fix: support Discord sign-in behind a trusted TLS proxy

.env.production.example

@@ -1,5 +1,8 @@
 # Loaded by bun start and bun run db:migrate:production.
 # Set your public origin, e.g. https://habits.example.com
 APP_ORIGIN=
+# Enable only behind a private proxy that overwrites X-Forwarded-Proto.
+TRUST_PROXY=false
+# Docker sets HOST=0.0.0.0; local startup defaults to 127.0.0.1.
 # Optional overrides: PORT, DATABASE_PATH, DISCORD_CLIENT_ID,
 # DISCORD_CLIENT_SECRET, AUTH_COOKIE_SECRET.

src/auth/auth.test.ts

@@ -55,6 +55,32 @@ beforeEach(() => {
 afterEach(() => sqlite.close());
 
 describe("Discord sign-in and sessions", () => {
+  test("trusted TLS proxy supports sign-in and secure callback cookies", async () => {
+    const publicOrigin = "https://habits.example";
+    const proxyApp = createApi(db, { ...config, origin: publicOrigin, trustProxy: true }, request, () => timestamp);
+    const response = await proxyApp.request("http://habits.example/api/auth/discord", { headers: { "X-Forwarded-Proto": "https" } });
+    const location = new URL(response.headers.get("Location")!);
+    expect(location.origin).toBe("https://discord.com");
+    expect(location.searchParams.get("redirect_uri")).toBe(`${publicOrigin}/api/auth/discord/callback`);
+    expect(response.headers.get("Set-Cookie")).toContain("Secure");
+    const callback = await proxyApp.request(`http://habits.example/api/auth/discord/callback?state=${location.searchParams.get("state")}&code=test-code`, {
+      headers: { "X-Forwarded-Proto": "https", Cookie: cookie(response, "__Host-minabot_oauth") },
+    });
+    expect(cookie(callback, "__Host-minabot_session")).not.toBe("");
+    expect(callback.headers.get("Set-Cookie")).toContain("Secure");
+  });
+
+  test("forwarded protocol is opt-in and cannot override the actual hostname", async () => {
+    for (const [trustProxy, host] of [[false, "habits.example"], [true, "wrong.example"]] as const) {
+      const proxyApp = createApi(db, { ...config, origin: "https://habits.example", trustProxy }, request, () => timestamp);
+      const response = await proxyApp.request(`http://${host}/api/auth/discord`, {
+        headers: { "X-Forwarded-Proto": "https", "X-Forwarded-Host": "habits.example" },
+      });
+      expect(new URL(response.headers.get("Location")!).origin).toBe("https://habits.example");
+      expect(response.headers.get("Set-Cookie")).toBeNull();
+    }
+  });
+
   test("me requires a valid session and health stays public", async () => {
     for (const value of ["", "minabot_session=malformed", `minabot_session=${"a".repeat(43)}`]) {
       const response = await app.request(`${origin}/api/me`, { headers: { Cookie: value } });

src/auth/config.ts

@@ -3,6 +3,7 @@ export type AuthConfig = {
   clientId: string;
   clientSecret: string;
   cookieSecret: string;
+  trustProxy?: boolean;
 };
 
 export function readAuthConfig(env = process.env): AuthConfig {
@@ -28,6 +29,7 @@ export function readAuthConfig(env = process.env): AuthConfig {
     clientId: env.DISCORD_CLIENT_ID ?? "",
     clientSecret: env.DISCORD_CLIENT_SECRET ?? "",
     cookieSecret: env.AUTH_COOKIE_SECRET ?? "",
+    trustProxy: env.TRUST_PROXY === "true",
   };
 }
 

src/auth/index.ts

@@ -75,7 +75,11 @@ export function createAuth(db: AppDatabase, config: AuthConfig, request: FetchDi
   routes.get("/discord", async c => {
     if (!isAuthConfigured(config)) return c.redirect("/?auth_error=not_configured");
     // Use one configured origin for redirects and cookie ownership.
-    if (new URL(c.req.url).origin !== config.origin) return c.redirect(`${config.origin}/api/auth/discord?${new URLSearchParams({ timezone: normalizeTimezone(c.req.query("timezone")) })}`);
+    const requestUrl = new URL(c.req.url);
+    // Enable only behind a private proxy that overwrites X-Forwarded-Proto.
+    // Keep the actual Host; never trust a client-supplied forwarded hostname.
+    if (config.trustProxy && c.req.header("X-Forwarded-Proto") === "https") requestUrl.protocol = "https:";
+    if (requestUrl.origin !== config.origin) return c.redirect(`${config.origin}/api/auth/discord?${new URLSearchParams({ timezone: normalizeTimezone(c.req.query("timezone")) })}`);
     const state = randomToken();
     await setSignedCookie(c, stateName, JSON.stringify({
       state,