NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions
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 returnsImpact
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
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | nltk | < 3.10.0 | 3.10.0 |
References
- http://www.openwall.com/lists/oss-security/2026/09/01/3
- https://www.vulncheck.com/advisories/nltk-before-3.10.0-redos-via-text-findall-unvalidated-regex
- https://github.com/nltk/nltk/security/advisories/GHSA-rrv8-h7p8-rx55
- https://nvd.nist.gov/vuln/detail/CVE-2026-80205
- https://github.com/nltk/nltk/pull/3674
- https://github.com/nltk/nltk/commit/d8e47539317b571ab1422981f5b9653d5eae1249
- https://github.com/nltk/nltk
- https://github.com/nltk/nltk/releases/tag/v3.10.0
- https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3750.yaml
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness (CWE)
CWE-1333- Platform
- Python (PyPI)
- Package
nltk- Disclosed
- 26.08.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-3750 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-rrv8-h7p8-rx55 - CC BY 4.0
Vulnerability history: nltk
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 52
- First → latest
- 08.2019 09.2026
- Without a fix
- 6
- Exploited in attacks
- 0
Severity breakdown
- Critical 5
- High 32
- Medium 13
- Low 2
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 2
- 2022: 1
- 2023: 0
- 2024: 1
- 2025: 0
- 2026: 47
-
NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions this entry
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