---
id: CVE-2020-6817
aliases:
  - GHSA-vqhp-cxgc-6wmm
  - PYSEC-2020-340
  - SNYK-PYTHON-BLEACH-561754
title: regular expression denial-of-service (ReDoS) in Bleach
summary: regular expression denial-of-service (ReDoS) in Bleach
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'
vendor: bleach
product: bleach
ecosystem: pip
affected:
  - bleach < 3.1.4
patched:
  - bleach 3.1.4
published: '2020-03-30'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:48:29.583864458Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-vqhp-cxgc-6wmm'
references:
  - url: 'https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2020-6817'
  - url: 'https://bugzilla.mozilla.org/show_bug.cgi?id=1623633'
  - url: 'https://github.com/mozilla/bleach'
  - url: 'https://github.com/mozilla/bleach/releases/tag/v3.1.4'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-340.yaml
  - url: 'https://snyk.io/vuln/SNYK-PYTHON-BLEACH-561754'
tags:
  - osv
  - pip
epss: 0.00718
epssPercentile: 0.52443
ingestedAt: '2026-09-12T03:13:01.731Z'
---

## Overview

### Impact

`bleach.clean` behavior parsing style attributes could result in a regular expression denial of service (ReDoS).

Calls to ``bleach.clean`` with an allowed tag with an allowed ``style`` attribute are vulnerable to ReDoS. For example, ``bleach.clean(..., attributes={'a': ['style']})``.

### Patches

3.1.4

### Workarounds

* do not whitelist the style attribute in `bleach.clean` calls

* limit input string length

### References

* https://bugzilla.mozilla.org/show_bug.cgi?id=1623633
* https://www.regular-expressions.info/redos.html
* https://blog.r2c.dev/posts/finding-python-redos-bugs-at-scale-using-dlint-and-r2c/
* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6817

### Credits

* Reported by schwag09 of r2c

### For more information
If you have any questions or comments about this advisory:

* Open an issue at https://github.com/mozilla/bleach/issues
* Email us at security@mozilla.org

## Affected packages

- `bleach < 3.1.4`

## Remediation

Upgrade to a patched release:

- `bleach 3.1.4`