diff --git a/lib/philomena_web/plugs/compromised_password_check_plug.ex b/lib/philomena_web/plugs/compromised_password_check_plug.ex
new file mode 100644
index 00000000..07aff0b4
--- /dev/null
+++ b/lib/philomena_web/plugs/compromised_password_check_plug.ex
@@ -0,0 +1,42 @@
+defmodule PhilomenaWeb.CompromisedPasswordCheckPlug do
+  import Phoenix.Controller
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    error_if_password_compromised(conn, conn.params)
+  end
+
+  defp error_if_password_compromised(conn, %{"user" => %{"password" => password}}) do
+    case password_compromised?(password) do
+      true ->
+        conn
+        |> put_flash(
+          :error,
+          "We've detected that the password you entered has been compromised during a data breach of another website. Please choose a different password."
+        )
+        |> redirect(external: conn.assigns.referrer)
+
+      false ->
+        conn
+    end
+  end
+
+  defp error_if_password_compromised(conn, _params),
+    do: conn
+
+  defp password_compromised?(password) do
+    <<prefix::binary-size(5), rest::binary>> =
+      :crypto.hash(:sha, password)
+      |> Base.encode16()
+
+    case HTTPoison.get(make_api_url(prefix)) do
+      {:ok, %HTTPoison.Response{body: body, status_code: 200}} -> String.contains?(body, rest)
+      _ -> false
+    end
+  end
+
+  defp make_api_url(prefix) do
+    "https://api.pwnedpasswords.com/range/#{prefix}"
+  end
+end
diff --git a/lib/philomena_web/router.ex b/lib/philomena_web/router.ex
index d8589364..fc5f8894 100644
--- a/lib/philomena_web/router.ex
+++ b/lib/philomena_web/router.ex
@@ -52,13 +52,23 @@ defmodule PhilomenaWeb.Router do
     plug PhilomenaWeb.FilterBannedUsersPlug
   end
 
+  pipeline :ensure_password_not_compromised do
+    plug PhilomenaWeb.CompromisedPasswordCheckPlug
+  end
+
   pipeline :protected do
     plug Pow.Plug.RequireAuthenticated,
       error_handler: Pow.Phoenix.PlugErrorHandler
   end
 
   scope "/" do
-    pipe_through [:browser, :ensure_totp, :ensure_not_banned, :ensure_tor_authorized]
+    pipe_through [
+      :browser,
+      :ensure_totp,
+      :ensure_not_banned,
+      :ensure_tor_authorized,
+      :ensure_password_not_compromised
+    ]
 
     pow_registration_routes()
   end