GHSA-8rfp-98v4-mmr6 Bleach: URI sanitization allows disallowed URI schemes with Unicode > U+00A0 in output

Not scored Published 16 Jun 2026

Bleach sanitizer fails to strip disallowed URI schemes containing Unicode characters, potentially allowing XSS if downstream systems normalize the output.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: A user has to click or open something
Fix: Available

What is GHSA-8rfp-98v4-mmr6?

GHSA-8rfp-98v4-mmr6 is a cross-site scripting vulnerability in bleach, published on 16 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

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 = &#39;&lt;a href=&#34;javascript\u200b:alert(document.cookie)&#34;&gt;Click me&lt;/a&gt;&#39;
result1 = bleach.clean(payload1)
print(f&#34;(ZWSP): {repr(result1)}&#34;)

Output:

(ZWSP): &#39;&lt;a href=&#34;javascript\u200b:alert(document.cookie)&#34;&gt;Click me&lt;/a&gt;&#39;

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/show_bug.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.

Which versions are affected by GHSA-8rfp-98v4-mmr6?

2 open source packages ship an affected version, including bleach.

Package	Affected	Fixed in
`bleach` PyPI	All versions	`6.4.0`
`bleach` PyPI	All versions	`6.4.0`

How do I fix GHSA-8rfp-98v4-mmr6?

Upgrade bleach to 6.4.0 or later.

Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.

Is GHSA-8rfp-98v4-mmr6 being exploited?

There is no known exploitation of GHSA-8rfp-98v4-mmr6 in the wild and no public exploit code.