diff --git a/apps/api/src/cors.spec.ts b/apps/api/src/cors.spec.ts
index 03bacff..b86928e 100644
--- a/apps/api/src/cors.spec.ts
+++ b/apps/api/src/cors.spec.ts
@@ -10,12 +10,59 @@ import { describe, it, expect } from "vitest";
  * - origin: must be specific origins, NOT wildcard (security requirement with credentials)
  * - Access-Control-Allow-Credentials: true header
  * - Access-Control-Allow-Origin: specific origin (not *)
+ * - No-origin requests blocked in production (SEC-API-26)
  */
 
+/**
+ * Replicates the CORS origin validation logic from main.ts
+ * so we can test it in isolation.
+ */
+function buildOriginValidator(nodeEnv: string | undefined): {
+  allowedOrigins: string[];
+  isDevelopment: boolean;
+  validate: (
+    origin: string | undefined,
+    callback: (err: Error | null, allow?: boolean) => void
+  ) => void;
+} {
+  const isDevelopment = nodeEnv !== "production";
+
+  const allowedOrigins = [
+    process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000",
+    "https://app.mosaicstack.dev",
+    "https://api.mosaicstack.dev",
+  ];
+
+  if (isDevelopment) {
+    allowedOrigins.push("http://localhost:3001");
+  }
+
+  const validate = (
+    origin: string | undefined,
+    callback: (err: Error | null, allow?: boolean) => void
+  ): void => {
+    if (!origin) {
+      if (isDevelopment) {
+        callback(null, true);
+      } else {
+        callback(new Error("CORS: Origin header is required"));
+      }
+      return;
+    }
+
+    if (allowedOrigins.includes(origin)) {
+      callback(null, true);
+    } else {
+      callback(new Error(`Origin ${origin} not allowed by CORS`));
+    }
+  };
+
+  return { allowedOrigins, isDevelopment, validate };
+}
+
 describe("CORS Configuration", () => {
   describe("Configuration requirements", () => {
     it("should document required CORS settings for cookie-based auth", () => {
-      // This test documents the requirements
       const requiredSettings = {
         origin: ["http://localhost:3000", "https://app.mosaicstack.dev"],
         credentials: true,
@@ -30,35 +77,25 @@ describe("CORS Configuration", () => {
     });
 
     it("should NOT use wildcard origin with credentials (security violation)", () => {
-      // Wildcard origin with credentials is a security violation
-      // This test ensures we never use that combination
       const validConfig1 = { origin: "*", credentials: false };
       const validConfig2 = { origin: "http://localhost:3000", credentials: true };
       const invalidConfig = { origin: "*", credentials: true };
 
-      // Valid configs
       expect(validConfig1.origin === "*" && !validConfig1.credentials).toBe(true);
       expect(validConfig2.origin !== "*" && validConfig2.credentials).toBe(true);
 
-      // Invalid config check - this combination should NOT be allowed
       const isInvalidCombination = invalidConfig.origin === "*" && invalidConfig.credentials;
-      expect(isInvalidCombination).toBe(true); // This IS an invalid combination
-      // We will prevent this in our CORS config
+      expect(isInvalidCombination).toBe(true);
     });
   });
 
   describe("Origin validation", () => {
     it("should define allowed origins list", () => {
-      const allowedOrigins = [
-        process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000",
-        "http://localhost:3001", // API origin (dev)
-        "https://app.mosaicstack.dev", // Production web
-        "https://api.mosaicstack.dev", // Production API
-      ];
+      const { allowedOrigins } = buildOriginValidator("development");
 
-      expect(allowedOrigins).toHaveLength(4);
       expect(allowedOrigins).toContain("http://localhost:3000");
       expect(allowedOrigins).toContain("https://app.mosaicstack.dev");
+      expect(allowedOrigins).toContain("https://api.mosaicstack.dev");
     });
 
     it("should match exact origins, not partial matches", () => {
@@ -77,4 +114,124 @@ describe("CORS Configuration", () => {
       expect(typeof envOrigin).toBe("string");
     });
   });
+
+  describe("Development mode CORS behavior", () => {
+    it("should allow requests with no origin in development", () => {
+      const { validate } = buildOriginValidator("development");
+
+      return new Promise<void>((resolve) => {
+        validate(undefined, (err, allow) => {
+          expect(err).toBeNull();
+          expect(allow).toBe(true);
+          resolve();
+        });
+      });
+    });
+
+    it("should include localhost:3001 in development origins", () => {
+      const { allowedOrigins } = buildOriginValidator("development");
+
+      expect(allowedOrigins).toContain("http://localhost:3001");
+    });
+
+    it("should allow valid origins in development", () => {
+      const { validate } = buildOriginValidator("development");
+
+      return new Promise<void>((resolve) => {
+        validate("http://localhost:3000", (err, allow) => {
+          expect(err).toBeNull();
+          expect(allow).toBe(true);
+          resolve();
+        });
+      });
+    });
+
+    it("should reject invalid origins in development", () => {
+      const { validate } = buildOriginValidator("development");
+
+      return new Promise<void>((resolve) => {
+        validate("http://evil.com", (err) => {
+          expect(err).toBeInstanceOf(Error);
+          expect(err?.message).toContain("not allowed by CORS");
+          resolve();
+        });
+      });
+    });
+  });
+
+  describe("Production mode CORS behavior (SEC-API-26)", () => {
+    it("should reject requests with no origin in production", () => {
+      const { validate } = buildOriginValidator("production");
+
+      return new Promise<void>((resolve) => {
+        validate(undefined, (err) => {
+          expect(err).toBeInstanceOf(Error);
+          expect(err?.message).toBe("CORS: Origin header is required");
+          resolve();
+        });
+      });
+    });
+
+    it("should NOT include localhost:3001 in production origins", () => {
+      const { allowedOrigins } = buildOriginValidator("production");
+
+      expect(allowedOrigins).not.toContain("http://localhost:3001");
+    });
+
+    it("should allow valid production origins", () => {
+      const { validate } = buildOriginValidator("production");
+
+      return new Promise<void>((resolve) => {
+        validate("https://app.mosaicstack.dev", (err, allow) => {
+          expect(err).toBeNull();
+          expect(allow).toBe(true);
+          resolve();
+        });
+      });
+    });
+
+    it("should reject invalid origins in production", () => {
+      const { validate } = buildOriginValidator("production");
+
+      return new Promise<void>((resolve) => {
+        validate("http://evil.com", (err) => {
+          expect(err).toBeInstanceOf(Error);
+          expect(err?.message).toContain("not allowed by CORS");
+          resolve();
+        });
+      });
+    });
+
+    it("should reject malicious origins that try partial matching", () => {
+      const { validate } = buildOriginValidator("production");
+
+      return new Promise<void>((resolve) => {
+        validate("https://app.mosaicstack.dev.evil.com", (err) => {
+          expect(err).toBeInstanceOf(Error);
+          expect(err?.message).toContain("not allowed by CORS");
+          resolve();
+        });
+      });
+    });
+  });
+
+  describe("ValidationPipe strict mode (SEC-API-25)", () => {
+    it("should document that forbidNonWhitelisted must be true", () => {
+      // This verifies the configuration intent:
+      // forbidNonWhitelisted: true rejects requests with unknown properties
+      // preventing mass-assignment vulnerabilities
+      const validationPipeConfig = {
+        transform: true,
+        whitelist: true,
+        forbidNonWhitelisted: true,
+        transformOptions: {
+          enableImplicitConversion: false,
+        },
+      };
+
+      expect(validationPipeConfig.forbidNonWhitelisted).toBe(true);
+      expect(validationPipeConfig.whitelist).toBe(true);
+      expect(validationPipeConfig.transformOptions.enableImplicitConversion).toBe(false);
+    });
+  });
 });
diff --git a/apps/api/src/main.ts b/apps/api/src/main.ts
index a32e51a..a706457 100644
--- a/apps/api/src/main.ts
+++ b/apps/api/src/main.ts
@@ -37,7 +37,7 @@ async function bootstrap() {
     new ValidationPipe({
       transform: true,
       whitelist: true,
-      forbidNonWhitelisted: false,
+      forbidNonWhitelisted: true,
       transformOptions: {
         enableImplicitConversion: false,
       },
@@ -48,21 +48,32 @@ async function bootstrap() {
 
   // Configure CORS for cookie-based authentication
   // SECURITY: Cannot use wildcard (*) with credentials: true
+  const isDevelopment = process.env.NODE_ENV !== "production";
+
   const allowedOrigins = [
     process.env.NEXT_PUBLIC_APP_URL ?? "http://localhost:3000",
-    "http://localhost:3001", // API origin (dev)
     "https://app.mosaicstack.dev", // Production web
     "https://api.mosaicstack.dev", // Production API
   ];
 
+  // Development-only origins (not allowed in production)
+  if (isDevelopment) {
+    allowedOrigins.push("http://localhost:3001"); // API origin (dev)
+  }
+
   app.enableCors({
     origin: (
       origin: string | undefined,
       callback: (err: Error | null, allow?: boolean) => void
     ): void => {
-      // Allow requests with no origin (e.g., mobile apps, Postman)
+      // SECURITY: In production, reject requests with no Origin header.
+      // In development, allow no-origin requests (Postman, curl, mobile apps).
       if (!origin) {
-        callback(null, true);
+        if (isDevelopment) {
+          callback(null, true);
+        } else {
+          callback(new Error("CORS: Origin header is required"));
+        }
         return;
       }