{
"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-2026-3752.yaml"
},
{
"type": "WEB",
"url": "https://www.vulncheck.com/advisories/nltk-before-3.10.3-regular-expression-denial-of-service-via-pl196xcorpusreader"
}
],
"schema_version": "1.7.3",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "NLTK: Pl196xCorpusReader has quadratic ReDoS on malformed TEI blocks",
"upstream": [
"PYSEC-2026-3752",
"CVE-2026-81725",
"GHSA-8mpw-7fpc-4gqj"
]
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Published Proof of Concept: A public proof of concept is available for this vulnerability.
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- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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