fix: reject duplicate Signature headers

test/pleroma/web/activity_pub/activity_pub_controller_test.exs

@@ -729,6 +729,22 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       assert Activity.get_by_ap_id(data["id"])
     end
 
+    test "rejects duplicate Signature headers before processing signed inbox requests", %{
+      conn: conn
+    } do
+      data = File.read!("test/fixtures/mastodon-post-activity.json") |> Jason.decode!()
+
+      conn =
+        conn
+        |> assign_valid_signature_for_actor(data["actor"])
+        |> put_req_header("content-type", "application/activity+json")
+        |> Map.update!(:req_headers, &[{"signature", "sig1=:fake-signature:"} | &1])
+        |> post("/inbox", data)
+
+      assert "error, unsupported HTTP Message Signature" == json_response(conn, 401)
+      assert [] == all_enqueued(worker: ReceiverWorker)
+    end
+
     test "it inserts an incoming activity into the database" <>
            "even if we can't fetch the user but have it in our db",
          %{conn: conn} do
@@ -794,6 +810,43 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       assert [] == all_enqueued(worker: SignatureRetryWorker)
     end
 
+    test "rejects duplicate Signature headers so clients can fall back", %{conn: conn} do
+      data = %{
+        "type" => "Create",
+        "actor" => "https://activitypubbot.example/user/ok",
+        "id" => "https://activitypubbot.example/activities/duplicate-signature-create",
+        "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+        "cc" => [],
+        "object" => %{
+          "type" => "Note",
+          "id" => "https://activitypubbot.example/objects/duplicate-signature-note",
+          "actor" => "https://activitypubbot.example/user/ok",
+          "attributedTo" => "https://activitypubbot.example/user/ok",
+          "content" => "hello from activitypub-bot",
+          "published" => "2026-05-29T13:33:31Z",
+          "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+          "cc" => []
+        }
+      }
+
+      legacy_signature =
+        "keyId=\"https://activitypubbot.example/user/ok#main-key\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest content-type\",signature=\"fake-signature\""
+
+      Mox.expect(Pleroma.StubbedHTTPSignaturesMock, :validate_conn, fn _conn -> false end)
+
+      conn =
+        conn
+        |> put_req_header("content-type", "application/activity+json")
+        |> put_req_header("date", "Fri, 29 May 2026 13:33:31 GMT")
+        |> put_req_header("digest", "SHA-256=fake-digest")
+        |> put_req_header("signature", "sig1=:fake-signature:")
+        |> Map.update!(:req_headers, &[{"signature", legacy_signature} | &1])
+        |> post("/inbox", data)
+
+      assert "error, unsupported HTTP Message Signature" == json_response(conn, 401)
+      assert [] == all_enqueued(worker: SignatureRetryWorker)
+    end
+
     test "keeps failed-signature retry for legacy signatures with Signature-Input", %{conn: conn} do
       data = %{
         "type" => "Create",
@@ -1397,6 +1450,20 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       assert Activity.get_by_ap_id(data["id"])
     end
 
+    test "rejects duplicate Signature headers", %{conn: conn, data: data} do
+      user = insert(:user)
+
+      conn =
+        conn
+        |> assign_valid_signature_for_actor(data["actor"])
+        |> put_req_header("content-type", "application/activity+json")
+        |> Map.update!(:req_headers, &[{"signature", "sig1=:fake-signature:"} | &1])
+        |> post("/users/#{user.nickname}/inbox", data)
+
+      assert "error, unsupported HTTP Message Signature" == json_response(conn, 401)
+      assert [] == all_enqueued(worker: ReceiverWorker)
+    end
+
     test "it accepts messages with to as string instead of array", %{conn: conn, data: data} do
       user = insert(:user)