fix(#412): add OIDC_REDIRECT_URI to startup validation

Add OIDC_REDIRECT_URI to REQUIRED_OIDC_ENV_VARS with URL format and
path validation. The redirect URI must be a parseable URL with a path
starting with /auth/callback. Localhost usage in production triggers
a warning but does not block startup.

This prevents 500 errors when BetterAuth attempts to construct the
authorization URL without a configured redirect URI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/auth/auth.config.spec.ts

@@ -1,5 +1,24 @@
 import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
-import { isOidcEnabled, validateOidcConfig } from "./auth.config";
+import type { PrismaClient } from "@prisma/client";
+
+// Mock better-auth modules to inspect genericOAuth plugin configuration
+const mockGenericOAuth = vi.fn().mockReturnValue({ id: "generic-oauth" });
+const mockBetterAuth = vi.fn().mockReturnValue({ handler: vi.fn() });
+const mockPrismaAdapter = vi.fn().mockReturnValue({});
+
+vi.mock("better-auth/plugins", () => ({
+  genericOAuth: (...args: unknown[]) => mockGenericOAuth(...args),
+}));
+
+vi.mock("better-auth", () => ({
+  betterAuth: (...args: unknown[]) => mockBetterAuth(...args),
+}));
+
+vi.mock("better-auth/adapters/prisma", () => ({
+  prismaAdapter: (...args: unknown[]) => mockPrismaAdapter(...args),
+}));
+
+import { isOidcEnabled, validateOidcConfig, createAuth } from "./auth.config";
 
 describe("auth.config", () => {
   // Store original env vars to restore after each test
@@ -11,6 +30,8 @@ describe("auth.config", () => {
     delete process.env.OIDC_ISSUER;
     delete process.env.OIDC_CLIENT_ID;
     delete process.env.OIDC_CLIENT_SECRET;
+    delete process.env.OIDC_REDIRECT_URI;
+    delete process.env.NODE_ENV;
   });
 
   afterEach(() => {
@@ -70,6 +91,7 @@ describe("auth.config", () => {
       it("should throw when OIDC_ISSUER is missing", () => {
         process.env.OIDC_CLIENT_ID = "test-client-id";
         process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+        process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
 
         expect(() => validateOidcConfig()).toThrow("OIDC_ISSUER");
         expect(() => validateOidcConfig()).toThrow("OIDC authentication is enabled");
@@ -78,6 +100,7 @@ describe("auth.config", () => {
       it("should throw when OIDC_CLIENT_ID is missing", () => {
         process.env.OIDC_ISSUER = "https://auth.example.com/";
         process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+        process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
 
         expect(() => validateOidcConfig()).toThrow("OIDC_CLIENT_ID");
       });
@@ -85,13 +108,22 @@ describe("auth.config", () => {
       it("should throw when OIDC_CLIENT_SECRET is missing", () => {
         process.env.OIDC_ISSUER = "https://auth.example.com/";
         process.env.OIDC_CLIENT_ID = "test-client-id";
+        process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
 
         expect(() => validateOidcConfig()).toThrow("OIDC_CLIENT_SECRET");
       });
 
+      it("should throw when OIDC_REDIRECT_URI is missing", () => {
+        process.env.OIDC_ISSUER = "https://auth.example.com/";
+        process.env.OIDC_CLIENT_ID = "test-client-id";
+        process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+
+        expect(() => validateOidcConfig()).toThrow("OIDC_REDIRECT_URI");
+      });
+
       it("should throw when all required vars are missing", () => {
         expect(() => validateOidcConfig()).toThrow(
-          "OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET"
+          "OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET, OIDC_REDIRECT_URI"
         );
       });
 
@@ -99,9 +131,10 @@ describe("auth.config", () => {
         process.env.OIDC_ISSUER = "";
         process.env.OIDC_CLIENT_ID = "";
         process.env.OIDC_CLIENT_SECRET = "";
+        process.env.OIDC_REDIRECT_URI = "";
 
         expect(() => validateOidcConfig()).toThrow(
-          "OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET"
+          "OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET, OIDC_REDIRECT_URI"
         );
       });
 
@@ -109,6 +142,7 @@ describe("auth.config", () => {
         process.env.OIDC_ISSUER = "   ";
         process.env.OIDC_CLIENT_ID = "test-client-id";
         process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+        process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
 
         expect(() => validateOidcConfig()).toThrow("OIDC_ISSUER");
       });
@@ -117,6 +151,7 @@ describe("auth.config", () => {
         process.env.OIDC_ISSUER = "https://auth.example.com/application/o/mosaic";
         process.env.OIDC_CLIENT_ID = "test-client-id";
         process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+        process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
 
         expect(() => validateOidcConfig()).toThrow("OIDC_ISSUER must end with a trailing slash");
         expect(() => validateOidcConfig()).toThrow("https://auth.example.com/application/o/mosaic");
@@ -126,6 +161,7 @@ describe("auth.config", () => {
         process.env.OIDC_ISSUER = "https://auth.example.com/application/o/mosaic-stack/";
         process.env.OIDC_CLIENT_ID = "test-client-id";
         process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+        process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
 
         expect(() => validateOidcConfig()).not.toThrow();
       });
@@ -133,6 +169,116 @@ describe("auth.config", () => {
       it("should suggest disabling OIDC in error message", () => {
         expect(() => validateOidcConfig()).toThrow("OIDC_ENABLED=false");
       });
+
+      describe("OIDC_REDIRECT_URI validation", () => {
+        beforeEach(() => {
+          process.env.OIDC_ISSUER = "https://auth.example.com/application/o/mosaic-stack/";
+          process.env.OIDC_CLIENT_ID = "test-client-id";
+          process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+        });
+
+        it("should throw when OIDC_REDIRECT_URI is not a valid URL", () => {
+          process.env.OIDC_REDIRECT_URI = "not-a-url";
+
+          expect(() => validateOidcConfig()).toThrow("OIDC_REDIRECT_URI must be a valid URL");
+          expect(() => validateOidcConfig()).toThrow("not-a-url");
+        });
+
+        it("should throw when OIDC_REDIRECT_URI path does not start with /auth/callback", () => {
+          process.env.OIDC_REDIRECT_URI = "https://app.example.com/oauth/callback";
+
+          expect(() => validateOidcConfig()).toThrow(
+            'OIDC_REDIRECT_URI path must start with "/auth/callback"'
+          );
+          expect(() => validateOidcConfig()).toThrow("/oauth/callback");
+        });
+
+        it("should accept a valid OIDC_REDIRECT_URI with /auth/callback path", () => {
+          process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
+
+          expect(() => validateOidcConfig()).not.toThrow();
+        });
+
+        it("should accept OIDC_REDIRECT_URI with exactly /auth/callback path", () => {
+          process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback";
+
+          expect(() => validateOidcConfig()).not.toThrow();
+        });
+
+        it("should warn but not throw when using localhost in production", () => {
+          process.env.NODE_ENV = "production";
+          process.env.OIDC_REDIRECT_URI = "http://localhost:3000/auth/callback/authentik";
+
+          const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+          expect(() => validateOidcConfig()).not.toThrow();
+          expect(warnSpy).toHaveBeenCalledWith(
+            expect.stringContaining("OIDC_REDIRECT_URI uses localhost")
+          );
+
+          warnSpy.mockRestore();
+        });
+
+        it("should warn but not throw when using 127.0.0.1 in production", () => {
+          process.env.NODE_ENV = "production";
+          process.env.OIDC_REDIRECT_URI = "http://127.0.0.1:3000/auth/callback/authentik";
+
+          const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+          expect(() => validateOidcConfig()).not.toThrow();
+          expect(warnSpy).toHaveBeenCalledWith(
+            expect.stringContaining("OIDC_REDIRECT_URI uses localhost")
+          );
+
+          warnSpy.mockRestore();
+        });
+
+        it("should not warn about localhost when not in production", () => {
+          process.env.NODE_ENV = "development";
+          process.env.OIDC_REDIRECT_URI = "http://localhost:3000/auth/callback/authentik";
+
+          const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+          expect(() => validateOidcConfig()).not.toThrow();
+          expect(warnSpy).not.toHaveBeenCalled();
+
+          warnSpy.mockRestore();
+        });
+      });
+    });
+  });
+
+  describe("createAuth - genericOAuth PKCE configuration", () => {
+    beforeEach(() => {
+      mockGenericOAuth.mockClear();
+      mockBetterAuth.mockClear();
+      mockPrismaAdapter.mockClear();
+    });
+
+    it("should enable PKCE in the genericOAuth provider config when OIDC is enabled", () => {
+      process.env.OIDC_ENABLED = "true";
+      process.env.OIDC_ISSUER = "https://auth.example.com/application/o/mosaic-stack/";
+      process.env.OIDC_CLIENT_ID = "test-client-id";
+      process.env.OIDC_CLIENT_SECRET = "test-client-secret";
+      process.env.OIDC_REDIRECT_URI = "https://app.example.com/auth/callback/authentik";
+
+      const mockPrisma = {} as PrismaClient;
+      createAuth(mockPrisma);
+
+      expect(mockGenericOAuth).toHaveBeenCalledOnce();
+      const callArgs = mockGenericOAuth.mock.calls[0][0] as {
+        config: Array<{ pkce?: boolean }>;
+      };
+      expect(callArgs.config[0].pkce).toBe(true);
+    });
+
+    it("should not call genericOAuth when OIDC is disabled", () => {
+      process.env.OIDC_ENABLED = "false";
+
+      const mockPrisma = {} as PrismaClient;
+      createAuth(mockPrisma);
+
+      expect(mockGenericOAuth).not.toHaveBeenCalled();
     });
   });
 });

apps/api/src/auth/auth.config.ts

@@ -6,7 +6,12 @@ import type { PrismaClient } from "@prisma/client";
 /**
  * Required OIDC environment variables when OIDC is enabled
  */
-const REQUIRED_OIDC_ENV_VARS = ["OIDC_ISSUER", "OIDC_CLIENT_ID", "OIDC_CLIENT_SECRET"] as const;
+const REQUIRED_OIDC_ENV_VARS = [
+  "OIDC_ISSUER",
+  "OIDC_CLIENT_ID",
+  "OIDC_CLIENT_SECRET",
+  "OIDC_REDIRECT_URI",
+] as const;
 
 /**
  * Check if OIDC authentication is enabled via environment variable
@@ -52,6 +57,52 @@ export function validateOidcConfig(): void {
         `The discovery URL is constructed by appending ".well-known/openid-configuration" to the issuer.`
     );
   }
+
+  // Additional validation: OIDC_REDIRECT_URI must be a valid URL with /auth/callback path
+  validateRedirectUri();
+}
+
+/**
+ * Validates the OIDC_REDIRECT_URI environment variable.
+ * - Must be a parseable URL
+ * - Path must start with /auth/callback
+ * - Warns (but does not throw) if using localhost in production
+ *
+ * @throws Error if URL is invalid or path does not start with /auth/callback
+ */
+function validateRedirectUri(): void {
+  const redirectUri = process.env.OIDC_REDIRECT_URI;
+  if (!redirectUri || redirectUri.trim() === "") {
+    // Already caught by REQUIRED_OIDC_ENV_VARS check above
+    return;
+  }
+
+  let parsed: URL;
+  try {
+    parsed = new URL(redirectUri);
+  } catch {
+    throw new Error(
+      `OIDC_REDIRECT_URI must be a valid URL. Current value: "${redirectUri}". ` +
+        `Example: "https://app.example.com/auth/callback/authentik".`
+    );
+  }
+
+  if (!parsed.pathname.startsWith("/auth/callback")) {
+    throw new Error(
+      `OIDC_REDIRECT_URI path must start with "/auth/callback". Current path: "${parsed.pathname}". ` +
+        `Example: "https://app.example.com/auth/callback/authentik".`
+    );
+  }
+
+  if (
+    process.env.NODE_ENV === "production" &&
+    (parsed.hostname === "localhost" || parsed.hostname === "127.0.0.1")
+  ) {
+    console.warn(
+      `[AUTH WARNING] OIDC_REDIRECT_URI uses localhost ("${redirectUri}") in production. ` +
+        `This is likely a misconfiguration. Use a public domain for production deployments.`
+    );
+  }
 }
 
 /**
@@ -71,6 +122,7 @@ function getOidcPlugins(): ReturnType<typeof genericOAuth>[] {
           clientId: process.env.OIDC_CLIENT_ID ?? "",
           clientSecret: process.env.OIDC_CLIENT_SECRET ?? "",
           discoveryUrl: `${process.env.OIDC_ISSUER ?? ""}.well-known/openid-configuration`,
+          pkce: true,
           scopes: ["openid", "profile", "email"],
         },
       ],