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+---
+title: Authenticate Server Actions Like API Routes
+impact: CRITICAL
+impactDescription: prevents unauthorized access to server mutations
+tags: server, server-actions, authentication, security, authorization
+---
+
+## Authenticate Server Actions Like API Routes
+
+**Impact: CRITICAL (prevents unauthorized access to server mutations)**
+
+Server Actions (functions with `"use server"`) are exposed as public endpoints, just like API routes. Always verify authentication and authorization **inside** each Server Action—do not rely solely on middleware, layout guards, or page-level checks, as Server Actions can be invoked directly.
+
+Next.js documentation explicitly states: "Treat Server Actions with the same security considerations as public-facing API endpoints, and verify if the user is allowed to perform a mutation."
+
+**Incorrect (no authentication check):**
+
+```typescript
+"use server";
+
+export async function deleteUser(userId: string) {
+  // Anyone can call this! No auth check
+  await db.user.delete({ where: { id: userId } });
+  return { success: true };
+}
+```
+
+**Correct (authentication inside the action):**
+
+```typescript
+"use server";
+
+import { verifySession } from "@/lib/auth";
+import { unauthorized } from "@/lib/errors";
+
+export async function deleteUser(userId: string) {
+  // Always check auth inside the action
+  const session = await verifySession();
+
+  if (!session) {
+    throw unauthorized("Must be logged in");
+  }
+
+  // Check authorization too
+  if (session.user.role !== "admin" && session.user.id !== userId) {
+    throw unauthorized("Cannot delete other users");
+  }
+
+  await db.user.delete({ where: { id: userId } });
+  return { success: true };
+}
+```
+
+**With input validation:**
+
+```typescript
+"use server";
+
+import { verifySession } from "@/lib/auth";
+import { z } from "zod";
+
+const updateProfileSchema = z.object({
+  userId: z.string().uuid(),
+  name: z.string().min(1).max(100),
+  email: z.string().email(),
+});
+
+export async function updateProfile(data: unknown) {
+  // Validate input first
+  const validated = updateProfileSchema.parse(data);
+
+  // Then authenticate
+  const session = await verifySession();
+  if (!session) {
+    throw new Error("Unauthorized");
+  }
+
+  // Then authorize
+  if (session.user.id !== validated.userId) {
+    throw new Error("Can only update own profile");
+  }
+
+  // Finally perform the mutation
+  await db.user.update({
+    where: { id: validated.userId },
+    data: {
+      name: validated.name,
+      email: validated.email,
+    },
+  });
+
+  return { success: true };
+}
+```
+
+Reference: [https://nextjs.org/docs/app/guides/authentication](https://nextjs.org/docs/app/guides/authentication)