The vulnerability stems from improper input sanitization in HTML processing. The pull request #860 specifically modifies the filterHTML function's regex patterns to address self-closing tags and event handler attributes. Multiple GitHub issues (#700, #715, etc.) demonstrate that malicious scripts/attributes persisted through the original filtering logic. The function's responsibility for HTML sanitization and the direct correlation between its regex shortcomings and the XSS payloads described in PoCs confirm its vulnerability.