Impact
A possible XSS bypass affects users calling bleach.clean with all of:
a in the allowed tagshref 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
Reported by
Reported by codeant from CodeAnt AI.
References
Impact
A possible XSS bypass affects users calling
bleach.cleanwith all of:ain the allowed tagshrefin allowed attributesThe
bleach.cleansanitizer 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:
Output:
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
Reported by
Reported by codeant from CodeAnt AI.
References