diff --git a/backend/server.js b/backend/server.js
index 364220a..239eb34 100644
--- a/backend/server.js
+++ b/backend/server.js
@@ -97,6 +97,94 @@ export const io = new Server(server, {
 export const userSockets = new Map() // userId -> Set<socketId>
 export const roomTimers = new Map() // roomId -> { timeLeft, state, intervalId, type }
 export const userLastActive = new Map() // userId -> timestamp
+export const roomMembers = new Map() // roomId -> Map<userId, { username, isGuest, socketIds }>
+export const roomHosts = new Map() // roomId -> userId
+
+const normalizeRoomId = (roomId) =>
+  typeof roomId === "string" ? roomId.trim() : ""
+
+const getRoomMembers = (roomId) => {
+  if (!roomMembers.has(roomId)) {
+    roomMembers.set(roomId, new Map())
+  }
+  return roomMembers.get(roomId)
+}
+
+const emitRoomError = (socket, event, reason) => {
+  socket.emit("roomError", { event, reason })
+}
+
+const canJoinRoom = (socket, roomId) => {
+  if (!roomId) return false
+  if (!socket.isGuest) return true
+  return socket.roomId === roomId
+}
+
+const addRoomMembership = (socket, roomId, userIdStr) => {
+  const members = getRoomMembers(roomId)
+  const existing = members.get(userIdStr)
+
+  if (existing) {
+    existing.socketIds.add(socket.id)
+  } else {
+    members.set(userIdStr, {
+      username: socket.data.username,
+      isGuest: socket.isGuest || false,
+      socketIds: new Set([socket.id]),
+    })
+  }
+
+  if (!socket.isGuest && !roomHosts.has(roomId)) {
+    roomHosts.set(roomId, userIdStr)
+  }
+
+  socket.join(roomId)
+}
+
+const isRoomMember = (roomId, userIdStr) =>
+  Boolean(roomMembers.get(roomId)?.has(userIdStr))
+
+const isRoomHost = (roomId, userIdStr) => roomHosts.get(roomId) === userIdStr
+
+const removeSocketFromRoomState = (socket, userIdStr) => {
+  for (const [roomId, members] of roomMembers.entries()) {
+    const member = members.get(userIdStr)
+    if (!member) continue
+
+    member.socketIds.delete(socket.id)
+    if (member.socketIds.size > 0) continue
+
+    members.delete(userIdStr)
+    if (roomHosts.get(roomId) === userIdStr) {
+      const nextHost = Array.from(members.entries()).find(
+        ([, candidate]) => !candidate.isGuest,
+      )
+      if (nextHost) {
+        roomHosts.set(roomId, nextHost[0])
+      } else {
+        roomHosts.delete(roomId)
+      }
+    }
+
+    if (members.size === 0) {
+      roomMembers.delete(roomId)
+      roomHosts.delete(roomId)
+    }
+  }
+}
+
+const removeRoomMember = (roomId, userIdStr) => {
+  const members = roomMembers.get(roomId)
+  if (!members?.has(userIdStr)) return false
+
+  members.delete(userIdStr)
+  if (members.size === 0) {
+    roomMembers.delete(roomId)
+    roomHosts.delete(roomId)
+  }
+
+  return true
+}
 
 // Socket rate limiting: max 20 concurrent connections per IP
 const socketConnectionCounts = new Map()
@@ -195,7 +283,7 @@ io.on("connection", async (socket) => {
 
     // If guest, auto-join their specific room
     if (socket.isGuest && socket.roomId) {
-      socket.join(socket.roomId)
+      addRoomMembership(socket, socket.roomId, userIdStr)
       socket.to(socket.roomId).emit("userJoinedRoom", {
         userId: userIdStr,
         username: socket.data.username,
@@ -244,14 +332,20 @@ io.on("connection", async (socket) => {
 
   // Room Handlers
   socket.on("joinRoom", (roomId) => {
-    socket.join(roomId)
-    console.log(`User ${userIdStr} joined room ${roomId}`)
-    socket.to(roomId).emit("userJoinedRoom", {
+    const safeRoomId = normalizeRoomId(roomId)
+    if (!canJoinRoom(socket, safeRoomId)) {
+      emitRoomError(socket, "joinRoom", "You are not allowed to join this room.")
+      return
+    }
+
+    addRoomMembership(socket, safeRoomId, userIdStr)
+    console.log(`User ${userIdStr} joined room ${safeRoomId}`)
+    socket.to(safeRoomId).emit("userJoinedRoom", {
       userId: userIdStr,
       username: socket.data.username,
       isGuest: socket.isGuest || false,
     })
-    const timer = roomTimers.get(roomId)
+    const timer = roomTimers.get(safeRoomId)
     if (timer) {
       socket.emit("pomodoroTick", { timeLeft: timer.timeLeft, state: timer.state, type: timer.type })
     }
@@ -287,35 +381,59 @@ io.on("connection", async (socket) => {
   })
 
   socket.on("roomMessage", (data) => {
+    const roomId = normalizeRoomId(data?.roomId)
+    if (!isRoomMember(roomId, userIdStr)) {
+      emitRoomError(socket, "roomMessage", "Join the room before sending messages.")
+      return
+    }
+
     // Ephemeral room message (not saved to DB)
-    io.to(data.roomId).emit("newRoomMessage", {
-      roomId: data.roomId,
+    io.to(roomId).emit("newRoomMessage", {
+      roomId,
       senderId: userIdStr,
       senderName: socket.data.username,
       isGuest: socket.isGuest || false,
-      text: data.text,
+      text: typeof data?.text === "string" ? data.text : "",
       createdAt: new Date().toISOString(),
     })
   })
 
   socket.on("removeGuest", (data) => {
-    // Only host should call this. Data = { roomId, guestId }
-    // In a real app, verify this socket is the host of the room
-    const targetSockets = userSockets.get(data.guestId)
+    const roomId = normalizeRoomId(data?.roomId)
+    const guestId = typeof data?.guestId === "string" ? data.guestId : ""
+
+    if (!isRoomHost(roomId, userIdStr)) {
+      emitRoomError(socket, "removeGuest", "Only the room host can remove guests.")
+      return
+    }
+
+    const target = roomMembers.get(roomId)?.get(guestId)
+    if (!target?.isGuest) {
+      emitRoomError(socket, "removeGuest", "Guest is not in this room.")
+      return
+    }
+
+    removeRoomMember(roomId, guestId)
+    const targetSockets = userSockets.get(guestId)
     if (targetSockets) {
       targetSockets.forEach((socketId) => {
         const targetSocket = io.sockets.sockets.get(socketId)
         if (targetSocket) {
-          targetSocket.leave(data.roomId)
-          targetSocket.emit("removedFromRoom", { roomId: data.roomId })
+          targetSocket.leave(roomId)
+          targetSocket.emit("removedFromRoom", { roomId })
         }
       })
     }
   })
 
   socket.on("pomodoroAction", (data) => {
-    if (socket.isGuest) return; // Only host can control timer
-    const { roomId, action, type } = data;
+    const roomId = normalizeRoomId(data?.roomId)
+    if (!isRoomHost(roomId, userIdStr)) {
+      emitRoomError(socket, "pomodoroAction", "Only the room host can control the timer.")
+      return
+    }
+
+    const { action, type } = data;
     let timer = roomTimers.get(roomId);
 
     if (action === "start") {
@@ -370,6 +488,7 @@ io.on("connection", async (socket) => {
       userLastActive.set(userIdStr, Date.now())
       console.log(`User disconnected: ${userIdStr} (socket: ${socket.id})`)
     }
+    removeSocketFromRoomState(socket, userIdStr)
     io.emit("getOnlineUsers", Array.from(userSockets.keys()))
   })
 })
@@ -437,3 +556,4 @@ if (isVercel()) {
 
 // Export for potential use in integration tests (e.g., supertest)
 export default app
+export { server }
diff --git a/backend/tests/roomSocketAuth.test.js b/backend/tests/roomSocketAuth.test.js
new file mode 100644
index 0000000..94eaaf1
--- /dev/null
+++ b/backend/tests/roomSocketAuth.test.js
@@ -0,0 +1,153 @@
+import { afterAll, beforeAll, beforeEach, describe, expect, it } from "vitest"
+import { io as Client } from "socket.io-client"
+import jwt from "jsonwebtoken"
+import mongoose from "mongoose"
+import {
+  server,
+  roomHosts,
+  roomMembers,
+  roomTimers,
+  userSockets,
+} from "../server.js"
+
+const waitFor = (socket, event) =>
+  new Promise((resolve) => {
+    socket.once(event, resolve)
+  })
+
+const waitForTick = () => new Promise((resolve) => setTimeout(resolve, 25))
+
+const closeSocket = (socket) =>
+  new Promise((resolve) => {
+    if (!socket?.connected) {
+      socket?.close()
+      resolve()
+      return
+    }
+    socket.once("disconnect", resolve)
+    socket.close()
+  })
+
+const connectSocket = async (url, payload) => {
+  const token = jwt.sign(payload, process.env.JWT_SECRET)
+  const socket = Client(url, {
+    auth: { token },
+    forceNew: true,
+    reconnection: false,
+    transports: ["websocket"],
+  })
+  await waitFor(socket, "connect")
+  return socket
+}
+
+describe("room socket authorization", () => {
+  let baseUrl
+  let hostSocket
+  let guestSocket
+  let outsiderSocket
+
+  beforeAll(async () => {
+    await new Promise((resolve) => {
+      server.listen(0, resolve)
+    })
+    const address = server.address()
+    baseUrl = `http://127.0.0.1:${address.port}`
+  })
+
+  afterAll(async () => {
+    await Promise.all(
+      [hostSocket, guestSocket, outsiderSocket].filter(Boolean).map(closeSocket),
+    )
+    await new Promise((resolve) => server.close(resolve))
+  })
+
+  beforeEach(async () => {
+    await Promise.all(
+      [hostSocket, guestSocket, outsiderSocket].filter(Boolean).map(closeSocket),
+    )
+    hostSocket = null
+    guestSocket = null
+    outsiderSocket = null
+    roomHosts.clear()
+    roomMembers.clear()
+    roomTimers.clear()
+    userSockets.clear()
+  })
+
+  it("rejects guest attempts to join arbitrary rooms", async () => {
+    guestSocket = await connectSocket(baseUrl, {
+      id: "guest-allowed",
+      name: "Guest Allowed",
+      isGuest: true,
+      roomId: "allowed-room",
+    })
+
+    const errorPromise = waitFor(guestSocket, "roomError")
+    guestSocket.emit("joinRoom", "other-room")
+
+    await expect(errorPromise).resolves.toEqual({
+      event: "joinRoom",
+      reason: "You are not allowed to join this room.",
+    })
+    expect(roomMembers.get("allowed-room")?.has("guest-allowed")).toBe(true)
+    expect(roomMembers.get("other-room")?.has("guest-allowed")).toBeFalsy()
+  })
+
+  it("rejects room messages from sockets that are not room members", async () => {
+    const hostId = new mongoose.Types.ObjectId().toString()
+    const outsiderId = new mongoose.Types.ObjectId().toString()
+    hostSocket = await connectSocket(baseUrl, { id: hostId })
+    outsiderSocket = await connectSocket(baseUrl, { id: outsiderId })
+
+    hostSocket.emit("joinRoom", "secure-room")
+    await waitForTick()
+
+    const errorPromise = waitFor(outsiderSocket, "roomError")
+    outsiderSocket.emit("roomMessage", {
+      roomId: "secure-room",
+      text: "I should not be delivered",
+    })
+
+    await expect(errorPromise).resolves.toEqual({
+      event: "roomMessage",
+      reason: "Join the room before sending messages.",
+    })
+  })
+
+  it("allows only the room host to remove guest members", async () => {
+    const hostId = new mongoose.Types.ObjectId().toString()
+    const outsiderId = new mongoose.Types.ObjectId().toString()
+    hostSocket = await connectSocket(baseUrl, { id: hostId })
+    outsiderSocket = await connectSocket(baseUrl, { id: outsiderId })
+    guestSocket = await connectSocket(baseUrl, {
+      id: "guest-removable",
+      name: "Guest Removable",
+      isGuest: true,
+      roomId: "host-room",
+    })
+
+    hostSocket.emit("joinRoom", "host-room")
+    await waitForTick()
+
+    const deniedPromise = waitFor(outsiderSocket, "roomError")
+    outsiderSocket.emit("removeGuest", {
+      roomId: "host-room",
+      guestId: "guest-removable",
+    })
+
+    await expect(deniedPromise).resolves.toEqual({
+      event: "removeGuest",
+      reason: "Only the room host can remove guests.",
+    })
+    expect(roomMembers.get("host-room")?.has("guest-removable")).toBe(true)
+
+    const removedPromise = waitFor(guestSocket, "removedFromRoom")
+    hostSocket.emit("removeGuest", {
+      roomId: "host-room",
+      guestId: "guest-removable",
+    })
+
+    await expect(removedPromise).resolves.toEqual({ roomId: "host-room" })
+    expect(roomMembers.get("host-room")?.has("guest-removable")).toBeFalsy()
+  })
+})