The problem is not XML by itself. XML adds a considerable amount of complexity to JSON[1] and when writing security-oriented software complexity matters quite a

The problem is not XML by itself. XML adds a considerable amount of complexity to JSON[1] and when writing security-oriented software complexity matters quite a bit[2]. But this is still a level of complexity that can be managed. Most other XML-based protocols aren't as bad as SAML. No, the main problem with SAML is that it relies on XML Signatures (XMLDSig). And the main problem with XML Signatures is that the signature needs to be embedded inside the XML it's signing, instead of being attached to it, like every other signature standard on the planet. [1] The added complexity is not just attributes and namespace, but also entities, DTD and processing instructions. If you want even the most basic type checking, XML schema becomes mandatory. This is important, since JSON doesn't need a schema for handling basic types, and in fact OAuth 2.0 and Open ID Connect do not rely on JSON Schema at all. [2] See the OWASP cheatsheet for all the ways accepting XML input from the internet can put your application in danger: https://cheatsheetseries.owasp.org/cheatsheets/XML_Security_Cheat_Sheet.html https://cheatsheetseries.owasp.org/cheatsheets/XML_Security_... No equivalent document exists for JSON.