StealEmoji: use Content-Type and reject non-images

E.g. *key’s emoji URLs typically don’t have file extensions, but
until now we just slapped ".png" at its end hoping for the best.

Furthermore, this gives us a chance to actually reject non-images,
which before was not feasible exatly due to those extension-less URLs

lib/pleroma/web/activity_pub/mrf/steal_emoji_policy.ex

@@ -33,16 +33,9 @@ defp reject_emoji?({shortcode, _url}, installed_emoji) do
     !valid_shortcode? or rejected_shortcode? or emoji_installed?
   end
 
-  defp steal_emoji(%{} = response, {shortcode, url}, emoji_dir_path) do
-    extension =
-      url
-      |> URI.parse()
-      |> Map.get(:path)
-      |> Path.basename()
-      |> Path.extname()
-
+  defp steal_emoji(%{} = response, {shortcode, extension}, emoji_dir_path) do
     shortcode = Path.basename(shortcode)
-    file_path = Path.join(emoji_dir_path, shortcode <> (extension || ".png"))
+    file_path = Path.join(emoji_dir_path, shortcode <> "." <> extension)
 
     case File.write(file_path, response.body) do
       :ok ->
@@ -54,14 +47,25 @@ defp steal_emoji(%{} = response, {shortcode, url}, emoji_dir_path) do
     end
   end
 
+  defp get_extension_if_safe(response) do
+    content_type =
+      :proplists.get_value("content-type", response.headers, MIME.from_path(response.url))
+
+    case content_type do
+      "image/" <> _ -> List.first(MIME.extensions(content_type))
+      _ -> nil
+    end
+  end
+
   defp maybe_steal_emoji({shortcode, url}, emoji_dir_path) do
     url = Pleroma.Web.MediaProxy.url(url)
 
     with {:ok, %{status: status} = response} when status in 200..299 <- Pleroma.HTTP.get(url) do
       size_limit = Config.get([:mrf_steal_emoji, :size_limit], 50_000)
+      extension = get_extension_if_safe(response)
 
-      if byte_size(response.body) <= size_limit do
-        steal_emoji(response, {shortcode, url}, emoji_dir_path)
+      if byte_size(response.body) <= size_limit and extension do
+        steal_emoji(response, {shortcode, extension}, emoji_dir_path)
       else
         Logger.debug(
           "MRF.StealEmojiPolicy: :#{shortcode}: at #{url} (#{byte_size(response.body)} B) over size limit (#{size_limit} B)"

test/pleroma/web/activity_pub/mrf/steal_emoji_policy_test.exs

@@ -45,7 +45,11 @@ test "Steals emoji on unknown shortcode from allowed remote host", %{
     refute File.exists?(path)
 
     Tesla.Mock.mock(fn %{method: :get, url: "https://example.org/emoji/firedfox.png"} ->
-      %Tesla.Env{status: 200, body: File.read!("test/fixtures/image.jpg")}
+      %Tesla.Env{
+        status: 200,
+        body: File.read!("test/fixtures/image.jpg"),
+        url: "https://example.org/emoji/firedfox.png"
+      }
     end)
 
     clear_config(:mrf_steal_emoji, hosts: ["example.org"], size_limit: 284_468)
@@ -72,7 +76,11 @@ test "rejects invalid shortcodes", %{path: path} do
     fullpath = Path.join(path, "fired/fox.png")
 
     Tesla.Mock.mock(fn %{method: :get, url: "https://example.org/emoji/firedfox"} ->
-      %Tesla.Env{status: 200, body: File.read!("test/fixtures/image.jpg")}
+      %Tesla.Env{
+        status: 200,
+        body: File.read!("test/fixtures/image.jpg"),
+        url: "https://example.org/emoji/firedfox.png"
+      }
     end)
 
     clear_config(:mrf_steal_emoji, hosts: ["example.org"], size_limit: 284_468)
@@ -86,6 +94,36 @@ test "rejects invalid shortcodes", %{path: path} do
     refute File.exists?(fullpath)
   end
 
+  test "prefers content-type header for extension", %{path: path} do
+    message = %{
+      "type" => "Create",
+      "object" => %{
+        "emoji" => [{"firedfox", "https://example.org/emoji/firedfox.fud"}],
+        "actor" => "https://example.org/users/admin"
+      }
+    }
+
+    Tesla.Mock.mock(fn %{method: :get, url: "https://example.org/emoji/firedfox.fud"} ->
+      %Tesla.Env{
+        status: 200,
+        body: File.read!("test/fixtures/image.jpg"),
+        url: "https://example.org/emoji/firedfox.wevp",
+        headers: [{"content-type", "image/gif"}]
+      }
+    end)
+
+    clear_config(:mrf_steal_emoji, hosts: ["example.org"], size_limit: 284_468)
+
+    assert {:ok, _message} = StealEmojiPolicy.filter(message)
+
+    assert "firedfox" in installed()
+    assert File.exists?(path)
+
+    assert path
+           |> Path.join("firedfox.gif")
+           |> File.exists?()
+  end
+
   test "reject regex shortcode", %{message: message} do
     refute "firedfox" in installed()
 
@@ -118,7 +156,11 @@ test "reject if size is above the limit", %{message: message} do
     refute "firedfox" in installed()
 
     Tesla.Mock.mock(fn %{method: :get, url: "https://example.org/emoji/firedfox.png"} ->
-      %Tesla.Env{status: 200, body: File.read!("test/fixtures/image.jpg")}
+      %Tesla.Env{
+        status: 200,
+        body: File.read!("test/fixtures/image.jpg"),
+        url: "https://example.org/emoji/firedfox.png"
+      }
     end)
 
     clear_config(:mrf_steal_emoji, hosts: ["example.org"], size_limit: 50_000)
@@ -132,7 +174,8 @@ test "reject if host returns error", %{message: message} do
     refute "firedfox" in installed()
 
     Tesla.Mock.mock(fn %{method: :get, url: "https://example.org/emoji/firedfox.png"} ->
-      {:ok, %Tesla.Env{status: 404, body: "Not found"}}
+      {:ok,
+       %Tesla.Env{status: 404, body: "Not found", url: "https://example.org/emoji/firedfox.png"}}
     end)
 
     clear_config(:mrf_steal_emoji, hosts: ["example.org"], size_limit: 284_468)