regular expression denial-of-service (ReDoS) in Bleach
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.cleancalls -
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 software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | bleach | < 3.1.4 | 3.1.4 |
References
- https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm
- https://blog.r2c.dev/posts/finding-python-redos-bugs-at-scale-using-dlint-and-r2c/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1623633
- https://github.com/mozilla/bleach/releases/tag/v3.1.4
- https://snyk.io/vuln/SNYK-PYTHON-BLEACH-561754
- https://nvd.nist.gov/vuln/detail/CVE-2020-6817
- https://github.com/mozilla/bleach
- https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-340.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, CWE-400- Platform
- Python (PyPI)
- Package
bleach- Disclosed
- 30.03.2020
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2020-340 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-vqhp-cxgc-6wmm - CC BY 4.0
Vulnerability history: bleach
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
- 8
- First → latest
- 03.2018 06.2026
- Without a fix
- 1
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 1
- Medium 5
- Low 1
Disclosures per year
- 2018: 1
- 2019: 0
- 2020: 3
- 2021: 1
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 3
-
regular expression denial-of-service (ReDoS) in Bleach this entry
Know which vulnerabilities affect your application
With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.