The fix for https://github.com/advisories/GHSA-gh4j-gqv2-49f6 in fast-xml-parser sanitizes -- sequences in XML comment content using .replace(/--/g, '- -'). This skip the values containing three consecutive dashes (e.g., --->...), allowing an attacker to break out of an XML comment and inject arbitrary XML/HTML content.
Impact
Any application with comment property enabled allow attacker to inject malicious or unwanted code like JS script tag in the XML/HTML output.
Workarounds
Check for the presence of 3 consecutive dashes externally in the property value used for comment tag.