docs(security): note DefaultOrigin is a single global, not per-tenant

Address the MEDIUM review point on #1323: operator-driven register/resend
point every tenant's e-mail link at the one DefaultOrigin SPA. Fits the
kit's single-dashboard model; a per-tenant-custom-domain deployment would
resolve the recipient tenant's own origin instead.

Author: marcelo-maciel

src/content/docs/changelog/index.mdx

@@ -13,7 +13,7 @@ Notable changes to the kit, newest first.
 
 ## 2026-08-10
 
-- **Identity: password-reset and e-mail-confirmation links now resolve the correct front-end per request.** The reset link was built from a single configured `OriginOptions.OriginUrl` - which points at the API and ships empty in production, so `forgot-password` threw `Origin URL is not configured` - and the confirmation link was built from the request host and pointed straight at the API's `GET /confirm-email` route. Neither could target the right SPA when the kit serves more than one front-end (the admin console and the tenant dashboard on different origins). Link resolution now goes through a dedicated **`FrontendOptions`** (`AllowedOrigins` + `DefaultOrigin`), kept separate from CORS. **Self-service** flows (`forgot-password`, `self-register`) build the link from the request `Origin` header, validated against `FrontendOptions:AllowedOrigins` and returned as the canonical entry - so each user gets a link back to the app they started from; because forgot-password is anonymous a forged or unlisted `Origin` is rejected with **`400`**, and a request with no `Origin` (curl, Scalar, mobile, server-to-server) falls back to `DefaultOrigin`. **Operator-driven** flows (`register`, `resend-confirmation-email`) target `DefaultOrigin` - the recipient's app - so a tenant user provisioned from the admin console gets a link into the tenant app, not the console. The confirmation e-mail now lands on the SPA `/confirm-email` page (which then calls the API) instead of the raw API route. **Action for deployments:** set `FrontendOptions:DefaultOrigin` to your tenant SPA, and list every SPA origin in `FrontendOptions:AllowedOrigins`. Both ship empty in production; **the host still starts without them** and logs a startup `Warning`, falling back to the API's own origin (`OriginOptions:OriginUrl`) for these links - serviceable, but not where your users expect to land. `CorsOptions:AllowedOrigins` and `OriginOptions:OriginUrl` keep their own roles (browser CORS; the API's public base for avatar URLs). See [#1323](https://github.com/fullstackhero/dotnet-starter-kit/pull/1323).
+- **Identity: password-reset and e-mail-confirmation links now resolve the correct front-end per request.** The reset link was built from a single configured `OriginOptions.OriginUrl` - which points at the API and ships empty in production, so `forgot-password` threw `Origin URL is not configured` - and the confirmation link was built from the request host and pointed straight at the API's `GET /confirm-email` route. Neither could target the right SPA when the kit serves more than one front-end (the admin console and the tenant dashboard on different origins). Link resolution now goes through a dedicated **`FrontendOptions`** (`AllowedOrigins` + `DefaultOrigin`), kept separate from CORS. **Self-service** flows (`forgot-password`, `self-register`) build the link from the request `Origin` header, validated against `FrontendOptions:AllowedOrigins` and returned as the canonical entry - so each user gets a link back to the app they started from; because forgot-password is anonymous a forged or unlisted `Origin` is rejected with **`400`**, and a request with no `Origin` (curl, Scalar, mobile, server-to-server) falls back to `DefaultOrigin`. **Operator-driven** flows (`register`, `resend-confirmation-email`) target `DefaultOrigin` - the recipient's app - so a tenant user provisioned from the admin console gets a link into the tenant app, not the console. The confirmation e-mail now lands on the SPA `/confirm-email` page (which then calls the API) instead of the raw API route. **Action for deployments:** set `FrontendOptions:DefaultOrigin` to your tenant SPA, and list every SPA origin in `FrontendOptions:AllowedOrigins`. Both ship empty in production; **the host still starts without them** and logs a startup `Warning`, falling back to the API's own origin for these links - `OriginOptions:OriginUrl`, or the request host when that is empty too - which is serviceable, and where confirmation links pointed before, but not where your users expect to land. `CorsOptions:AllowedOrigins` and `OriginOptions:OriginUrl` keep their own roles (browser CORS; the API's public base for avatar URLs). See [#1323](https://github.com/fullstackhero/dotnet-starter-kit/pull/1323).
 
 ## 2026-07-13
 

src/content/docs/modules/identity.mdx

@@ -1,6 +1,6 @@
 ---
 title: Identity module
-lastUpdated: 2026-07-02
+lastUpdated: 2026-08-10
 description: JWT bearer + refresh tokens, ASP.NET Identity with roles + permissions, user groups, operator impersonation, two-factor TOTP, sessions, and password-policy enforcement.
 sidebar:
   label: Identity
@@ -150,7 +150,7 @@ endpoints.MapPost("/users", handler)
 All 51 endpoints are under `/api/v1/identity/`. The rate-limited `auth` policy covers `POST /token/issue`, `POST /token/refresh`, `GET /confirm-email`, `POST /users/{id}/resend-confirmation-email`, `POST /forgot-password`, `POST /reset-password`, and `POST /self-register`. Full table:
 
 <Callout type="note" title="Where reset & confirmation e-mails point">
-Auth e-mail links resolve through `FrontendOptions` (a dedicated config, separate from CORS). **Self-service** flows (`forgot-password`, `self-register`) link back to the front-end that made the request - the base URL comes from the request `Origin` header, validated against `FrontendOptions:AllowedOrigins` - so with more than one SPA each user gets a link to the app they started from; a forged or unlisted origin is rejected with `400`, and a request with no `Origin` (non-browser callers) falls back to `FrontendOptions:DefaultOrigin`. **Operator-driven** flows (`register`, `resend-confirmation-email`) target `DefaultOrigin` (the recipient's app), so a tenant user provisioned from the admin console gets a link into the tenant app, not the console. The confirmation link lands on the SPA `/confirm-email` page (which then calls `GET /confirm-email`), not the API route directly. Left unset, the host still boots and these links fall back to the API's own origin, which is rarely what you want - configure it, see [CORS & headers](/docs/security/cors-and-headers/).
+Auth e-mail links resolve through `FrontendOptions` (a dedicated config, separate from CORS). **Self-service** flows (`forgot-password`, `self-register`) link back to the front-end that made the request - the base URL comes from the request `Origin` header, validated against `FrontendOptions:AllowedOrigins` - so with more than one SPA each user gets a link to the app they started from; a forged or unlisted origin is rejected with `400`, and a request with no `Origin` (non-browser callers) falls back to `FrontendOptions:DefaultOrigin`. **Operator-driven** flows (`register`, `resend-confirmation-email`) target `DefaultOrigin` (the recipient's app), so a tenant user provisioned from the admin console gets a link into the tenant app, not the console. The confirmation link lands on the SPA `/confirm-email` page (which then calls `GET /confirm-email`), not the API route directly. Left unset, the host still boots and these links fall back to the API's own origin (`OriginOptions:OriginUrl`, or the request host), which is rarely what you want - configure it, see [CORS & headers](/docs/security/cors-and-headers/).
 </Callout>
 
 | Verb | Route | What it does |

src/content/docs/security/cors-and-headers.mdx

@@ -1,6 +1,6 @@
 ---
 title: CORS & security headers
-lastUpdated: 2026-06-11
+lastUpdated: 2026-08-10
 description: CORS-before-HTTPS-redirect ordering, the SignalR-credentialed-CORS gotcha, and the production security headers the kit emits by default.
 sidebar:
   label: CORS & headers
@@ -54,6 +54,24 @@ Pipeline order (relevant slice):
 6. ...
 ```
 
+## Front-end origin for auth e-mail links
+
+Links that land on a front-end SPA - the password-reset and e-mail-confirmation e-mails - are **not** built from the CORS list. They resolve through a dedicated `FrontendOptions`, kept separate from CORS on purpose: the CORS allowlist governs which browsers may *call* the API, while this list governs which origins may appear *inside an outbound link*. The two often overlap but carry different duties, and coupling them breaks same-origin / reverse-proxy topologies (SPA + API on one domain need no CORS entries, yet the browser still sends `Origin` on the POST).
+
+```jsonc
+  "FrontendOptions": {
+    "AllowedOrigins": [ "http://localhost:5173", "http://localhost:5174" ],
+    "DefaultOrigin": "http://localhost:5174"   // the tenant SPA
+  }
+```
+
+- **Self-service flows** (`forgot-password`, `self-register`) build the link from the request `Origin` header, validated against `AllowedOrigins` and returned as the canonical list entry - so with more than one SPA each user gets a link back to the app they started from. Because forgot-password is anonymous this is a security boundary: a **forged or unlisted `Origin` is rejected with `400`** rather than turned into a link. A request with **no** `Origin` header (curl, the Scalar try-it UI, mobile, server-to-server) falls back to `DefaultOrigin` instead of failing.
+- **Operator-driven flows** (`register`, `resend-confirmation-email`) target `DefaultOrigin` - the recipient's app - not the calling operator's origin, so a tenant user provisioned from the admin console gets a link into the tenant app, not the console.
+- Matching is component-wise (scheme + host + port, port exact). `appsettings.Production.json` ships both settings empty. The host **still starts** without them: it logs one startup `Warning` naming `FrontendOptions:DefaultOrigin`, and link resolution falls back to the API's own origin - `OriginOptions:OriginUrl`, or the request's own host when that is empty too. E-mails keep going out, but their links point at the API rather than your SPA (which is where confirmation links pointed before this resolver existed). A background job, having no request to derive a host from, fails instead. Configure both (see the [production checklist](/docs/security/production-checklist/)) to send users where they expect to land.
+- `DefaultOrigin` is a **single global**, not per-tenant or custom-domain aware, so operator-driven `register` / `resend-confirmation-email` point every tenant's link at that one SPA. That fits the kit's single-dashboard model; a deployment with per-tenant custom domains would need to resolve the recipient tenant's own origin instead.
+
+(`OriginOptions:OriginUrl` has a second, unrelated role: it's the API's own public base for back-end-served assets such as avatar URLs, exposed via `IRequestContext.Origin`.)
+
 ## Why not AllowAnyOrigin for SignalR
 
 CORS spec says: when a response has `Access-Control-Allow-Credentials: true`, the `Access-Control-Allow-Origin` must be an explicit origin, not `*`. SignalR's negotiate request is credentialed (it carries `Cookie` or the JWT via `accessTokenFactory`'s query-param fallback). With `AllowAnyOrigin()`, the server emits `Allow-Origin: *`, which violates the spec - the browser silently refuses to use the response, and SignalR's `HubConnection` fails to start with a confusing CORS error.
@@ -126,23 +144,6 @@ services.ConfigureApplicationCookie(o =>
 
 The cookie should be HttpOnly (no JS access - limits XSS impact), Secure (HTTPS only), and SameSite=Strict for the strongest CSRF defence.
 
-## Front-end origin for auth e-mail links
-
-Links that land on a front-end SPA - the password-reset and e-mail-confirmation e-mails - are **not** built from the CORS list. They resolve through a dedicated `FrontendOptions`, kept separate from CORS on purpose: the CORS allowlist governs which browsers may *call* the API, while this list governs which origins may appear *inside an outbound link*. The two often overlap but carry different duties, and coupling them breaks same-origin / reverse-proxy topologies (SPA + API on one domain need no CORS entries, yet the browser still sends `Origin` on the POST).
-
-```jsonc
-  "FrontendOptions": {
-    "AllowedOrigins": [ "http://localhost:5173", "http://localhost:5174" ],
-    "DefaultOrigin": "http://localhost:5174"   // the tenant SPA
-  }
-```
-
-- **Self-service flows** (`forgot-password`, `self-register`) build the link from the request `Origin` header, validated against `AllowedOrigins` and returned as the canonical list entry - so with more than one SPA each user gets a link back to the app they started from. Because forgot-password is anonymous this is a security boundary: a **forged or unlisted `Origin` is rejected with `400`** rather than turned into a link. A request with **no** `Origin` header (curl, the Scalar try-it UI, mobile, server-to-server) falls back to `DefaultOrigin` instead of failing.
-- **Operator-driven flows** (`register`, `resend-confirmation-email`) target `DefaultOrigin` - the recipient's app - not the calling operator's origin, so a tenant user provisioned from the admin console gets a link into the tenant app, not the console.
-- Matching is component-wise (scheme + host + port, port exact). `appsettings.Production.json` ships both settings empty. The host **still starts** without them: it logs one startup `Warning` naming `FrontendOptions:DefaultOrigin`, and link resolution falls back to the API's own origin (`OriginOptions:OriginUrl`), so e-mails keep going out but point at the API rather than your SPA. Configure them (see the [production checklist](/docs/security/production-checklist/)) to send users where they expect to land.
-
-(`OriginOptions:OriginUrl` has a second, unrelated role: it's the API's own public base for back-end-served assets such as avatar URLs, exposed via `IRequestContext.Origin`.)
-
 ## Common mistakes
 
 - **Setting `AllowAll = true` in production.** CORS exists to give browsers a sanity check on cross-origin calls. Opening to the world removes the check (it doesn't directly compromise auth - auth still gates the request - but it removes the browser-enforced "is this site allowed to call you?" layer).

src/content/docs/security/production-checklist.mdx

@@ -1,6 +1,6 @@
 ---
 title: Production security checklist
-lastUpdated: 2026-06-11
+lastUpdated: 2026-08-10
 description: Ten configuration items you must check before shipping fullstackhero to production. Skip none.
 sidebar:
   label: Production checklist
@@ -59,7 +59,7 @@ Adjust for your industry. Healthcare (HIPAA) and finance (PCI-DSS) tend to requi
 
 `CorsOptions:AllowAll = true` (and the `SetIsOriginAllowed(_ => true)` policy it enables) is **dev only**. Production needs the explicit lists - and note that `appsettings.Production.json` ships `AllowedOrigins` empty, which means **no CORS middleware mounts at all** until you fill it in; your front-ends on other origins will be blocked by the browser. See [CORS & security headers](/docs/security/cors-and-headers/).
 
-Separately, set **`FrontendOptions`** (`AllowedOrigins` + `DefaultOrigin`) - the allowlist and fallback the Identity module uses to build password-reset and e-mail-confirmation links. It ships empty in production too. The host still boots without it, logging a startup `Warning` and falling back to the API's own origin (`OriginOptions:OriginUrl`), so auth e-mails keep arriving but their links point at the API instead of your SPA. `DefaultOrigin` is the tenant SPA: it's the fallback for non-browser callers and the target for operator-driven register/resend links.
+Separately, set **`FrontendOptions`** (`AllowedOrigins` + `DefaultOrigin`) - the allowlist and fallback the Identity module uses to build password-reset and e-mail-confirmation links. It ships empty in production too. The host still boots without it, logging a startup `Warning` and falling back to the API's own origin (`OriginOptions:OriginUrl`, or the request host when that is empty as well), so auth e-mails keep arriving but their links point at the API instead of your SPA. `DefaultOrigin` is the tenant SPA: it's the fallback for non-browser callers and the target for operator-driven register/resend links.
 
 ```jsonc
 {