Affected: nltk

NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.

Affected: PyPI: nltk

### 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 aga…

Affected: PyPI: nltk

NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.