Fail faster.

lib/pleroma/plugs/http_signature.ex

@@ -14,19 +14,26 @@ def call(%{assigns: %{valid_signature: true}} = conn, opts) do
   def call(conn, opts) do
     user = conn.params["actor"]
     Logger.debug("Checking sig for #{user}")
+    [signature | _] = get_req_header(conn, "signature")
 
-    if get_req_header(conn, "signature") do
-      conn =
-        conn
-        |> put_req_header(
-          "(request-target)",
-          String.downcase("#{conn.method}") <> " #{conn.request_path}"
-        )
+    cond do
+      signature && String.contains?(signature, user) ->
+        conn =
+          conn
+          |> put_req_header(
+            "(request-target)",
+            String.downcase("#{conn.method}") <> " #{conn.request_path}"
+          )
+
+        assign(conn, :valid_signature, HTTPSignatures.validate_conn(conn))
 
-      assign(conn, :valid_signature, HTTPSignatures.validate_conn(conn))
-    else
-      Logger.debug("No signature header!")
-      conn
+      signature ->
+        Logger.debug("Signature not from actor")
+        assign(conn, :valid_signature, false)
+
+      true ->
+        Logger.debug("No signature header!")
+        conn
     end
   end
 end

test/plugs/http_signature_plug_test.exs

+defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
+  use Pleroma.Web.ConnCase
+  alias Pleroma.Web.HTTPSignatures
+  alias Pleroma.Web.Plugs.HTTPSignaturePlug
+
+  import Plug.Conn
+  import Mock
+
+  test "it call HTTPSignatures to check validity if the actor sighed it" do
+    params = %{"actor" => "http://mastodon.example.org/users/admin"}
+    conn = build_conn(:get, "/doesntmattter", params)
+
+    with_mock HTTPSignatures, validate_conn: fn _ -> true end do
+      conn =
+        conn
+        |> put_req_header(
+          "signature",
+          "keyId=\"http://mastodon.example.org/users/admin#main-key"
+        )
+        |> HTTPSignaturePlug.call(%{})
+
+      assert conn.assigns.valid_signature == true
+      assert called(HTTPSignatures.validate_conn(:_))
+    end
+  end
+
+  test "bails out early if the signature isn't by the activity actor" do
+    params = %{"actor" => "https://mst3k.interlinked.me/users/luciferMysticus"}
+    conn = build_conn(:get, "/doesntmattter", params)
+
+    with_mock HTTPSignatures, validate_conn: fn _ -> false end do
+      conn =
+        conn
+        |> put_req_header(
+          "signature",
+          "keyId=\"http://mastodon.example.org/users/admin#main-key"
+        )
+        |> HTTPSignaturePlug.call(%{})
+
+      assert conn.assigns.valid_signature == false
+      refute called(HTTPSignatures.validate_conn(:_))
+    end
+  end
+end