diff --git a/.changeset/fix-serve-reject-origin-null.md b/.changeset/fix-serve-reject-origin-null.md
new file mode 100644
index 00000000..4cbff62a
--- /dev/null
+++ b/.changeset/fix-serve-reject-origin-null.md
@@ -0,0 +1,5 @@
+---
+"@stainless-code/codemap": patch
+---
+
+Reject opaque `Origin: null` in `codemap serve` CSRF checks.
diff --git a/src/application/http-server.test.ts b/src/application/http-server.test.ts
index 3a870510..9205ad51 100644
--- a/src/application/http-server.test.ts
+++ b/src/application/http-server.test.ts
@@ -579,14 +579,19 @@ describe("http-server — CSRF + DNS-rebinding guard", () => {
     expect(body.error).toContain("Origin");
   });
 
-  it("allows Origin: null (file:// pages, sandboxed iframes — non-attack vector)", async () => {
+  it("rejects Origin: null (opaque browser contexts)", async () => {
     serverHandle = await startServer();
-    const r = await fetch(`http://127.0.0.1:${serverHandle.port}/tool/query`, {
-      method: "POST",
-      headers: { "Content-Type": "application/json", Origin: "null" },
-      body: JSON.stringify({ sql: "SELECT 1" }),
-    });
-    expect(r.status).toBe(200);
+    const r = await fetch(
+      `http://127.0.0.1:${serverHandle.port}/tool/save_baseline`,
+      {
+        method: "POST",
+        headers: { "Content-Type": "application/json", Origin: "null" },
+        body: JSON.stringify({ name: "csrf-test", sql: "SELECT 1" }),
+      },
+    );
+    expect(r.status).toBe(403);
+    const body = (await r.json()) as { error: string };
+    expect(body.error).toContain("Origin: null");
   });
 
   it("rejects POST with mismatched Host header (DNS rebinding)", async () => {
diff --git a/src/application/http-server.ts b/src/application/http-server.ts
index 02078dce..5e2d1361 100644
--- a/src/application/http-server.ts
+++ b/src/application/http-server.ts
@@ -571,7 +571,8 @@ function validate<T extends ZodRawShape>(
  *    that interface.
  * 3. **`Origin`** — fallback for older browsers that don't send
  *    `Sec-Fetch-Site`. Browsers send `Origin` on every non-GET request
- *    (and most GETs); non-browser clients don't. Reject if present.
+ *    (and most GETs); non-browser clients don't. Reject if present,
+ *    including the opaque value `null` (file://, sandboxed contexts).
  *
  * Returns a reason string (becomes the 403 body) or `undefined` to allow.
  */
@@ -601,7 +602,7 @@ function csrfCheck(
   }
 
   const origin = req.headers.origin;
-  if (origin !== undefined && origin !== "" && origin !== "null") {
+  if (origin !== undefined && origin !== "") {
     return `cross-origin request rejected (Origin: ${origin}). codemap serve does not accept browser-driven cross-origin requests.`;
   }