Debugging SAML SSO: How to Decode a SAMLResponse (and Why It's Sometimes Not XML)

You're debugging a broken SSO login. The identity provider (IdP) redirects back to your app, and somewhere in the request is a big blob called SAMLResponse . You grab it, Base64-decode it, and expect to see clean XML. Sometimes you do. Sometimes you get binary garbage that starts with bytes like 0x78 0x9c and looks nothing like markup. Both outcomes are correct. The difference is which SAML binding the IdP used, and once you know the two encoding chains, SAML debugging stops being guesswork. The two bindings, and their two encodings SAML sends its messages ( SAMLResponse , SAMLRequest ) using one of two HTTP bindings, and they encode the payload differently: HTTP-POST binding — the message rides in a hidden form field that auto-submits via POST. The value is simply: Base64(XML) Decode the Base64 and you get the assertion XML directly. This is the common case for the response coming back from the IdP. HTTP-Redirect binding — the message rides in a URL query string, so it has to be small and URL-safe. The value is: URLEncode( Base64( DEFLATE( XML ) ) ) That's three layers. If you only Base64-decode it, you're staring at the raw output of a DEFLATE compressor — which is exactly the binary garbage people report. This binding is typically used for SAMLRequest (the AuthnRequest your app sends to the IdP) and for Single Logout. Critically, the redirect binding uses raw DEFLATE (RFC 1951) with no zlib header and no checksum . That's the single most common thing people get wrong — they reach for a normal zlib/gzip inflate, it chokes on the missing header, and they conclude the blob is corrupt. It isn't; it just needs a raw inflate. Decoding both in Python import base64 import zlib from urllib.parse import unquote # --- HTTP-POST binding: Base64(XML) --- def decode_post ( saml_response : str ) -> str : return base64 . b64decode ( saml_response ). decode ( " utf-8 " ) # --- HTTP-Redirect binding: URLEncode(Base64(DEFLATE(XML))) --- def decode_redirect ( saml_param : str ) -> s