CVE-2016-9909: The serializer in html5lib before 0.99999999 might allow remote attackers to conduct cross-site scripting (...

Security readout for executives and security teams

CVE-2016-9909 is a potential cross-site scripting issue in html5lib before 0.99999999. If an application used the vulnerable serializer on attacker-influenced HTML, mishandled attribute escaping could let unsafe markup survive into a page. No source in the bundle shows active exploitation. Exposure is most likely in Python applications or dependencies that use html5lib serialization for untrusted or semi-trusted HTML content. Risk depends on whether serialized output is later rendered in browsers and whether attacker-controlled attribute values can reach that path. Treat this as a targeted dependency remediation item, not an emergency. Prioritize systems that process user-supplied HTML, comments, rich text, email content, or imported markup and render the serialized result to users. Mitigation focus: Upgrade html5lib to 0.99999999 or later where used.; Check vendor or distribution advisories for packaged html5lib fixes.; Avoid serializing attacker-controlled HTML with vulnerable versions..

Prepared

Jun 13, 2026, 06:02 UTC (UTC+00:00)

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.