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docs(android): spec Android App Links (assetlinks.json + build-time host)

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Promote APP_LINKS.md from a deferred design note into an implementation spec
matching the website `feat/mobile-app-links` and android `feat/app-links`
branches: the server-side `/.well-known/assetlinks.json` route + `mobile_app_links_enabled`
toggle + additive redirect-allowlist entry, and the app-side `autoVerify`
intent-filter driven by a build-time `appLinkHost` (a single multi-tenant APK
cannot autoVerify open-ended shard domains, so App Links are a white-label /
first-party build opt-in; the custom scheme stays the permanent fallback).

Update PLAN.md §9 (M9 follow-up) and the §14 open item, and the redirect-URI
allowlist section of website/BACKEND_DESIGN.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NgyHnrNa8WwG3doxvxjuCr
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claude
2026-07-20 18:37:04 -05:00
parent 44544dc3bc
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223
android/APP_LINKS.md
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@@ -1,17 +1,18 @@
# Architecture note — App Links & the multi-tenant callback problem
# Android App Links — implementation spec
Status: **design note; not yet implemented.** Written before the App Links work begins so the
multi-tenancy question is decided on paper first (per the mobile-SSO spec). The native SSO bridge
ships with the **custom-scheme** callback only (`runicgateway://auth/callback`); everything below is
the *later* hardening path and its open design questions.
Status: **implementation spec (M9 follow-up).** Stacks on the native SSO bridge (M9 Part 2):
the app already handles the **custom-scheme** callback `runicgateway://auth/callback`, and that stays
the permanent default and universal fallback. App Links are an **opt-in hardening** layered on top —
a verified `https://` callback that only the domain's real owner can claim.
Read alongside: the "Mobile SSO Authorization Bridge" section of
[`../website/BACKEND_DESIGN.md`](../website/BACKEND_DESIGN.md) (the endpoints/tables), and
[`PLAN.md`](./PLAN.md) §4.2 / §9 (the app milestone).
[`../website/BACKEND_DESIGN.md`](../website/BACKEND_DESIGN.md) (endpoints/tables/allowlist), and
[`PLAN.md`](./PLAN.md) §4.2 / §9 (the app milestone). This spec matches what ships on the
`feat/mobile-app-links` (website) and `feat/app-links` (android) branches.
---
## 1. The problem
## 1. The problem it solves
The mobile SSO bridge redirects the browser back to the app with a one-time code:
@@ -19,12 +20,11 @@ The mobile SSO bridge redirects the browser back to the app with a one-time code
runicgateway://auth/callback?code=…&state=…
```
A **custom URI scheme** is fine for a self-hosted, single-tenant, internal client, but it is *not*
owned by anyone: any other Android app can also register an intent-filter for
`runicgateway://auth/callback` and, if chosen by the user (or if it registers more specifically),
intercept the callback. The code is single-use, PKCE-bound, and short-lived — so an interceptor
still cannot complete `/exchange` without the app's `code_verifier` — but a hijacked callback is
still a denial-of-service and a phishing surface we would rather close.
A **custom URI scheme** is fine for a self-hosted internal client, but it is not *owned* by anyone:
any other Android app can register an intent-filter for `runicgateway://auth/callback` and, if chosen
by the user, intercept the callback. The code is single-use, PKCE-bound (Layer B), and short-lived —
so an interceptor still cannot complete `/exchange` without the app's `code_verifier` — but a hijacked
callback is still a denial-of-service and a phishing surface we would rather close.
**Android App Links** (verified `https://` deep links) close it: the OS only routes an `https://`
link to an app that has proven, via a file served from *that domain*, that it owns the app. An
@@ -40,64 +40,157 @@ Links verification is **per-domain**: the domain must serve
https://<shard-domain>/.well-known/assetlinks.json
```
asserting the Android app's **package name** + **signing-certificate SHA-256 fingerprint**. The one
published app binary (one package name, one signing cert) must therefore be verifiable against
**every** shard domain that wants App Links — a domain set that is open-ended and not known at build
time.
asserting the Android app's **package name** + **signing-certificate SHA-256 fingerprint**.
Two consequences:
That is only half the problem. The other half is an Android platform constraint that decides the whole
shape of the app side:
1. **The shard must serve `assetlinks.json`.** Shard owners will not hand-edit a JSON file with a
cert fingerprint. The website has to **auto-serve** it from an admin setting.
2. **The app must know which shard domain it is paired to** before it can trust an App Link for that
domain. This is a *pairing/bootstrapping* problem, not just a callback-security detail — it is the
part that makes App Links more than a drop-in swap for the custom scheme.
> **`android:autoVerify` needs a *literal* host at build time.** An intent-filter's `<data android:host>`
> is a static string in the merged manifest; there is no "any host" or runtime host. A **single
> published multi-tenant APK therefore cannot autoVerify an open-ended set of shard domains** — the set
> is not known when the APK is built.
## 3. Proposed shape (when we build it)
So App Links here are **not** a drop-in replacement for the custom scheme. They split into two pieces
that ship independently:
### 3.1 Server: auto-served `assetlinks.json`
1. **Server (`assetlinks.json`) — shippable now, benefits any App-Links-capable build.** Every shard
can auto-serve its Digital Asset Links statement behind an admin toggle. This is a pure add and is
implemented on `feat/mobile-app-links`.
2. **App (`autoVerify` intent-filter) — a *build-time* opt-in.** Because the host must be baked in,
App Links are available to:
- a **white-label / first-party build** that bakes one shard's host (`-PappLinkHost=play.myshard.com`);
- a future **canonical relay domain** (`runicgateway.app`, PLAN §14 — *not yet secured*) that all
shards could bounce their final callback through, autoVerified by the generic build.
- One published app ⇒ one package name (`com.runicgateway.app`) and one release signing cert. Its
SHA-256 fingerprint is a **constant of the published app**, not shard-specific.
- Add a website route `GET /.well-known/assetlinks.json` (served at the **web root**, outside
`/api/v1`) that emits the Digital Asset Links statement for that fixed package + fingerprint.
- Gate it behind an admin setting `mobile_app_links_enabled` (default **off**). Off ⇒ the route 404s
and the app stays on the custom scheme for that shard. On ⇒ the shard opts into App Links.
- The fingerprint is the same for every shard, so it can be a shipped constant / env default
(`MOBILE_APP_CERT_SHA256`) rather than something each owner types. The **only** per-shard action is
flipping the setting on.
- When enabled, the shard also registers its `https://<domain>/mobile/callback` URL into the mobile
redirect-URI allowlist (see the bridge's exact-match allowlist) **in addition to** the custom
scheme — the custom scheme is never removed, it is the universal fallback.
The **generic multi-tenant build bakes no host and stays custom-scheme-only** — correct and safe.
### 3.2 App: which domain do I trust?
The custom scheme is never removed. It is the fallback on every build, for every shard, always.
- The app already stores the shard **base URL** it is paired to (first-run connect flow, PLAN §3).
That base URL's host is the *only* domain the app should accept an App Link callback from.
- The intent-filter for `https://…/mobile/callback` cannot be scoped to a runtime host in the
manifest (intent-filters are static). Options, in order of preference:
1. **Custom scheme stays the default**; App Links are an *opt-in* the app only relies on after it
has (a) a paired base URL and (b) confirmed that host serves a valid `assetlinks.json`. Until
both hold, the app requests the custom-scheme `redirect_uri` at `/start`. This keeps a single
code path and avoids trusting an unverified `https` callback.
2. Register a broad `https` autoVerify intent-filter and **reject at runtime** any callback whose
host ≠ the paired base-URL host. AutoVerify only succeeds for domains that actually serve the
file, so in practice only real, opted-in shard domains route to the app; the runtime host check
is defense-in-depth.
- **Decision to make at build time:** whether to ship the `https` autoVerify intent-filter at all in
v1 of the native SSO client, or defer it entirely and ship custom-scheme-only. Given the spec's
guidance ("custom scheme is the practical default; App Links can be layered on per-instance"),
**custom-scheme-only for the first native-SSO release** is the recommended path.
## 3. Server design — `feat/mobile-app-links`
## 4. Recommendation
### 3.1 Auto-served `assetlinks.json`
- **This round:** custom scheme only. No `assetlinks.json` route, no autoVerify intent-filter, no
pairing changes. The bridge's redirect-URI allowlist contains exactly the one fixed
application-owned callback (`runicgateway://auth/callback`).
- **Follow-up (opt-in hardening), only if/when the app is published publicly:** implement §3.1
(auto-served `assetlinks.json` behind an admin toggle) and §3.2 option 1 (App Links relied on only
after the paired host is verified). Keep the custom scheme as the permanent fallback.
- **Route:** `GET /.well-known/assetlinks.json`, served at the **web root** (outside `/api/v1`, before
the SPA catch-all) in `server/src/app.js`.
- **Gate:** the admin setting `mobile_app_links_enabled` (default **off**). Off ⇒ the route **404s** and
the app stays on the custom scheme for that shard. On ⇒ the shard opts into App Links.
- **Body:** the Digital Asset Links statement for the fixed package `com.runicgateway.app` and the
release signing cert SHA-256 fingerprint(s):
Nothing in the bridge's server design has to change to add App Links later: it is purely *more
entries in the redirect-URI allowlist* plus a static file route. That is the point of keeping the
allowlist exact-match and configurable from day one.
```json
[
{
"relation": ["delegate_permission/common.handle_all_urls"],
"target": {
"namespace": "android_app",
"package_name": "com.runicgateway.app",
"sha256_cert_fingerprints": ["AB:CD:…"]
}
}
]
```
- **Fingerprint source:** env `MOBILE_APP_CERT_SHA256` — comma-separated (supports **cert rotation** and
a debug + release cert during testing). It is a **constant of the published app**, identical for every
shard, so it is a shipped/env default, not something each owner types. The package name is likewise
fixed (`MOBILE_APP_PACKAGE`, default `com.runicgateway.app`).
- **Enabled but no fingerprint configured ⇒ 404** (+ a one-time warn): serving a statement with no
fingerprint asserts nothing and would only mislead the verifier.
- Response is `application/json`, `Cache-Control: public, max-age=3600` (the Play verifier and the OS
re-fetch it; it changes only on a cert rotation).
### 3.2 Redirect-URI allowlist extension
`mobileSso.controller` validates the app's `redirect_uri` by **exact match** against
`MOBILE_AUTH_REDIRECT_URIS` (default `runicgateway://auth/callback`). App Links add exactly one more
acceptable value, and **only when the toggle is on**:
- When `mobile_app_links_enabled`, `/start` additionally accepts the **self-origin** HTTPS callback
`https://<request-host>/mobile/callback` (derived from the request/`APP_BASE_URL`, never from
attacker-controlled input). Still **exact match** — never a prefix match.
- The static custom-scheme allowlist is never narrowed; the HTTPS entry is *additive*.
- No new table or schema: the check reads the one boolean setting.
### 3.3 Public settings advertise the capability
`settings.getPublic()` gains `mobileAppLinks: <bool>` (mirrors the toggle) so a client can tell whether
a shard opted in before requesting an HTTPS `redirect_uri` (a white-label build uses it to avoid asking
for a callback the server would reject).
## 4. App design — `feat/app-links`
### 4.1 Build-time host (`appLinkHost`)
- Gradle property `appLinkHost` (default empty). Wired in `app/build.gradle.kts` into **both**:
- `BuildConfig.APP_LINK_HOST` — read by `SsoAuthManager` to decide the redirect;
- `manifestPlaceholders["appLinkHost"]` — substituted into the App Link intent-filter's host.
- **Default (generic build):** empty ⇒ `BuildConfig.APP_LINK_HOST = ""` and the placeholder falls back
to the reserved sentinel `runic-gateway.invalid` (RFC 6761 — never resolves), so the `autoVerify`
filter is **inert**: it matches no real link and verification simply never succeeds. No custom-scheme
behaviour changes.
- **White-label build:** `./gradlew assembleRelease -PappLinkHost=play.myshard.com` bakes that one host
into the filter and enables the HTTPS redirect for that host.
### 4.2 Manifest
A second intent-filter on `MainActivity`, alongside the unchanged custom-scheme one:
```xml
<intent-filter android:autoVerify="true">
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.BROWSABLE" />
<data android:scheme="https"
android:host="${appLinkHost}"
android:path="/mobile/callback" />
</intent-filter>
```
### 4.3 `SsoAuthManager` (pure Kotlin, unit-tested on the JVM)
- **Redirect selection in `buildStartUrl`:** request the HTTPS `redirect_uri`
`https://<pairedHost>/mobile/callback` **iff** `BuildConfig.APP_LINK_HOST` is non-blank *and* equals
the paired base-URL host (case-insensitive); otherwise the fixed custom-scheme `REDIRECT_URI`. A
white-label build that bakes the host is responsible for enabling the server toggle too (§3.2).
- **Verified-callback matcher + host-trust check:** a new `matchesAppLinkCallback(scheme, host, path)`
accepts only `scheme == https`, `path == /mobile/callback`, and **`host == the paired base-URL host`**.
The paired-host equality is defense-in-depth: even though `autoVerify` already means only a real,
opted-in shard domain can route here, the app still refuses any HTTPS callback whose host isn't the
shard it is currently paired to.
- The rest is unchanged: both matchers feed the *same* `complete(state, code, error)` → `/exchange` →
`SessionManager.onSignedIn`. There is no second auth path.
### 4.4 `MainActivity`
`handleSsoCallback` routes a VIEW intent through **`matchesCallback(...) || matchesAppLinkCallback(...)`**;
everything downstream (state check, exchange, sign-in) is shared. Custom-scheme and App Link callbacks
are indistinguishable past the edge.
## 5. Turning it on for a shard
1. Publish/point the app build at the shard host (`-PappLinkHost=<host>`) — or use the generic build and
leave App Links off.
2. Set `MOBILE_APP_CERT_SHA256` (release cert fingerprint) in the website env.
3. Admin → Shard/Settings: enable **App Links** (`mobile_app_links_enabled`).
4. Verify `https://<host>/.well-known/assetlinks.json` returns the statement; confirm Android verifies
(`adb shell pm get-app-links com.runicgateway.app`).
If any step is skipped the app transparently keeps using the custom scheme — nothing breaks.
## 6. Testing
- **Server (`node --test`):** route 404s when the toggle is off; 404s when on but no fingerprint;
returns the correct statement + content-type when on and configured; the redirect allowlist accepts
`https://<host>/mobile/callback` only when enabled and rejects it otherwise (custom scheme always
accepted).
- **App (JVM unit tests):** `matchesAppLinkCallback` accepts only https + `/mobile/callback` + the paired
host and rejects a foreign host / http / wrong path; `buildStartUrl` requests the HTTPS redirect only
when the baked host matches the paired host, else the custom scheme.
## 7. What does *not* change
- The bridge's server design (PKCE Layer A/B, single-use codes, `/start` + `/exchange`) is untouched;
App Links are *one more allowlist entry* + *one static file route*. That is the whole point of keeping
the allowlist exact-match and configurable from day one.
- The custom scheme remains on every build and is the permanent fallback.
- No change to `servuo-plugins/` — App Links are entirely a website ↔ app concern.

17
android/PLAN.md
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@@ -794,8 +794,14 @@ push, and Play (M6–M8) follow the designed app.
`code_verifier`/`code_challenge` + `state`; open the Custom Tab at `/auth/mobile/sso/start`;
verify `state` on the callback; `POST …/exchange`; store the returned pair in the existing
`TokenStore` (M3). No new token-storage or refresh code — it feeds the M3 session machinery.
- **Deferred:** App Links / per-shard `assetlinks.json` / pairing — custom scheme only for now
([`APP_LINKS.md`](./APP_LINKS.md)).
- **Follow-up — App Links (opt-in hardening on top of Part 2):** a website `GET
/.well-known/assetlinks.json` route behind the `mobile_app_links_enabled` admin toggle, a
self-origin HTTPS entry added to the redirect-URI allowlist when enabled, and an app-side
`autoVerify` intent-filter for `https://<host>/mobile/callback` driven by a **build-time**
`appLinkHost` (a single multi-tenant APK cannot autoVerify open-ended shard domains, so the
generic build stays custom-scheme; white-label/first-party builds bake one host). The custom
scheme remains the permanent fallback on every build. Full spec + rollout in
[`APP_LINKS.md`](./APP_LINKS.md).
---
@@ -915,7 +921,12 @@ it is a UX convenience, not a v1 requirement — deferred at M3, descoped at M6;
`runicgateway.app` domain (needed for a verified app-link host and a matching package namespace). Also
the fixed launcher name (baked at build even though in-app branding is per-shard — one APK, any shard).
Since SSO/invite/reset are website-handled, the app mostly *opens* website URLs rather than needing its
own verified app links — confirm whether any deep-link-back is wanted at all for v1.
own verified app links. **App Links resolved (M9 follow-up):** the SSO callback is the one place a
verified deep-link-back helps; the server side (`assetlinks.json` + toggle) ships for any shard, but
the app-side `autoVerify` needs a **literal build-time host**, so it is a white-label/first-party build
opt-in (`-PappLinkHost=<host>`) — the generic multi-tenant build stays custom-scheme. A canonical
`runicgateway.app` relay host, if secured, would let the generic build autoVerify one central domain.
See [`APP_LINKS.md`](./APP_LINKS.md).
- ntfy: exact upstream image + pinned tag (Part-1 landed the compose service — confirm the tag), and
its reverse-proxy hostname/path. The hostname must land in `NTFY_ALLOWED_ORIGINS` before M7 Part 2 is
end-to-end testable (the app registers an endpoint on that origin; the SSRF guard rejects others). No