The short version of why SAML is bad is because it’s based on XML. This means that SAML is not only subject to the usual vulnerabilities that you’d expect, but also XML vulnerabilities!

A slightly longer explanation is that SAML is based on a garbage cryptographic signature standard called “XML-DSig” which, among other issues, allows for partial signatures and stores the signature inside the XML element that it signed.

Honestly, SAML is something of an info hazard. I don’t recommend learning more about it. But if you really want to, this blog post goes into more detail: https://dev.to/workos/fun-with-saml-sso-vulnerabilities-and-...