The Flash browser plugin supported interactive apps & games befor the modern web could handle them.
It were a such frequent source of vulnerabilities that browsers eventually banned it.
SAML feels like that, but for authentication. SAMLStorm is the latest in an ongoing history of XML-parsing vulns, and probably not the last.
https://workos.com/blog/samlstorm
An alternative, OIDC, is based on REST and JSON, avoiding that class of problems.