GHSA-8rfp-98v4-mmr6: XSS
Impact
A possible XSS bypass affects users calling bleach.clean with all of:
a in the allowed tags href in allowed attributes
The bleach.clean sanitizer outputs URIs containing disallowed scheme patterns that it should be stripping. However, because the inserted Unicode characters make the scheme invalid per RFC 3986, modern browsers do not execute these as javascript: URIs. The practical security impact is limited to:
- Bleach's output contains URI values that violate the caller's protocol allowlist, breaking the sanitizer's contract. - If a downstream system performs its own Unicode normalization on bleach's output (stripping invisible characters before rendering), the javascript: scheme could become valid. This is a non-standard processing chain but represents a theoretical secondary risk.
This is not a direct XSS vulnerability.
Python code example from reporter with Bleach v6.3.0 and Python 3.13:
import bleach payload1 = '<a href="javascript\u200b:alert(document.cookie)">Click me</a>' result1 = bleach.clean(payload1) print(f"(ZWSP): {repr(result1)}")
Output:
(ZWSP): '<a href="javascript\u200b:alert(document.cookie)">Click me</a>'
Patches
Users should upgrade to Bleach 6.4.0.
Workarounds
Pre-process content removing non-ASCII characters from URI schemes before sanitizing with bleach.clean.
A strong Content-Security-Policy without unsafe-inline and unsafe-eval script-srcs will also help mitigate the risk.
References
https://bugzilla.mozilla.org/showbug.cgi?id=2023812 RFC 3986, Section 3.1 (URI Scheme syntax): scheme characters are restricted to ALPHA ( ALPHA / DIGIT / "+" / "-" / "." )
Reported by
Reported by codeant from CodeAnt AI.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/bleachto a version that resolves this vulnerability.Fixed in 6.4.0 - Configuration
Pre-process content to remove non-ASCII (invisible/zero-width) characters from URI schemes before calling bleach.clean.
pre-processing before bleach.clean remove_non_ascii_from_uri_schemes = true - Compensating control
Deploy a strong Content-Security-Policy that does not allow 'unsafe-inline' or 'unsafe-eval' in script-src to mitigate risks from potentially malformed javascript: URIs.
Event History
Frequently Asked Questions
What is the severity of GHSA-8rfp-98v4-mmr6?
The severity of GHSA-8rfp-98v4-mmr6 is rated at 15.
What is the nature of the vulnerability in GHSA-8rfp-98v4-mmr6?
GHSA-8rfp-98v4-mmr6 is an XSS bypass vulnerability in the bleach.clean sanitizer.
How does GHSA-8rfp-98v4-mmr6 affect users?
Users calling bleach.clean with specific allowed tags and attributes may experience uncensored disallowed scheme patterns.
How do I mitigate GHSA-8rfp-98v4-mmr6?
To mitigate GHSA-8rfp-98v4-mmr6, avoid using disallowed scheme patterns in URIs within bleach.clean.
What versions of bleach are affected by GHSA-8rfp-98v4-mmr6?
GHSA-8rfp-98v4-mmr6 affects versions of the bleach library that allow specified tags and attributes in sanitization.