A possible XSS bypass affects users calling bleach.clean with all of:
a in the allowed tagshref in allowed attributesThe 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:
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>'
Users should upgrade to Bleach 6.4.0.
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.
Reported by codeant from CodeAnt AI.
| Software | From | Fixed in |
|---|---|---|
bleach
|
- | 6.4.0 |
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
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