saas-sso-scim-enterprise-auth

SaaS Enterprise Auth — SSO, SCIM, Custom Domain

Acknowledgement: Shared by Peter Bamuhigire, techguypeter.com, +256 784 464178.

Use When

• Adding enterprise auth to a B2B SaaS — SAML 2.0 (Okta, Azure AD, Ping, OneLogin), OIDC (Auth0, Cognito, custom OPs).
• Implementing SCIM 2.0 user provisioning/deprovisioning so the buyer's IdP can lifecycle users into your platform.
• Letting tenants connect their own IdP per tenant (multi-IdP-per-platform).
• Supporting custom domains (app.customer.com) with automated cert provisioning.
• Adding IP allowlists per tenant for enterprise-tier security policy.
• Migrating an existing email-password tenant to IdP-enforced login.
• Exposing an audit-log API the customer can pull into their SIEM.

Do Not Use When

• The task is the customer-facing tenant-admin auth screen — use dual-auth-rbac.
• The task is internal staff auth — use dual-auth-rbac + MFA + the back-office privileged-access workflow in saas-admin-backoffice-tooling.
• The task is OAuth as a client (calling Google/Stripe APIs) — use vibe-security-skill.

Required Inputs

• Current auth implementation (which provider / framework — Cognito, Auth0, Clerk, Supabase Auth, custom, etc.).
• Identity model from saas-control-plane-engineering (users + memberships).
• Enterprise-tier definition from subscription-billing (which plan gates enterprise auth).
• Custom-domain strategy from saas-deployment-models / multi-tenant-saas-architecture.

Workflow

1. Read this SKILL.md.
2. Decide build-vs-buy (§2). For most teams, buy via WorkOS / Ory / Auth0 Enterprise Connections / Clerk / Frontegg.
3. Design the per-tenant IdP configuration model (§3).
4. Implement SAML 2.0 SP-initiated and IdP-initiated SSO (§4).
5. Implement SCIM 2.0 provisioning endpoints (§5).
6. Implement custom domain + auto-TLS (§6).
7. Implement IP allowlists per tenant (§7).
8. Implement audit-log API (§8).
9. Migration plan: email-password → IdP-enforced (§9).
10. Apply anti-patterns (§10).

Quality Standards

• SAML metadata XML validated; signatures verified; assertion replay-protected (InResponseTo, recent timestamp).
• SCIM endpoints conform to RFC 7644 (Users + Groups resources, PATCH with Operations).
• Custom domain TLS auto-renewed (cert-manager / ACM / Let's Encrypt).
• IP allowlists enforced at the edge (gateway / WAF), not in the app.
• Audit log API supports pagination, time-range, structured JSON, cursor + idempotent reads.
• Enterprise-auth features gated by plan via saas-entitlements-and-plan-gating.

Anti-Patterns

• Rolling custom SAML — auth code is full of subtle bugs (XML Signature Wrapping, comment-injection, certificate pinning). Use a vetted library or buy.
• One global IdP for the whole platform when the platform should support per-tenant IdP.
• SCIM as "we read your IdP nightly" — fragile, slow. SCIM is push from IdP; implement the receiving endpoints.
• IP allowlists enforced only in app code (bypass via direct API call).
• Email-password permanently bypassable on enterprise-IdP-enforced tenants ("emergency login" door). If you need a break-glass, make it audited and time-limited.

Outputs

• Build-vs-buy ADR.
• Per-tenant IdP configuration schema.
• SAML endpoints (ACS, metadata, SLO).
• SCIM endpoints (Users + Groups CRUD).
• Custom domain provisioning automation.
• IP allowlist policy + enforcement layer.
• Audit log API spec.

Evidence Produced

Category	Artifact	Format	Example
Architecture	Enterprise auth ADR	ADR markdown	docs/adr/0015-enterprise-auth.md
Security	Per-tenant IdP config schema	Markdown doc + SQL	docs/saas/tenant-idp-config.md
Release evidence	SCIM endpoint spec	Markdown doc with example payloads	docs/saas/scim-spec.md

References

• references/saml-implementation.md — SAML 2.0 flow, attribute mapping, validation pitfalls.
• references/scim-provisioning.md — SCIM 2.0 endpoints, PATCH semantics, Okta/Azure quirks.
• references/custom-domain-and-tls.md — DNS verification, cert-manager, fingerprint validation.
• references/audit-log-api.md — API surface, retention, format.
• Companion: multi-tenant-saas-architecture, dual-auth-rbac, saas-control-plane-engineering, saas-entitlements-and-plan-gating, vibe-security-skill.

§1 The Enterprise-Auth Pack

Selling to enterprises requires:

1. SSO via SAML 2.0 (and/or OIDC) with per-tenant IdP.
2. SCIM 2.0 so the buyer can provision/deprovision via their IdP.
3. Custom domain (app.customer.com CNAME).
4. IP allowlists per tenant.
5. Audit log API the buyer's SIEM pulls.
6. MFA enforcement policies per tenant.
7. Session control — max session length, idle timeout, forced re-auth on privileged actions.

Mersch (Hacking SaaS) is emphatic — these are not edge cases; they are price of entry to the enterprise segment. Shipping the first enterprise deal without them stalls a four-week deal into a four-month deal.

§2 Build vs Buy

Option	Cost	Time	Notes
WorkOS	$$$	1-2 weeks	Best-in-class enterprise auth API; SAML + SCIM + Directory Sync; charges per active connection
Frontegg	$$$	1-2 weeks	All-in-one B2B identity, includes admin portal for tenants
Ory (open source / managed)	$-$$$	2-4 weeks	Self-hostable; more control; more ops
Auth0 Enterprise	$$$$	1-2 weeks	Mature; expensive at scale
Clerk	$$$	1 week	Strong DX; expanding enterprise feature set
AWS Cognito	$-$$	2-4 weeks	If already on AWS; SAML works; SCIM limited
Roll custom	Eng time	2-3 months	Don't. The auth surface is enormous and one bug is a CVE.

Default recommendation for SaaS up to $10M ARR: WorkOS (best ROI on enterprise-auth complexity). Migrate to in-house only if cost + control require it at $50M+ ARR.

§3 Per-Tenant IdP Configuration

CREATE TABLE tenant_idp_configurations (
    tenant_id          BIGINT UNSIGNED PRIMARY KEY,
    enforced           BOOLEAN NOT NULL DEFAULT FALSE,  -- if true, email-password blocked
    protocol           ENUM('saml','oidc','none') NOT NULL DEFAULT 'none',

    -- SAML
    saml_metadata_xml  TEXT,
    saml_entity_id     VARCHAR(255),
    saml_sso_url       VARCHAR(512),
    saml_certificate   TEXT,
    saml_attr_email    VARCHAR(128) DEFAULT 'NameID',
    saml_attr_name     VARCHAR(128) DEFAULT 'displayName',
    saml_attr_groups   VARCHAR(128) DEFAULT 'groups',

    -- OIDC
    oidc_issuer        VARCHAR(512),
    oidc_client_id     VARCHAR(255),
    oidc_client_secret VARCHAR(512),    -- encrypted at rest

    -- SCIM
    scim_enabled       BOOLEAN NOT NULL DEFAULT FALSE,
    scim_bearer_token_hash CHAR(64),    -- SCIM endpoint auth
    scim_group_mapping JSON,            -- IdP group → tenant role

    -- Misc
    domain_verified    BOOLEAN NOT NULL DEFAULT FALSE, -- buyer proved ownership of email domain
    email_domain       VARCHAR(255),     -- e.g., 'acme.com'

    configured_by      BIGINT UNSIGNED,
    configured_at      DATETIME,
    updated_at         DATETIME
);

Domain claim flow is essential — if any user with email @acme.com can claim the Acme tenant, you have a takeover risk. Domain verification via DNS TXT record or a verified-by-IT click flow.

§4 SAML 2.0 Implementation

Endpoints

Endpoint	Purpose
GET /sso/saml/{tenant}/metadata	Your platform's SP metadata XML
POST /sso/saml/{tenant}/acs	Assertion Consumer Service — IdP posts SAMLResponse here
GET /sso/saml/{tenant}/login	SP-initiated login (redirect to IdP)
GET /sso/saml/{tenant}/logout	Single Logout (SLO) initiation
POST /sso/saml/{tenant}/sls	Single Logout Service — IdP posts logout here

Flow

User → app.example.com → "Sign in with SSO" → enters email → email domain → resolve tenant
     → redirect to /sso/saml/{tenant}/login
     → SAMLRequest built, signed, redirect to tenant.saml_sso_url
     → User authenticates at IdP
     → IdP posts SAMLResponse to /sso/saml/{tenant}/acs
     → Validate signature, audience, recipient, NotBefore/NotOnOrAfter, InResponseTo
     → Extract attributes (email, name, groups)
     → Match or create user; bind to tenant; resolve roles from group mapping
     → Issue platform session
     → Redirect to app

Validation pitfalls (these are CVE-tier when wrong)

• Always verify the XML signature against the configured certificate.
• Reject if NotOnOrAfter in the past.
• Reject if Recipient does not match your ACS URL.
• Reject if Audience does not match your SP entity ID.
• Reject if the assertion has been seen before (replay protection: cache Assertion.ID for the validity window).
• Beware XML Signature Wrapping — validate the signed element is the one you're consuming.
• Beware XXE — parse with external-entity resolution disabled.

Library recommendations

• Python: python3-saml (OneLogin) — well-maintained.
• Node: passport-saml / samlify.
• Go: crewjam/saml.
• PHP: onelogin/php-saml.
• Or: don't roll — use WorkOS / Auth0 / Cognito and avoid these pitfalls.

§5 SCIM 2.0 Implementation

SCIM is push from IdP to platform — provisioning/deprovisioning runs on the IdP's schedule.

Endpoints (under /scim/v2/{tenant}/)

Endpoint	Method	Purpose
/Users	GET	List users (paginated)
/Users	POST	Create user
/Users/{id}	GET	Read user
/Users/{id}	PUT	Replace user
/Users/{id}	PATCH	Partial update (most-used)
/Users/{id}	DELETE	Deprovision user
/Groups	GET / POST / etc.	Group CRUD
/ServiceProviderConfig	GET	Platform capabilities
/Schemas	GET	Resource schemas
/ResourceTypes	GET	Resource types

Auth

Bearer token per tenant. Tenant admin generates it once in the platform UI; pastes into Okta/Azure SCIM connector.

PATCH semantics

PATCH /Users/{id} body:

{
  "schemas": ["urn:ietf:params:scim:api:messages:2.0:PatchOp"],
  "Operations": [
    {"op": "replace", "path": "active", "value": false}
  ]
}

Implementing PATCH correctly is the hardest SCIM problem. Test against Okta + Azure AD + OneLogin payloads — they all send slightly different shapes.

Group mapping

Map IdP groups (e.g., acme-saas-admins) to your platform roles (e.g., admin). Stored in tenant_idp_configurations.scim_group_mapping.

§6 Custom Domain + Auto-TLS

Customer wants their users to log in at app.acme.com instead of acme.your-saas.com.

Flow

1. Tenant admin enters app.acme.com in your platform's domain settings.
2. Platform displays a CNAME to add: app.acme.com CNAME tenant-acme.your-saas.com.
3. Tenant adds CNAME at their DNS.
4. Platform verifies CNAME resolves to your CNAME target (polling).
5. Platform provisions TLS cert via Let's Encrypt / ACM / cert-manager (HTTP-01 or DNS-01 challenge).
6. Once cert issued and CNAME verified, custom domain becomes active for that tenant.
7. Cert auto-renews; failures alert.

Routing

Edge layer (nginx / Cloudflare / ALB) accepts the custom domain; SNI extracts hostname; resolves to tenant; routes to backend with X-Tenant-Id header. The backend trusts the header only because it came from the trusted edge.

§7 IP Allowlists Per Tenant

Enforced at the edge (WAF / ALB / Cloudflare / Cloudfront). App-layer enforcement is a fallback only.

CREATE TABLE tenant_ip_allowlists (
    id           BIGINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    tenant_id    BIGINT UNSIGNED NOT NULL,
    cidr         VARCHAR(43) NOT NULL,
    description  VARCHAR(255),
    added_by     BIGINT UNSIGNED,
    added_at     DATETIME NOT NULL,
    INDEX idx_tenant (tenant_id)
);

Edge consults the allowlist (cached) and rejects with 403 before the request hits the app. App-layer enforcement is a safety net but never the sole gate.

Caveats:

• Mobile-app users have rotating IPs; allowlists usually only apply to web + SCIM endpoints.
• API tokens may need a separate allowlist (often more permissive than web).

§8 Audit Log API

Customers want to pull audit logs into Splunk/Datadog/Elastic.

GET /api/v1/admin/audit-log?from=2026-05-01T00:00:00Z&cursor=<opaque>
Authorization: Bearer <tenant_admin_api_key>

→ 200 OK
{
  "data": [ { id, occurred_at, actor_email, action, target_resource, target_id, ip, metadata } ],
  "cursor": "<next-page-opaque>",
  "has_more": true
}

Pagination: cursor-based, not page-based (avoid skip-N inefficiency at large scale).

Retention exposed via /api/v1/admin/audit-log/retention-policy.

What to include: tenant-scoped events the customer cares about — user login/logout, MFA changes, role changes, plan changes, member invited/removed, data export, integration created/revoked.

§9 Migration: Email-Password → IdP-Enforced

Once tenant configures SSO, decide:

• Optional SSO — users can choose SSO or email-password.
• Enforced SSO — email-password blocked; all logins go through IdP.

Enforced is the enterprise expectation. Migration steps:

1. Tenant admin configures IdP, verifies metadata, tests SP-initiated login.
2. Tenant admin invites the rest of the team (or relies on SCIM provisioning).
3. Tenant admin schedules enforcement date.
4. Platform sends email warning to all tenant users.
5. On enforcement date, set tenant_idp_configurations.enforced = TRUE.
6. Email-password logins return: "Your organisation requires SSO. Sign in via your company SSO."

Break-glass: super-admin in the back-office can temporarily disable enforcement for a tenant (audited, time-boxed, justified).

§10 Anti-Patterns

• Rolling custom SAML. Use a library; ideally use WorkOS / Auth0.
• SCIM as nightly pull. SCIM is push from IdP; your endpoints must accept the push.
• IP allowlist enforced only in app code. Trivially bypassable. Edge-level enforcement is required.
• One SAML configuration globally. Each tenant has its own IdP; per-tenant config is mandatory.
• Email-password "emergency login" left enabled silently. Audit logs hidden; security catastrophe. If kept, audited + time-boxed + alert-on-use.
• No domain claim. Anyone can SSO-link a tenant they're not part of via email-domain assertion.
• Audit log API rate-limited too aggressively. Customers pull large windows; need pagination + reasonable rate.

§11 Read Next

• dual-auth-rbac — base auth implementation this skill extends.
• multi-tenant-saas-architecture — tenant binding model.
• saas-control-plane-engineering — identity service this skill plugs into.
• saas-entitlements-and-plan-gating — gate enterprise auth behind enterprise plan.
• saas-admin-backoffice-tooling — staff break-glass workflow.
• vibe-security-skill — security review baseline.