feat(auth): sign in with Steam (MODULE_API 1.17.0)

Steam is a built-in provider beside Google and Discord (STEAM_SIGNIN.md,
D392-D396). OpenID 2.0: the assertion is verified server to server with
check_authentication, return_to must match byte for byte, and the claimed id
must be a 17-digit SteamID64. A Steam Web API key is required to enable it
(D394); persona name and avatar are stored on the identity and refreshed at
each sign-in, and a failed lookup never blocks one. Steam never signs anybody
up (D395), on the web or the mobile bridge. Several Steam accounts may be
linked; DELETE /auth/me/account/identities/:provider takes ?id= to unlink one.

MODULE_API 1.17.0: ctx.identities.list, api.onIdentityLinking (refuse-only,
fails closed, module text never shown) and api.onIdentityLinked. No unlink
hook (D393). Schema additive; OpenAPI, routes and engagement manifests
regenerated with no modules installed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/src/auth/providers/steam.provider.js

@@ -0,0 +1,172 @@
+// Built-in Steam provider (OpenID 2.0, STEAM_SIGNIN.md §3).
+//
+// Steam is not OAuth2: there is no client id, no token endpoint and no `state`
+// parameter. The browser goes to steamcommunity.com with an OpenID 2.0
+// `checkid_setup` request and comes back carrying `openid.*` fields, and the ONLY
+// proof those fields are genuine is asking Steam itself — a server-to-server
+// `check_authentication` POST of the same fields, which must answer
+// `is_valid:true`. Our CSRF `state` rides inside `return_to`, where the existing
+// single-use tx cookie verifies it exactly as it does for the OAuth providers.
+//
+// The admin-configured "secret" is a Steam Web API key (D394), used for one thing:
+// the persona name and avatar shown beside a linked Steam account. That lookup
+// decorates an identity and never decides a sign-in — if it fails, the sign-in
+// goes ahead with the bare id.
+
+const BaseProvider = require('./base.provider')
+const log = require('../../utils/logger')('sso')
+
+const OP_ENDPOINT = 'https://steamcommunity.com/openid/login'
+const OPENID_NS = 'http://specs.openid.net/auth/2.0'
+const IDENTIFIER_SELECT = 'http://specs.openid.net/auth/2.0/identifier_select'
+const CLAIMED_ID_RE = /^https:\/\/steamcommunity\.com\/openid\/id\/(\d{17})$/
+const SUMMARIES_URL = 'https://api.steampowered.com/ISteamUser/GetPlayerSummaries/v2/'
+// Steam serves avatars from its own CDNs. Anything else in that field is not
+// something we render.
+const AVATAR_RE = /^https:\/\/([a-z0-9-]+\.)*(steamstatic\.com|akamaihd\.net)\/[^\s"'<>]{1,400}$/i
+
+const VERIFY_TIMEOUT_MS = 10_000
+const SUMMARY_TIMEOUT_MS = 5_000
+
+// A failure the controller turns into a sentence rather than a 500.
+class SteamAssertionError extends Error {
+  constructor(reason) {
+    super(`steam assertion refused: ${reason}`)
+    this.reason = reason
+  }
+}
+
+// The 17-digit SteamID64 at the end of a claimed id, or null.
+function parseClaimedId(claimedId) {
+  const m = typeof claimedId === 'string' ? CLAIMED_ID_RE.exec(claimedId) : null
+  return m ? m[1] : null
+}
+
+// The `return_to` we send, and must get back byte for byte: the callback URL with
+// our tx nonce as `state`.
+function returnToFor(redirectUri, state) {
+  const sep = redirectUri.includes('?') ? '&' : '?'
+  return `${redirectUri}${sep}state=${encodeURIComponent(state)}`
+}
+
+// The realm is the origin the return_to lives under — what Steam shows the user
+// as "you are signing in to".
+function realmFor(redirectUri) {
+  return new URL(redirectUri).origin
+}
+
+// Every `openid.*` field from a parsed query, as strings. Anything else is ours
+// (`state`) and does not go back to Steam.
+function openidFields(query = {}) {
+  const out = {}
+  for (const [k, v] of Object.entries(query)) {
+    if (k.startsWith('openid.') && typeof v === 'string') out[k] = v
+  }
+  return out
+}
+
+class SteamProvider extends BaseProvider {
+  constructor(config = {}) {
+    super({ kind: 'steam', name: 'Steam', ...config, id: config.id || 'steam' })
+    // Stored in client_secret_enc like any secret; it is a Web API key here.
+    this.apiKey = config.clientSecret ?? config.client_secret ?? null
+  }
+
+  getAuthorizationUrl(state, { redirectUri } = {}) {
+    const params = new URLSearchParams({
+      'openid.ns': OPENID_NS,
+      'openid.mode': 'checkid_setup',
+      'openid.return_to': returnToFor(redirectUri, state),
+      'openid.realm': realmFor(redirectUri),
+      'openid.identity': IDENTIFIER_SELECT,
+      'openid.claimed_id': IDENTIFIER_SELECT,
+    })
+    return `${OP_ENDPOINT}?${params.toString()}`
+  }
+
+  // `query` is the callback's parsed query string; `redirectUri` and `state` are
+  // what we sent, so `return_to` can be matched exactly. Resolves to the usual
+  // { subject, email, emailVerified, name } plus displayName/avatarUrl, or throws
+  // SteamAssertionError.
+  async handleCallback({ query, redirectUri, state } = {}) {
+    const fields = openidFields(query)
+    if (fields['openid.mode'] !== 'id_res') throw new SteamAssertionError('mode')
+    if (fields['openid.ns'] !== OPENID_NS) throw new SteamAssertionError('ns')
+    if (fields['openid.op_endpoint'] !== OP_ENDPOINT) throw new SteamAssertionError('op_endpoint')
+    if (fields['openid.return_to'] !== returnToFor(redirectUri, state)) throw new SteamAssertionError('return_to')
+    const subject = parseClaimedId(fields['openid.claimed_id'])
+    if (!subject) throw new SteamAssertionError('claimed_id')
+    if (fields['openid.identity'] !== fields['openid.claimed_id']) throw new SteamAssertionError('identity')
+
+    if (!(await this.verifyAssertion(fields))) throw new SteamAssertionError('not_valid')
+
+    const summary = await this.playerSummary(subject)
+    return {
+      subject,
+      email: null, // Steam never sends one
+      emailVerified: false,
+      name: summary.displayName,
+      displayName: summary.displayName,
+      avatarUrl: summary.avatarUrl,
+    }
+  }
+
+  // The one proof (§5): send the same fields back with mode check_authentication.
+  // Never skipped; any transport failure is a refusal, not a pass.
+  async verifyAssertion(fields) {
+    const body = new URLSearchParams({ ...fields, 'openid.mode': 'check_authentication' })
+    let text
+    try {
+      const res = await fetch(OP_ENDPOINT, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+        body,
+        signal: AbortSignal.timeout(VERIFY_TIMEOUT_MS),
+      })
+      if (!res.ok) {
+        log.warn('steam check_authentication failed', { status: res.status })
+        return false
+      }
+      text = await res.text()
+    } catch (err) {
+      log.warn('steam check_authentication unreachable', { message: err.message })
+      return false
+    }
+    return /^is_valid:true$/m.test(text)
+  }
+
+  // Persona name + avatar through the Web API key. Decoration only: every failure
+  // answers nulls and the sign-in carries on (D394). The key rides in the URL, so
+  // the URL is never logged.
+  async playerSummary(steamId) {
+    const none = { displayName: null, avatarUrl: null }
+    if (!this.apiKey) return none
+    try {
+      const url = `${SUMMARIES_URL}?key=${encodeURIComponent(this.apiKey)}&steamids=${steamId}`
+      const res = await fetch(url, { headers: { Accept: 'application/json' }, signal: AbortSignal.timeout(SUMMARY_TIMEOUT_MS) })
+      if (!res.ok) {
+        log.warn('steam player summary failed; signing in with the bare id', { status: res.status })
+        return none
+      }
+      const data = await res.json()
+      const player = data && data.response && Array.isArray(data.response.players) ? data.response.players[0] : null
+      if (!player || player.steamid !== steamId) return none
+      const name = typeof player.personaname === 'string' ? player.personaname.trim().slice(0, 191) : ''
+      const avatar = player.avatarfull || player.avatarmedium || player.avatar
+      return {
+        displayName: name || null,
+        avatarUrl: typeof avatar === 'string' && AVATAR_RE.test(avatar) ? avatar : null,
+      }
+    } catch (err) {
+      log.warn('steam player summary unreachable; signing in with the bare id', { message: err.message })
+      return none
+    }
+  }
+}
+
+SteamProvider.SteamAssertionError = SteamAssertionError
+SteamProvider.parseClaimedId = parseClaimedId
+SteamProvider.returnToFor = returnToFor
+SteamProvider.OP_ENDPOINT = OP_ENDPOINT
+
+module.exports = SteamProvider