diff --git a/changelog.d/host-header-verification.security b/changelog.d/host-header-verification.security
new file mode 100644
index 000000000..11a0784b2
--- /dev/null
+++ b/changelog.d/host-header-verification.security
@@ -0,0 +1 @@
+Ensure Host header is present and matches instance URI
diff --git a/lib/pleroma/web/activity_pub/activity_pub_controller.ex b/lib/pleroma/web/activity_pub/activity_pub_controller.ex
index 2bfff6968..3d1875599 100644
--- a/lib/pleroma/web/activity_pub/activity_pub_controller.ex
+++ b/lib/pleroma/web/activity_pub/activity_pub_controller.ex
@@ -303,7 +303,12 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
     end
   end
 
-  def inbox(%{assigns: %{valid_signature: true}} = conn, %{"nickname" => nickname} = params) do
+  def inbox(
+        %{
+          assigns: %{valid_signature: true, valid_host_header: true}
+        } = conn,
+        %{"nickname" => nickname} = params
+      ) do
     with {:recipient_exists, %User{} = recipient} <-
            {:recipient_exists, User.get_cached_by_nickname(nickname)},
          {:sender_exists, {:ok, %User{} = actor}} <-
@@ -342,7 +347,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
     end
   end
 
-  def inbox(%{assigns: %{valid_signature: true}} = conn, params) do
+  def inbox(%{assigns: %{valid_signature: true, valid_host_header: true}} = conn, params) do
     Federator.incoming_ap_doc(params)
     json(conn, "ok")
   end
diff --git a/lib/pleroma/web/plugs/ensure_host_matches_plug.ex b/lib/pleroma/web/plugs/ensure_host_matches_plug.ex
new file mode 100644
index 000000000..ab2129f9e
--- /dev/null
+++ b/lib/pleroma/web/plugs/ensure_host_matches_plug.ex
@@ -0,0 +1,63 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2026 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.EnsureHostMatchesPlug do
+  @moduledoc "Ensures Host header matches instance"
+
+  alias Pleroma.Web.Endpoint
+
+  import Plug.Conn
+
+  def init(options), do: options
+
+  @spec call(Plug.Conn.t(), Plug.opts()) :: Plug.Conn.t()
+  def call(%Plug.Conn{assigns: %{valid_signature: true}} = conn, _opts) do
+    # Host header is scheme-less, URI.parse needs the //
+    host_header = get_req_header(conn, "host")
+    host_uri = URI.parse("//#{host_header}")
+    instance_uri = URI.parse(Endpoint.url())
+
+    case host_header do
+      [host] ->
+        cond do
+          host == "" ->
+            resp(conn, 400, "Host header not provided") |> halt()
+
+          true ->
+            if host_matches?(host_uri, instance_uri),
+              do: assign(conn, :valid_host_header, true),
+              else: resp(conn, 400, "Host header does not match this instance") |> halt()
+        end
+
+      [_head | _rest] ->
+        conn
+        |> resp(400, "More than one Host header provided")
+        |> halt()
+
+      [] ->
+        conn
+        |> resp(400, "Host header not provided")
+        |> halt()
+    end
+  end
+
+  # Host header may not be provided, but signature verification failed anyway
+  def call(conn, _opts), do: conn
+
+  defp case_insensitive_compare(checked, authority) do
+    String.downcase(checked) == String.downcase(authority)
+  end
+
+  # Host header did not provide port
+  # Host header is scheme-less, URI.parse does not provide default port
+  defp host_matches?(%URI{host: req_host, port: nil}, %URI{host: instance_host}),
+    do: case_insensitive_compare(req_host, instance_host)
+
+  # Host header provided a port
+  # Any port specified in the Endpoint url configuration is valid here
+  defp host_matches?(%URI{host: req_host, port: port}, %URI{host: instance_host, port: port}),
+    do: case_insensitive_compare(req_host, instance_host)
+
+  defp host_matches?(_, _), do: false
+end
diff --git a/lib/pleroma/web/router.ex b/lib/pleroma/web/router.ex
index c91ca8c97..813c5c482 100644
--- a/lib/pleroma/web/router.ex
+++ b/lib/pleroma/web/router.ex
@@ -216,6 +216,7 @@ defmodule Pleroma.Web.Router do
   pipeline :http_signature do
     plug(Pleroma.Web.Plugs.HTTPSignaturePlug)
     plug(Pleroma.Web.Plugs.MappedSignatureToIdentityPlug)
+    plug(Pleroma.Web.Plugs.EnsureHostMatchesPlug)
   end
 
   pipeline :inbox_guard do
diff --git a/lib/pleroma/workers/signature_retry_worker.ex b/lib/pleroma/workers/signature_retry_worker.ex
index 2c4c097dd..28958faff 100644
--- a/lib/pleroma/workers/signature_retry_worker.ex
+++ b/lib/pleroma/workers/signature_retry_worker.ex
@@ -8,6 +8,7 @@ defmodule Pleroma.Workers.SignatureRetryWorker do
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.Utils
   alias Pleroma.Web.Federator
+  alias Pleroma.Web.Plugs.EnsureHostMatchesPlug
   alias Pleroma.Web.Plugs.MappedSignatureToIdentityPlug
 
   require Logger
@@ -48,6 +49,7 @@ defmodule Pleroma.Workers.SignatureRetryWorker do
              {:ok, _public_key} <- Signature.refetch_public_key(conn_data),
              {:signature, true} <- {:signature, validate_signature(conn_data)},
              {:same_actor, true} <- {:same_actor, validate_same_actor(conn_data)},
+             {:host_header, true} <- {:host_header, validate_host_header(conn_data)},
              {:ok, res} <- Federator.perform(:incoming_ap_doc, params) do
           unless Instances.reachable?(params["actor"]) do
             domain = URI.parse(params["actor"]).host
@@ -103,6 +105,16 @@ defmodule Pleroma.Workers.SignatureRetryWorker do
     end
   end
 
+  defp validate_host_header(conn_data) do
+    case EnsureHostMatchesPlug.call(conn_data, []) do
+      %Plug.Conn{assigns: %{valid_signature: true, valid_host_header: true}} ->
+        true
+
+      _ ->
+        false
+    end
+  end
+
   defp validate_same_actor(conn_data) do
     case MappedSignatureToIdentityPlug.call(conn_data, []) do
       %Plug.Conn{assigns: %{valid_signature: true}} ->
@@ -170,6 +182,10 @@ defmodule Pleroma.Workers.SignatureRetryWorker do
         {:same_actor, false} ->
           {:cancel, :actor_signature_mismatch}
 
+        # Host header from request not for us
+        {:host_header, false} ->
+          {:cancel, :host_header_mismatch}
+
         # Origin / URL validation failed somewhere possibly due to spoofing
         {:error, :origin_containment_failed} ->
           {:cancel, :origin_containment_failed}
@@ -234,6 +250,7 @@ defmodule Pleroma.Workers.SignatureRetryWorker do
   defp log_signature_retry_rejection({:cancel, reason}, context)
        when reason in [
               :actor_signature_mismatch,
+              :host_header_mismatch,
               :invalid_signature,
               :invalid_signature_retry_metadata,
               :missing_signature_retry_metadata,
diff --git a/test/pleroma/web/activity_pub/activity_pub_controller_test.exs b/test/pleroma/web/activity_pub/activity_pub_controller_test.exs
index 3988c3912..68e39b3a0 100644
--- a/test/pleroma/web/activity_pub/activity_pub_controller_test.exs
+++ b/test/pleroma/web/activity_pub/activity_pub_controller_test.exs
@@ -950,6 +950,50 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       refute Activity.get_by_ap_id(data["id"])
     end
 
+    test "does not process post with Host header not for us", %{conn: conn} do
+      alice = insert(:user, local: false, ap_id: "https://one.com/users/alice")
+      object_id = "https://one.com/objects/inbox-forged-note"
+
+      data = %{
+        "type" => "Create",
+        "actor" => alice.ap_id,
+        "id" => "https://one.com/activities/inbox-forged-create",
+        "context" => "https://one.com/contexts/inbox-forged-create",
+        "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+        "cc" => [],
+        "object" => %{
+          "type" => "Note",
+          "id" => object_id,
+          "actor" => alice.ap_id,
+          "attributedTo" => alice.ap_id,
+          "context" => "https://one.com/contexts/inbox-forged-create",
+          "content" => "forged post",
+          "published" => "2024-07-25T13:33:31Z",
+          "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+          "cc" => []
+        }
+      }
+
+      # Plug will complain when replacing raw host header with put_req_header.
+      # The Plug way is updating conn.host, but that isn't the raw header
+      # and that isn't used in the EnsureHostMatchesPlug, because it doesn't include the port.
+      conn =
+        conn
+        |> assign_valid_signature_for_actor(alice)
+        |> delete_req_header("host")
+        |> put_req_header("content-type", "application/activity+json")
+
+      conn = %{conn | req_headers: conn.req_headers ++ [{"host", "invalid.example.com"}]}
+      conn = post(conn, "/inbox", data)
+
+      assert "Host header does not match this instance" == conn.resp_body
+      assert 400 == conn.status
+      assert true == conn.halted
+
+      refute Activity.get_by_ap_id(data["id"])
+      refute Object.get_by_ap_id(object_id)
+    end
+
     test "accept follow activity", %{conn: conn} do
       clear_config([:instance, :federating], true)
       relay = Relay.get_actor()
diff --git a/test/pleroma/web/plugs/ensure_host_matches_plug_test.exs b/test/pleroma/web/plugs/ensure_host_matches_plug_test.exs
new file mode 100644
index 000000000..8ace74dfb
--- /dev/null
+++ b/test/pleroma/web/plugs/ensure_host_matches_plug_test.exs
@@ -0,0 +1,121 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2026 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.EnsureHostMatchesPlugTest do
+  use Pleroma.Web.ConnCase
+
+  alias Pleroma.Web.Endpoint
+  alias Pleroma.Web.Plugs.EnsureHostMatchesPlug
+
+  import Plug.Conn
+  import Tesla.Mock
+
+  setup do
+    mock(fn env -> apply(HttpRequestMock, :request, [env]) end)
+    :ok
+  end
+
+  defp set_host(conn, host), do: %{conn | req_headers: conn.req_headers ++ [{"host", host}]}
+
+  describe "EnsureHostMatchesPlug" do
+    setup do
+      conn = build_conn(:post, "/cofe") |> assign(:valid_signature, true)
+      [conn: conn]
+    end
+
+    test "gracefully handles no Host header", %{conn: conn} do
+      conn = EnsureHostMatchesPlug.call(conn, %{})
+
+      assert conn.status == 400
+      assert conn.halted == true
+      assert conn.resp_body == "Host header not provided"
+    end
+
+    test "gracefully handles empty Host header", %{conn: conn} do
+      conn =
+        conn
+        |> set_host("")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.status == 400
+      assert conn.halted == true
+      assert conn.resp_body == "Host header not provided"
+    end
+
+    test "it rejects Host header not matching Endpoint URL", %{conn: conn} do
+      conn =
+        conn
+        |> set_host("invalid.example.com")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.status == 400
+      assert conn.halted == true
+      assert conn.resp_body == "Host header does not match this instance"
+    end
+
+    test "it rejects Host header not matching Endpoint with port", %{conn: conn} do
+      endpoint = URI.parse(Endpoint.url())
+
+      conn =
+        conn
+        |> set_host("invalid.example.com:#{endpoint.port}")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.status == 400
+      assert conn.halted == true
+      assert conn.resp_body == "Host header does not match this instance"
+    end
+
+    test "it rejects Host header not matching Endpoint port", %{conn: conn} do
+      endpoint = URI.parse(Endpoint.url())
+
+      conn =
+        conn
+        |> set_host("#{endpoint.host}:25")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.status == 400
+      assert conn.halted == true
+      assert conn.resp_body == "Host header does not match this instance"
+    end
+
+    test "it rejects multiple Host headers", %{conn: conn} do
+      conn =
+        conn
+        |> set_host("host1.example.com")
+        |> set_host("host2.example.com")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.status == 400
+      assert conn.halted == true
+      assert conn.resp_body == "More than one Host header provided"
+    end
+
+    test "it works for Host header without port", %{conn: conn} do
+      endpoint = URI.parse(Endpoint.url())
+
+      conn =
+        conn
+        |> set_host("#{endpoint.host}")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.halted == false
+      assert Map.get(conn.assigns, :valid_host_header, nil)
+    end
+
+    test "it works for Host header with port same as Endpoint", %{
+      conn: conn
+    } do
+      endpoint = URI.parse(Endpoint.url())
+
+      conn =
+        conn
+        |> set_host("#{endpoint.host}:#{endpoint.port}")
+        |> EnsureHostMatchesPlug.call(%{})
+
+      assert conn.halted == false
+      assert Map.get(conn.assigns, :valid_host_header, nil)
+    end
+  end
+end
diff --git a/test/pleroma/workers/signature_retry_worker_test.exs b/test/pleroma/workers/signature_retry_worker_test.exs
index 94dd5f6c1..3806ecac9 100644
--- a/test/pleroma/workers/signature_retry_worker_test.exs
+++ b/test/pleroma/workers/signature_retry_worker_test.exs
@@ -16,12 +16,13 @@ defmodule Pleroma.Workers.SignatureRetryWorkerTest do
   alias Pleroma.Signature
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.UserView
+  alias Pleroma.Web.Endpoint
   alias Pleroma.Web.Federator
   alias Pleroma.Workers.SignatureRetryWorker
 
   defp signature_headers_for(%User{} = signer) do
     [
-      {"host", "local.test"},
+      {"host", "#{URI.parse(Endpoint.url()).host}"},
       {"date", "Thu, 25 Jul 2024 13:33:31 GMT"},
       {"digest", "SHA-256=fake-digest"},
       {"content-type", "application/activity+json"},
@@ -245,6 +246,66 @@ defmodule Pleroma.Workers.SignatureRetryWorkerTest do
     refute Activity.get_by_ap_id(create["id"])
   end
 
+  test "cancels when the Host header does not match Endpoint" do
+    alice = insert(:user, local: false, ap_id: "https://one.com/users/alice")
+
+    create = %{
+      "type" => "Create",
+      "actor" => alice.ap_id,
+      "id" => "https://one.com/activities/invalid-signature-create",
+      "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+      "cc" => [],
+      "object" => %{
+        "type" => "Note",
+        "id" => "https://one.com/objects/invalid-signature-note",
+        "actor" => alice.ap_id,
+        "attributedTo" => alice.ap_id,
+        "content" => "forged post",
+        "published" => "2024-07-25T13:33:31Z",
+        "to" => ["https://www.w3.org/ns/activitystreams#Public"],
+        "cc" => []
+      }
+    }
+
+    expect_signature_from(alice)
+
+    headers =
+      [
+        {"host", "invalid.example.com"},
+        {"date", "Thu, 25 Jul 2024 13:33:31 GMT"},
+        {"digest", "SHA-256=fake-digest"},
+        {"content-type", "application/activity+json"},
+        {
+          "signature",
+          "keyId=\"#{alice.ap_id}#main-key\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest content-type\",signature=\"fake-signature\""
+        }
+      ]
+
+    assert {:ok, oban_job} =
+             Federator.incoming_failed_signature_ap_doc(%{
+               method: "POST",
+               req_headers: headers,
+               request_path: "/inbox",
+               params: create,
+               query_string: ""
+             })
+
+    log =
+      capture_log([level: :warning], fn ->
+        assert {:cancel, :host_header_mismatch} = SignatureRetryWorker.perform(oban_job)
+      end)
+
+    assert log =~ "Failed-signature inbox retry rejected"
+    assert log =~ "reason=:host_header_mismatch"
+    assert log =~ "payload_actor=\"https://one.com/users/alice\""
+    assert log =~ "signature_actor=\"https://one.com/users/alice\""
+    assert log =~ "activity_id=\"https://one.com/activities/invalid-signature-create\""
+    assert log =~ "type=\"Create\""
+    assert log =~ "request_path=\"/inbox\""
+
+    refute Activity.get_by_ap_id(create["id"])
+  end
+
   test "processes the activity after refetching a valid matching signature" do
     alice = insert(:user, local: false, ap_id: "https://one.com/users/alice")
 
@@ -309,11 +370,11 @@ defmodule Pleroma.Workers.SignatureRetryWorkerTest do
         "content-type" => "application/activity+json",
         date: date,
         digest: digest,
-        host: "local.test"
+        host: "#{URI.parse(Endpoint.url()).host}"
       })
 
     req_headers = [
-      ["host", "local.test"],
+      ["host", "#{URI.parse(Endpoint.url()).host}"],
       ["date", date],
       ["digest", digest],
       ["content-type", "application/activity+json"],
diff --git a/test/support/conn_case.ex b/test/support/conn_case.ex
index f010fec33..c01516169 100644
--- a/test/support/conn_case.ex
+++ b/test/support/conn_case.ex
@@ -119,7 +119,10 @@ defmodule Pleroma.Web.ConnCase do
     DataCase.stub_pipeline()
 
     Mox.verify_on_exit!()
+    endpoint = URI.parse(Pleroma.Web.Endpoint.url())
+    conn = Phoenix.ConnTest.build_conn()
+    conn = %{conn | req_headers: [{"host", "#{endpoint.host}:#{endpoint.port}"}]}
 
-    {:ok, conn: Phoenix.ConnTest.build_conn()}
+    {:ok, conn: conn}
   end
 end