XML Signatures are a typical example of a security protocol invented in the early 2000’s. They suffer from high complexity, a large attack surface and a wealth of configurable features that can weaken or break its security guarantees in surprising ways. Modern usage of XML signatures is mostly restricted to somewhat obscure protocols and legacy applications, but there is one important exception: SAML. SAML, which stands for Security Assertion Markup Language, is one of the two main Single-Sign-On standards used in modern web applications. While its alternative, the OAuth based OpenID Connect (OIDC) is gaining popularity, SAML is still the de-facto standard for large enterprises and complex integrations.
SAML relies on XML signatures to protect messages forwarded through the browser. This turns XML signature verification into a very interesting external attack surface for attacking modern multi-tenant SaaS applications. While you don’t need a detailed understanding of SAML to follow this post, interested readers can take a look at Okta’s Understanding SAML writeup or the SAML 2.0 wiki entry to get a better understanding of the protocol.
Source: Project Zero