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CVE-2020-6817

2026-09-29T04:56:00.119764+00:00

Affected: Mozilla Bleach

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']}).

GHSA-vqhp-cxgc-6wmm — regular expression denial-of-service (ReDoS) in Bleach

2026-09-29T04:56:00.119940+00:00

Affected: PyPI: 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.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

PYSEC-2020-340

2026-09-29T04:56:00.120035+00:00

Affected: PyPI: bleach

In Mozilla Bleach before 3.1.4, `bleach.clean` behavior parsing style attributes could result in a regular expression denial of service (ReDoS).