Webfinger: Add test showing wrong webfinger behavior

test/fixtures/webfinger/graf-imposter-webfinger.json

@@ -0,0 +1,41 @@
+{
+    "subject": "acct:graf@poa.st",
+    "aliases": [
+        "https://fba.ryona.agenc/webfingertest"
+    ],
+    "links": [
+        {
+            "rel": "http://webfinger.net/rel/profile-page",
+            "type": "text/html",
+            "href": "https://fba.ryona.agenc/webfingertest"
+        },
+        {
+            "rel": "self",
+            "type": "application/activity+json",
+            "href": "https://fba.ryona.agenc/webfingertest"
+        },
+        {
+            "rel": "http://ostatus.org/schema/1.0/subscribe",
+            "template": "https://fba.ryona.agenc/contact/follow?url={uri}"
+        },
+        {
+            "rel": "http://schemas.google.com/g/2010#updates-from",
+            "type": "application/atom+xml",
+            "href": ""
+        },
+        {
+            "rel": "salmon",
+            "href": "https://fba.ryona.agenc/salmon/friendica"
+        },
+        {
+            "rel": "http://microformats.org/profile/hcard",
+            "type": "text/html",
+            "href": "https://fba.ryona.agenc/hcard/friendica"
+        },
+        {
+            "rel": "http://joindiaspora.com/seed_location",
+            "type": "text/html",
+            "href": "https://fba.ryona.agenc"
+        }
+    ]
+}

test/pleroma/web/web_finger_test.exs

@@ -204,4 +204,19 @@ test "refuses to process XML remote entities" do
       assert :error = WebFinger.finger("pekorino@pawoo.net")
     end
   end
+
+  test "prevents forgeries" do
+    Tesla.Mock.mock(fn
+      %{url: "https://fba.ryona.agency/.well-known/webfinger?resource=acct:graf@fba.ryona.agency"} ->
+        fake_webfinger =
+          File.read!("test/fixtures/webfinger/graf-imposter-webfinger.json") |> Jason.decode!()
+
+        Tesla.Mock.json(fake_webfinger)
+
+      %{url: "https://fba.ryona.agency/.well-known/host-meta"} ->
+        {:ok, %Tesla.Env{status: 404}}
+    end)
+
+    refute {:ok, _} = WebFinger.finger("graf@fba.ryona.agency")
+  end
 end