Bound what a request can make the server hold

Cap JSON bodies at 64 KB on every API route except JMAP and uploads, which
bound themselves. Sign-in used to read a body of any size before its rate
limits ran; the flood ceiling now also runs before the body is read.

For sessions whose JMAP requests are checked, lower the read cap from 16 MB
to 4 MB, allow four such reads per session at once, and turn requests away
with a 503 once 32 MB is held across everyone.

Count sign-in limits per /64 for IPv6, since one host holds a whole /64.

Bind the compose example to loopback, and run it read-only with no
capabilities and no-new-privileges. Keep .env.* out of git and the image
build context.

server/src/request-limits.test.ts

@@ -0,0 +1,115 @@
+import { test, before, after } from "node:test";
+import assert from "node:assert/strict";
+
+/**
+ * How much a request may make the proxy hold in memory.
+ *
+ * Routes that read JSON take a small body and no more, whether or not anyone
+ * is signed in. The JMAP route streams straight through for a session that may
+ * administer; for one that may not, it reads the body to check it, and that
+ * read is capped in size, in how many one session runs at once, and in bytes
+ * across everyone.
+ */
+
+const PORT = 18813;
+process.env.MOCK_PORT = String(PORT);
+process.env.MOCK_USER = "[email protected]";
+process.env.MOCK_PASS = "demo-password";
+process.env.STALWART_URL = `http://127.0.0.1:${PORT}`;
+process.env.APP_SECRET = "test-secret-for-request-limits";
+
+const mock = await import("./mock/index.js");
+const { createApp } = await import("./app.js");
+const { rateLimitKey } = await import("./clientip.js");
+
+const app = createApp();
+const HEADERS = { "content-type": "application/json", "x-requested-with": "ihasmail" };
+let cookie = "";
+
+/** A body that arrives in chunks with no content-length, as a chunked upload does. */
+function chunked(size: number, chunk = 256 * 1024): ReadableStream<Uint8Array> {
+  let sent = 0;
+  return new ReadableStream({
+    pull(controller) {
+      if (sent >= size) return controller.close();
+      const n = Math.min(chunk, size - sent);
+      controller.enqueue(new Uint8Array(n).fill(0x20));
+      sent += n;
+    },
+  });
+}
+
+const jmap = (body: BodyInit) =>
+  app.request("/api/jmap", { method: "POST", headers: { ...HEADERS, cookie }, body, duplex: "half" } as RequestInit);
+
+before(async () => {
+  // Not remembered: a device that is not the person's own, so JMAP is checked.
+  const res = await app.request("/api/auth/login", { method: "POST", headers: HEADERS, body: JSON.stringify({ username: "[email protected]", password: "demo-password" }) });
+  assert.equal(res.status, 200, "login should succeed against the mock");
+  cookie = res.headers.get("set-cookie")!.split(";")[0]!;
+});
+
+after(() => {
+  (mock as { server?: { close(): void } }).server?.close();
+});
+
+test("sign-in refuses a large body by its length, before reading it", async () => {
+  const res = await app.request("/api/auth/login", {
+    method: "POST",
+    headers: { ...HEADERS, "content-length": String(200 * 1024 * 1024) },
+    body: "{}",
+  });
+  assert.equal(res.status, 413);
+});
+
+test("sign-in refuses a large chunked body without holding all of it", async () => {
+  const res = await app.request("/api/auth/login", { method: "POST", headers: HEADERS, body: chunked(2 * 1024 * 1024), duplex: "half" } as RequestInit);
+  assert.equal(res.status, 413);
+});
+
+test("other JSON routes are limited too", async () => {
+  const res = await app.request("/api/account/password", { method: "POST", headers: { ...HEADERS, cookie }, body: chunked(1024 * 1024), duplex: "half" } as RequestInit);
+  assert.equal(res.status, 413);
+});
+
+test("an ordinary checked JMAP request still goes through", async () => {
+  const res = await jmap(JSON.stringify({ using: ["urn:ietf:params:jmap:core", "urn:ietf:params:jmap:mail"], methodCalls: [["Mailbox/get", { accountId: "a1", ids: [] }, "0"]] }));
+  assert.equal(res.status, 200);
+});
+
+test("a JMAP request larger than the check allows is refused", async () => {
+  assert.equal((await jmap(chunked(5 * 1024 * 1024))).status, 413);
+});
+
+test("a JMAP request larger than a sign-in body is not caught by the small-body limit", async () => {
+  // 200 KB of whitespace around a real request: valid JSON, well past 64 KB.
+  const body = `${" ".repeat(200 * 1024)}{"using":["urn:ietf:params:jmap:core"],"methodCalls":[["Core/echo",{},"0"]]}`;
+  assert.equal((await jmap(body)).status, 200);
+});
+
+test("one session cannot hold more than a few checked reads at once", async () => {
+  // Bodies that never finish: each holds its slot until its stream fails.
+  const controllers: ReadableStreamDefaultController<Uint8Array>[] = [];
+  const pending: Promise<Response>[] = [];
+  for (let i = 0; i < 4; i++) {
+    const s = new ReadableStream<Uint8Array>({ start(c) { controllers.push(c); c.enqueue(new TextEncoder().encode("{")); } });
+    pending.push(jmap(s));
+  }
+  await new Promise((r) => setTimeout(r, 50));
+  const fifth = await jmap("{}");
+  assert.equal(fifth.status, 429);
+  assert.ok(fifth.headers.get("retry-after"));
+  for (const c of controllers) c.error(new Error("client went away"));
+  await Promise.allSettled(pending);
+  // The slots are given back once those requests end.
+  const again = await jmap(JSON.stringify({ using: ["urn:ietf:params:jmap:core"], methodCalls: [["Core/echo", {}, "0"]] }));
+  assert.equal(again.status, 200);
+});
+
+test("IPv6 addresses share a rate-limit key across their /64", () => {
+  assert.equal(rateLimitKey("2001:db8:1:2:aaaa::1"), rateLimitKey("2001:db8:1:2:ffff:ffff:ffff:ffff"));
+  assert.notEqual(rateLimitKey("2001:db8:1:2::1"), rateLimitKey("2001:db8:1:3::1"));
+  assert.equal(rateLimitKey("2001:db8:1:2::1"), "2001:db8:1:2::/64");
+  assert.equal(rateLimitKey("198.51.100.7"), "198.51.100.7");
+  assert.equal(rateLimitKey("unknown"), "unknown");
+});