NLTKGHSA-rrv8-h7p8-rx55

NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions

CVE-2026-80205 · Published Sep 8, 2026

High7.5

Fix: upgrade to 3.10.0 or later

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:

```python

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

```python

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...

Affected versions

Package	Affected	Fixed in
nltk PyPI	< 3.10.0	3.10.0

Details and references

### 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: ```python 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 ```python 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](https://www.offgridsec.com)

CVSS 3.1: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Severity from: GitHub (reviewed advisory)
Weakness: CWE-1333
Also known as: CVE-2026-80205, PYSEC-2026-3750

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