diff --git a/assets/js/lib/user.js b/assets/js/lib/user.js
index 7f4f33a1a..a9de85826 100644
--- a/assets/js/lib/user.js
+++ b/assets/js/lib/user.js
@@ -1,9 +1,9 @@
 import { decodeBase64, encodeBase64 } from "./utils";
 
-const USER_DATA_COOKIE = "user_data";
+const USER_DATA_COOKIE = "lb:user_data";
 
 /**
- * Stores user data in the `"user_data"` cookie.
+ * Stores user data in the `"lb:user_data"` cookie.
  */
 export function storeUserData(userData) {
   const json = JSON.stringify(userData);
@@ -12,7 +12,7 @@ export function storeUserData(userData) {
 }
 
 /**
- * Loads user data from the `"user_data"` cookie.
+ * Loads user data from the `"lb:user_data"` cookie.
  */
 export function loadUserData() {
   const encoded = getCookieValue(USER_DATA_COOKIE);
diff --git a/config/dev.exs b/config/dev.exs
index 8becc9eb9..0edd88065 100644
--- a/config/dev.exs
+++ b/config/dev.exs
@@ -8,7 +8,7 @@ import Config
 config :livebook, LivebookWeb.Endpoint,
   # Binding to loopback ipv4 address prevents access from other machines.
   # Change to `ip: {0, 0, 0, 0}` to allow access from other machines.
-  http: [ip: {127, 0, 0, 1}, port: 4000],
+  http: [ip: {127, 0, 0, 1}, port: 4000, protocol_options: [max_header_value_length: 32768]],
   code_reloader: true,
   debug_errors: true,
   check_origin: false,
diff --git a/config/prod.exs b/config/prod.exs
index 2b1572a1c..22220e130 100644
--- a/config/prod.exs
+++ b/config/prod.exs
@@ -2,7 +2,7 @@ import Config
 
 # Default bind and port for production
 config :livebook, LivebookWeb.Endpoint,
-  http: [ip: {127, 0, 0, 1}, port: 8080],
+  http: [ip: {127, 0, 0, 1}, port: 8080, protocol_options: [max_header_value_length: 32768]],
   server: true
 
 config :livebook, :iframe_port, 8081
diff --git a/lib/livebook_web/endpoint.ex b/lib/livebook_web/endpoint.ex
index 56b8bdb4d..c3a8e8ac1 100644
--- a/lib/livebook_web/endpoint.ex
+++ b/lib/livebook_web/endpoint.ex
@@ -6,7 +6,7 @@ defmodule LivebookWeb.Endpoint do
   # Set :encryption_salt if you would also like to encrypt it.
   @session_options [
     store: :cookie,
-    key: "_livebook_key",
+    key: "lb:session",
     signing_salt: "deadbook"
   ]
 
@@ -78,6 +78,7 @@ defmodule LivebookWeb.Endpoint do
   plug Plug.MethodOverride
   plug Plug.Head
   plug :session
+  plug :purge_cookies
 
   # Run custom plugs from the app configuration
   plug LivebookWeb.ConfiguredPlug
@@ -103,6 +104,30 @@ defmodule LivebookWeb.Endpoint do
     end
   end
 
+  # Because we run on localhost, we may accumulate
+  # cookies from several other apps. Our header limit
+  # is set to 32kB. Once we are 75% of said limit,
+  # we clear other cookies to make sure we don't go
+  # over the limit.
+  def purge_cookies(conn, _opts) do
+    cookie_size =
+      conn
+      |> Plug.Conn.get_req_header("cookie")
+      |> Enum.map(&byte_size/1)
+      |> Enum.sum()
+
+    if cookie_size > 24576 do
+      conn.cookies
+      |> Enum.reject(fn {key, _value} -> String.starts_with?(key, "lb:") end)
+      |> Enum.take(10)
+      |> Enum.reduce(conn, fn {key, _value}, conn ->
+        Plug.Conn.delete_resp_cookie(conn, key)
+      end)
+    else
+      conn
+    end
+  end
+
   def access_struct_url() do
     base =
       case struct_url() do
diff --git a/lib/livebook_web/plugs/user_plug.ex b/lib/livebook_web/plugs/user_plug.ex
index 8d71ff86f..1b51d4078 100644
--- a/lib/livebook_web/plugs/user_plug.ex
+++ b/lib/livebook_web/plugs/user_plug.ex
@@ -40,14 +40,14 @@ defmodule LivebookWeb.UserPlug do
   end
 
   defp ensure_user_data(conn) do
-    if Map.has_key?(conn.req_cookies, "user_data") do
+    if Map.has_key?(conn.req_cookies, "lb:user_data") do
       conn
     else
       user_data = user_data(User.new())
       encoded = user_data |> Jason.encode!() |> Base.encode64()
       # Set `http_only` to `false`, so that it can be accessed on the client
       # Set expiration in 5 years
-      put_resp_cookie(conn, "user_data", encoded, http_only: false, max_age: 157_680_000)
+      put_resp_cookie(conn, "lb:user_data", encoded, http_only: false, max_age: 157_680_000)
     end
   end
 
@@ -60,7 +60,7 @@ defmodule LivebookWeb.UserPlug do
   # Copies user_data from cookie to session, so that it's
   # accessible to LiveViews
   defp mirror_user_data_in_session(conn) do
-    user_data = conn.cookies["user_data"] |> Base.decode64!() |> Jason.decode!()
+    user_data = conn.cookies["lb:user_data"] |> Base.decode64!() |> Jason.decode!()
     put_session(conn, :user_data, user_data)
   end
 end
diff --git a/test/livebook_web/plugs/user_plug_test.exs b/test/livebook_web/plugs/user_plug_test.exs
index f689089be..219195976 100644
--- a/test/livebook_web/plugs/user_plug_test.exs
+++ b/test/livebook_web/plugs/user_plug_test.exs
@@ -33,7 +33,7 @@ defmodule LivebookWeb.UserPlugTest do
       |> fetch_cookies()
       |> call()
 
-    assert conn.cookies["user_data"] != nil
+    assert conn.cookies["lb:user_data"] != nil
   end
 
   test "keeps user_data cookie if present" do
@@ -43,10 +43,10 @@ defmodule LivebookWeb.UserPlugTest do
     conn =
       conn(:get, "/")
       |> init_test_session(%{})
-      |> put_req_cookie("user_data", cookie_value)
+      |> put_req_cookie("lb:user_data", cookie_value)
       |> fetch_cookies()
       |> call()
 
-    assert conn.cookies["user_data"] == cookie_value
+    assert conn.cookies["lb:user_data"] == cookie_value
   end
 end