diff --git a/.changeset/server-function-csrf.md b/.changeset/server-function-csrf.md
new file mode 100644
index 000000000..f897d71f4
--- /dev/null
+++ b/.changeset/server-function-csrf.md
@@ -0,0 +1,7 @@
+---
+"@solidjs/start": patch
+---
+
+Reject cross-site server function requests to prevent CSRF.
+
+A `"use server"` function could be invoked by another site with the visitor's cookies, over a GET or a form POST, because the request was not checked. Requests to server functions are now allowed only from the same origin or same site. The check trusts the `Sec-Fetch-Site` header and falls back to comparing `Origin` against the request host, so a cross-site page can no longer trigger a server function. Same-origin calls, user-initiated navigations, and no-JS form submissions are unaffected. A separate origin that needs to call your backend should use an API route with explicit CORS.
diff --git a/packages/start/src/fns/handler.spec.ts b/packages/start/src/fns/handler.spec.ts
index 5ba67d3e2..ac5dedd43 100644
--- a/packages/start/src/fns/handler.spec.ts
+++ b/packages/start/src/fns/handler.spec.ts
@@ -336,3 +336,87 @@ describe("seroval stream response headers", () => {
     expect(h3Event.res.headers.get("content-type")).toBe("text/plain; charset=utf-8");
   });
 });
+
+describe("cross-site request rejection (CSRF)", () => {
+  const call = async (headers: Record<string, string>, method = "POST") => {
+    const request = new Request("http://localhost/_server?id=fn", { method, headers });
+    const h3Event = { res: { headers: new Headers(), status: 200 } };
+    vi.mocked(getFetchEvent).mockReturnValue({
+      request,
+      response: { headers: { getSetCookie: () => [] } },
+      nativeEvent: h3Event,
+      locals: {},
+    } as unknown as FetchEvent);
+    const fn = vi.fn(() => ({ ok: true }));
+    vi.mocked(getServerFunction).mockReturnValue(fn as never);
+    const { handleServerFunction } = await import("./handler.ts");
+    const response = (await handleServerFunction(h3Event as never)) as Response;
+    return { response, fn };
+  };
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("rejects a cross-site request by Sec-Fetch-Site without running the function", async () => {
+    const { response, fn } = await call({
+      "sec-fetch-site": "cross-site",
+      "x-server-instance": "server-fn:1",
+    });
+    expect(response.status).toBe(403);
+    expect(fn).not.toHaveBeenCalled();
+  });
+
+  it("rejects a cross-site GET navigation (Sec-Fetch-Site, no Origin)", async () => {
+    const { response, fn } = await call({ "sec-fetch-site": "cross-site" }, "GET");
+    expect(response.status).toBe(403);
+    expect(fn).not.toHaveBeenCalled();
+  });
+
+  it("rejects when Origin host differs and Sec-Fetch-Site is absent", async () => {
+    const { response, fn } = await call({
+      origin: "https://evil.example",
+      "x-server-instance": "server-fn:1",
+    });
+    expect(response.status).toBe(403);
+    expect(fn).not.toHaveBeenCalled();
+  });
+
+  it("allows a same-origin request", async () => {
+    const { response, fn } = await call({
+      "sec-fetch-site": "same-origin",
+      "x-server-instance": "server-fn:1",
+    });
+    expect(response.status).not.toBe(403);
+    expect(fn).toHaveBeenCalled();
+  });
+
+  it("allows a same-site request", async () => {
+    const { response, fn } = await call({
+      "sec-fetch-site": "same-site",
+      "x-server-instance": "server-fn:1",
+    });
+    expect(fn).toHaveBeenCalled();
+    expect(response.status).not.toBe(403);
+  });
+
+  it("allows a user-initiated navigation (Sec-Fetch-Site: none)", async () => {
+    const { response, fn } = await call({ "sec-fetch-site": "none" }, "GET");
+    expect(fn).toHaveBeenCalled();
+    expect(response.status).not.toBe(403);
+  });
+
+  it("allows a matching Origin when Sec-Fetch-Site is absent", async () => {
+    const { response, fn } = await call({
+      origin: "http://localhost",
+      "x-server-instance": "server-fn:1",
+    });
+    expect(fn).toHaveBeenCalled();
+    expect(response.status).not.toBe(403);
+  });
+
+  it("allows a request with neither header (non-browser client)", async () => {
+    const { fn } = await call({ "x-server-instance": "server-fn:1" });
+    expect(fn).toHaveBeenCalled();
+  });
+});
diff --git a/packages/start/src/fns/handler.ts b/packages/start/src/fns/handler.ts
index bb866b606..b40887fb4 100644
--- a/packages/start/src/fns/handler.ts
+++ b/packages/start/src/fns/handler.ts
@@ -19,14 +19,53 @@ import { applyServerFunctionErrorHandler } from "./error-handler.ts";
 import type { FetchEvent, PageEvent } from "../server/types.ts";
 import { getExpectedRedirectStatus } from "../server/util.ts";
 
+/**
+ * Server functions are same-origin RPC. A cross-site page must not be able to
+ * invoke one with the visitor's cookies, so reject cross-site requests before
+ * the function runs. This is the token-less CSRF defense used by other
+ * frameworks: trust `Sec-Fetch-Site` when the browser sends it, and fall back
+ * to comparing `Origin` against the request host.
+ *
+ * `same-origin` and `same-site` are allowed, matching the reach of a
+ * `SameSite=Lax`/`Strict` cookie. `none` is a user-initiated navigation
+ * (typed URL, bookmark), not a request forged by another site.
+ */
+function isCrossSiteRequest(request: Request, url: URL): boolean {
+  const secFetchSite = request.headers.get("sec-fetch-site");
+  if (secFetchSite) {
+    return secFetchSite === "cross-site";
+  }
+  // Older browsers omit Sec-Fetch-Site. They still send Origin on the
+  // cross-site requests that matter (form and fetch POSTs), so compare it.
+  const origin = request.headers.get("origin");
+  if (origin && origin !== "null") {
+    try {
+      return new URL(origin).host !== url.host;
+    } catch {
+      return true;
+    }
+  }
+  // No Origin either (a same-origin GET, or a non-browser client): nothing to
+  // reject on.
+  return false;
+}
+
 export async function handleServerFunction(h3Event: H3Event) {
   const event = getFetchEvent(h3Event);
   const request = event.request;
 
+  const url = new URL(request.url);
+
+  if (isCrossSiteRequest(request, url)) {
+    return new Response(
+      import.meta.env.DEV ? "Cross-site server function requests are not allowed" : null,
+      { status: 403 },
+    );
+  }
+
   const serverReference = request.headers.get("X-Server-Id");
   const instance = request.headers.get("X-Server-Instance");
   const singleFlight = request.headers.has("X-Single-Flight");
-  const url = new URL(request.url);
   let functionId: string | undefined | null;
   if (serverReference) {
     // invariant(typeof serverReference === "string", "Invalid server function");