regular expression denial-of-service (ReDoS) in Bleach (CVE-2020-6817) | HOL Guard CVE

High · CVSS 7.5CVE-2020-6817GHSA-VQHP-CXGC-6WMM

regular expression denial-of-service (ReDoS) in Bleach

Answer in brief

CVE-2020-6817 records a High severity (CVSS 7.5) vulnerability in regular expression denial-of-service (ReDoS) in Bleach. The current sources do not mark it as known exploited. The current feed maps bleach (pypi). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Mar 30, 2020Updated Sep 10, 2026Source checked Sep 10, 2026First seen by HOL Aug 7, 2026Material review Sep 10, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.5
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps bleach (pypi). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
bleachpypi	>=0 <3.1.4	3.1.4

Recommended response

Check inventory. Check lockfiles and deployed manifests for bleach.
Review the reported fix. Update bleach to 3.1.4 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Mar 30, 2020
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Aug 7, 2026

Sources and claim methodology

Upstream source description

### 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 [email protected]

Quoted source text, attributed separately from HOL analysis.