fix(saml): exempt ACS endpoint from CSRF Origin enforcement#666
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The SAML POST binding delivers the assertion via an auto-submitting form from the IdP's own origin — a legitimate cross-origin POST the Origin allow-list always rejected with 403, making the shipped SAML SP unusable for real browser flows (found while building the with-org-saml example, verified live). ACS defends itself with XML signature validation, InResponseTo binding, and the replay cache — same rationale as the /oauth_callback/ exemption. Exemption is suffix-scoped to /acs; a regression test proves other /oauth/saml/ POSTs stay protected.
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What
Found while live-verifying the new
with-org-samlexample against a dockerized Keycloak IdP: the CSRF middleware has no exemption forPOST /oauth/saml/:org_slug/acs, so the IdP's auto-submitting form POST (SAML POST binding, IdP origin, form-urlencoded) gets403 csrf_validation_failed— the shipped SAML SP feature is unusable for real browser flows (headless scripts only worked by faking allow-listedOriginheaders).Fix
Exempt
/oauth/saml/*/acs(prefix + suffix scoped) from Origin enforcement — the same rationale as the existing/oauth_callback/exemption: the ACS handler defends itself with XML signature validation, InResponseTo binding, and the assertion replay cache; CSRF Origin checks cannot apply to a flow whose legitimate POST is always cross-origin.Testing
TestCSRFExemptsSAMLACS: cross-origin IdP form POST reaches the ACS handler; other/oauth/saml/POST routes (e.g./login) remain CSRF-protected — exemption doesn't leak.go build, gofmt clean.Related: the MAI follow-ups memory lists "SAML handler-level tests" as a deferred item — this is the first regression from that gap.