---
id: CVE-2026-80205
title: >-
  nltk: NLTK: Denial of Service via unvalidated regular expressions
  (CVE-2026-80205)
summary: >-
  A flaw was found in NLTK. A remote attacker can exploit a regular expression
  denial of service (ReDoS) vulnerability in the `Text.findall()` and
  `TokenSearcher.findall()` methods. These methods accept user-supplied regular
  expressions with…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cvssSource: vendor
cwe: CWE-1333
vendor: Red Hat
product: Red Hat OpenShift AI (RHOAI)
affected:
  - exploit_intelligence
  - lightspeed_core
  - openshift_lightspeed
  - ansible_automation_platform 2
  - openshift_ai_rhoai
  - openshift_ai 3.5
patched:
  - openshift_ai 3.5
published: '2026-08-26'
updated: '2026-09-22'
sourceUpdated: '2026-09-22T05:58:45+00:00'
source: CSAF
sourceUrl: 'https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-80205.json'
references:
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-80205.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-80205'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2524331'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-80205'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-80205'
  - url: 'https://github.com/nltk/nltk/security/advisories/GHSA-rrv8-h7p8-rx55'
  - url: >-
      https://www.vulncheck.com/advisories/nltk-before-3.10.0-redos-via-text-findall-unvalidated-regex
  - url: 'https://access.redhat.com/errata/RHSA-2026:69539'
  - url: 'https://github.com/nltk/nltk/pull/3674'
  - url: >-
      https://github.com/nltk/nltk/commit/d8e47539317b571ab1422981f5b9653d5eae1249
  - url: 'https://github.com/nltk/nltk'
  - url: 'https://github.com/nltk/nltk/releases/tag/v3.10.0'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3750.yaml
  - url: 'http://www.openwall.com/lists/oss-security/2026/09/01/3'
  - url: 'https://github.com/advisories/GHSA-rrv8-h7p8-rx55'
tags:
  - csaf
  - vex
  - red-hat
  - osv
  - pip
  - ghsa
epss: 0.00489
epssPercentile: 0.41221
aliases:
  - GHSA-rrv8-h7p8-rx55
  - PYSEC-2026-3750
ecosystem: pip
ingestedAt: '2026-09-02T19:31:25.079Z'
---

## Overview

A flaw was found in NLTK. A remote attacker can exploit a regular expression denial of service (ReDoS) vulnerability in the `Text.findall()` and `TokenSearcher.findall()` methods. These methods accept user-supplied regular expressions without validation, allowing an attacker to provide crafted patterns that cause catastrophic backtracking. This can lead to indefinite CPU saturation, resulting in a denial of service to all users of the Python process.

## Vendor advisories

- **RHSA-2026:69539** · Red Hat · fixed in: Red Hat OpenShift AI 3.5 · released 2026-09-21 · [advisory](https://access.redhat.com/errata/RHSA-2026:69539)
- **Red Hat VEX** · Important · affected: Exploit Intelligence, Lightspeed Core, OpenShift Lightspeed, Red Hat Ansible Automation Platform 2, Red Hat OpenShift AI (RHOAI) · no fix planned: Exploit Intelligence, Red Hat Ansible Automation Platform 2, Lightspeed Core, OpenShift Lightspeed, … · updated 2026-09-22 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-80205.json)

**nltk: NLTK: Denial of Service via unvalidated regular expressions** — rated Important by Red Hat. Released 2026-08-26, updated 2026-09-22.

Affected:

- Exploit Intelligence
- Lightspeed Core
- OpenShift Lightspeed
- Red Hat Ansible Automation Platform 2
- Red Hat OpenShift AI (RHOAI)

Fixed:

- Red Hat OpenShift AI 3.5

No fix planned:

- Exploit Intelligence
- Red Hat Ansible Automation Platform 2
- Lightspeed Core
- OpenShift Lightspeed
- Red Hat OpenShift AI (RHOAI)

Not affected:

- Red Hat OpenShift AI 3.5
- OpenShift Lightspeed

## Remediation

For Red Hat OpenShift AI 3.5.1 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this errata update:

https://docs.redhat.com/en/documentation/red_hat_openshift_ai/ https://access.redhat.com/errata/RHSA-2026:69539

Workarounds / mitigations:

- Sanitize all untrusted input passed to `Text.findall()` or `TokenSearcher.findall()` to prevent execution of arbitrary or unvalidated regular expressions, or enforce execution timeouts via worker process isolation to bound CPU consumption.

## Package advisory (CVE-2026-80205)

Affected packages:

- `nltk < 3.10.0`

Patched in:

- `nltk 3.10.0`

Source: https://osv.dev/vulnerability/GHSA-rrv8-h7p8-rx55