Vulnerability Database

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Total vulnerabilities in the database

Bleach: URI sanitization allows disallowed URI schemes with Unicode > U+00A0 in output — bleach

Incomplete List of Disallowed Inputs

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/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.

  • Published: Jun 16, 2026
  • Updated: Jun 17, 2026
  • GHSA: GHSA-8rfp-98v4-mmr6
  • Severity: Low
  • Exploit:
  • CISA KEV:

CVSS v3:

  • Severity: Unknown
  • Score: 0
  • AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:N

CWEs:

Frequently Asked Questions

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CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

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Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

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