Only allow exact id matches

This protects us from falling for obvious spoofs as from the current
upload exploit (unfortunately we can’t reasonably do anything about
spoofs with exact matches as was possible via emoji and proxy).

Such objects being invalid is supported by the spec, sepcifically
sections 3.1 and 3.2: https://www.w3.org/TR/activitypub/#obj-id

Anonymous objects are not relevant here (they can only exists within
parent objects iiuc) and neither is client-to-server or transient objects
(as those cannot be fetched in the first place).
This leaves us with the requirement for `id` to (a) exist and
(b) be a publicly dereferencable URI from the originating server.
This alone does not yet demand strict equivalence, but the spec then
further explains objects ought to be fetchable _via their ID_.
Meaning an object not retrievable via its ID, is invalid.

This reading is supported by the fact, e.g. GoToSocial (recently) and
Mastodon (for 6+ years) do already implement such strict ID checks,
additionally proving this doesn’t cause federation issues in practice.

However, apart from canonical IDs there can also be additional display
URLs. *omas first redirect those to their canonical location, but *keys
and Mastodon directly serve the AP representation without redirects.

Mastodon and GTS deal with this in two different ways,
but both constitute an effective countermeasure:
 - Mastodon:
   Unless it already is a known AP id, two fetches occur.
   The first fetch just reads the `id` property and then refetches from
   the id. The last fetch requires the returned id to exactly match the
   URL the content was fetched from. (This can be optimised by skipping
   the second fetch if it already matches)
   05eda8d193/app/helpers/jsonld_helper.rb (L168)
   63f0979799

 - GTS:
   Only does a single fetch and then checks if _either_ the id
   _or_ url property (which can be an object) match the original fetch
   URL. This relies on implementations always including their display URL
   as "url" if differing from the id. For actors this is true for all
   investigated implementations, for posts only Mastodon includes an
   "url", but it is also the only one with a differing display URL.
   2bafd7daf5 (diff-943bbb02c8ac74ac5dc5d20807e561dcdfaebdc3b62b10730f643a20ac23c24fR222)

Albeit Mastodon’s refetch offers higher compatibility with theoretical
implmentations using either multiple different display URL or not
denoting any of them as "url" at all, for now we chose to adopt a
GTS-like refetch-free approach to avoid additional implementation
concerns wrt to whether redirects should be allowed when fetching a
canonical AP id and potential for accidentally loosening some checks
(e.g. cross-domain refetches) for one of the fetches.
This may be reconsidered in the future.

lib/pleroma/object/containment.ex

@@ -11,6 +11,9 @@ defmodule Pleroma.Object.Containment do
   Object containment is an important step in validating remote objects to prevent
   spoofing, therefore removal of object containment functions is NOT recommended.
   """
+
+  alias Pleroma.Web.ActivityPub.Transmogrifier
+
   def get_actor(%{"actor" => actor}) when is_binary(actor) do
     actor
   end
@@ -47,6 +50,18 @@ def get_object(_) do
   defp compare_uris(%URI{host: host} = _id_uri, %URI{host: host} = _other_uri), do: :ok
   defp compare_uris(_id_uri, _other_uri), do: :error
 
+  defp compare_uris_exact(uri, uri), do: :ok
+
+  defp compare_uris_exact(%URI{} = id, %URI{} = other),
+    do: compare_uris_exact(URI.to_string(id), URI.to_string(other))
+
+  defp compare_uris_exact(id_uri, other_uri)
+       when is_binary(id_uri) and is_binary(other_uri) do
+    norm_id = String.replace_suffix(id_uri, "/", "")
+    norm_other = String.replace_suffix(other_uri, "/", "")
+    if norm_id == norm_other, do: :ok, else: :error
+  end
+
   @doc """
   Checks whether an URL to fetch from is from the local server.
 
@@ -77,6 +92,26 @@ def contain_origin(id, %{"attributedTo" => actor} = params),
 
   def contain_origin(_id, _data), do: :ok
 
+  @doc """
+  Check whether the fetch URL (after redirects) exactly (sans tralining slash) matches either
+  the canonical ActivityPub id or the objects url field (for display URLs from *key and Mastodon)
+
+  Since this is meant to be used for fetches, anonymous or transient objects are not accepted here.
+  """
+  def contain_id_to_fetch(url, %{"id" => id} = data) when is_binary(id) do
+    with {:id, :error} <- {:id, compare_uris_exact(id, url)},
+         # "url" can be a "Link" object and this is checked before full normalisation
+         display_url <- Transmogrifier.fix_url(data)["url"],
+         true <- display_url != nil do
+      compare_uris_exact(display_url, url)
+    else
+      {:id, :ok} -> :ok
+      _ -> :error
+    end
+  end
+
+  def contain_id_to_fetch(_url, _data), do: :error
+
   @doc """
   Check whether the object id is from the same host as another id
   """

lib/pleroma/object/fetcher.ex

@@ -263,7 +263,7 @@ def fetch_and_contain_remote_object_from_id(id) when is_binary(id) do
          {_, :ok} <- {:local_fetch, Containment.contain_local_fetch(id)},
          {:ok, final_id, body} <- get_object(id),
          {:ok, data} <- safe_json_decode(body),
-         {_, :ok} <- {:containment, Containment.contain_origin_from_id(final_id, data)},
+         {_, :ok} <- {:strict_id, Containment.contain_id_to_fetch(final_id, data)},
          {_, :ok} <- {:containment, Containment.contain_origin(final_id, data)} do
       unless Instances.reachable?(final_id) do
         Instances.set_reachable(final_id)
@@ -271,6 +271,9 @@ def fetch_and_contain_remote_object_from_id(id) when is_binary(id) do
 
       {:ok, data}
     else
+      {:strict_id, _} ->
+        {:error, "Object's ActivityPub id/url does not match final fetch URL"}
+
       {:scheme, _} ->
         {:error, "Unsupported URI scheme"}
 

test/fixtures/tesla_mock/https__info.pleroma.site_activity3.json

@@ -3,7 +3,7 @@
         "attributedTo": "http://mastodon.example.org/users/admin",
         "attachment": [],
         "content": "<p>this post was not actually written by Haelwenn</p>",
-        "id": "https://info.pleroma.site/activity2.json",
+        "id": "https://info.pleroma.site/activity3.json",
         "published": "2018-09-01T22:15:00Z",
         "tag": [],
         "to": [

test/fixtures/tesla_mock/relay@mastdon.example.org.json

@@ -11,7 +11,7 @@
     "toot": "http://joinmastodon.org/ns#",
     "Emoji": "toot:Emoji"
   }],
-  "id": "http://mastodon.example.org/users/admin",
+  "id": "http://mastodon.example.org/users/relay",
   "type": "Application",
   "invisible": true,
   "following": "http://mastodon.example.org/users/admin/following",
@@ -24,8 +24,8 @@
   "url": "http://mastodon.example.org/@admin",
   "manuallyApprovesFollowers": false,
   "publicKey": {
-    "id": "http://mastodon.example.org/users/admin#main-key",
+    "id": "http://mastodon.example.org/users/relay#main-key",
-    "owner": "http://mastodon.example.org/users/admin",
+    "owner": "http://mastodon.example.org/users/relay",
     "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAtc4Tir+3ADhSNF6VKrtW\nOU32T01w7V0yshmQei38YyiVwVvFu8XOP6ACchkdxbJ+C9mZud8qWaRJKVbFTMUG\nNX4+6Q+FobyuKrwN7CEwhDALZtaN2IPbaPd6uG1B7QhWorrY+yFa8f2TBM3BxnUy\nI4T+bMIZIEYG7KtljCBoQXuTQmGtuffO0UwJksidg2ffCF5Q+K//JfQagJ3UzrR+\nZXbKMJdAw4bCVJYs4Z5EhHYBwQWiXCyMGTd7BGlmMkY6Av7ZqHKC/owp3/0EWDNz\nNqF09Wcpr3y3e8nA10X40MJqp/wR+1xtxp+YGbq/Cj5hZGBG7etFOmIpVBrDOhry\nBwIDAQAB\n-----END PUBLIC KEY-----\n"
   },
   "attachment": [{

test/pleroma/collections/collections_fetcher_test.exs

@@ -12,11 +12,14 @@ defmodule Akkoma.Collections.FetcherTest do
   end
 
   test "it should extract items from an embedded array in a Collection" do
+    ap_id = "https://example.com/collection/ordered_array"
+
     unordered_collection =
       "test/fixtures/collections/unordered_array.json"
       |> File.read!()
-
+      |> Jason.decode!()
-    ap_id = "https://example.com/collection/ordered_array"
+      |> Map.put("id", ap_id)
+      |> Jason.encode!(pretty: true)
 
     Tesla.Mock.mock(fn
       %{

test/pleroma/object/containment_test.exs

@@ -105,6 +105,56 @@ test "contain_origin_from_id() allows matching IDs" do
         )
     end
 
+    test "contain_id_to_fetch() refuses alternate IDs within the same origin domain" do
+      data = %{
+        "id" => "http://example.com/~alyssa/activities/1234.json",
+        "url" => "http://example.com/@alyssa/status/1234"
+      }
+
+      :error =
+        Containment.contain_id_to_fetch(
+          "http://example.com/~alyssa/activities/1234",
+          data
+        )
+    end
+
+    test "contain_id_to_fetch() allows matching IDs" do
+      data = %{
+        "id" => "http://example.com/~alyssa/activities/1234.json/"
+      }
+
+      :ok =
+        Containment.contain_id_to_fetch(
+          "http://example.com/~alyssa/activities/1234.json/",
+          data
+        )
+
+      :ok =
+        Containment.contain_id_to_fetch(
+          "http://example.com/~alyssa/activities/1234.json",
+          data
+        )
+    end
+
+    test "contain_id_to_fetch() allows display URLs" do
+      data = %{
+        "id" => "http://example.com/~alyssa/activities/1234.json",
+        "url" => "http://example.com/@alyssa/status/1234"
+      }
+
+      :ok =
+        Containment.contain_id_to_fetch(
+          "http://example.com/@alyssa/status/1234",
+          data
+        )
+
+      :ok =
+        Containment.contain_id_to_fetch(
+          "http://example.com/@alyssa/status/1234/",
+          data
+        )
+    end
+
     test "users cannot be collided through fake direction spoofing attempts" do
       _user =
         insert(:user, %{

test/pleroma/object/fetcher_test.exs

@@ -57,6 +57,46 @@ defp spoofed_object_with_ids(
           body: spoofed_object_with_ids("https://patch.cx/objects/spoof_content_type")
         }
 
+      # Spoof: mismatching ids
+      # Variant 1: Non-exisitng fake id
+      %{
+        method: :get,
+        url:
+          "https://patch.cx/media/03ca3c8b4ac3ddd08bf0f84be7885f2f88de0f709112131a22d83650819e36c2.json"
+      } ->
+        %Tesla.Env{
+          status: 200,
+          url:
+            "https://patch.cx/media/03ca3c8b4ac3ddd08bf0f84be7885f2f88de0f709112131a22d83650819e36c2.json",
+          headers: [{"content-type", "application/activity+json"}],
+          body: spoofed_object_with_ids()
+        }
+
+      %{method: :get, url: "https://patch.cx/objects/spoof"} ->
+        %Tesla.Env{
+          status: 404,
+          url: "https://patch.cx/objects/spoof",
+          headers: [],
+          body: "Not found"
+        }
+
+      # Varaint 2: two-stage payload
+      %{method: :get, url: "https://patch.cx/media/spoof_stage1.json"} ->
+        %Tesla.Env{
+          status: 200,
+          url: "https://patch.cx/media/spoof_stage1.json",
+          headers: [{"content-type", "application/activity+json"}],
+          body: spoofed_object_with_ids("https://patch.cx/media/spoof_stage2.json")
+        }
+
+      %{method: :get, url: "https://patch.cx/media/spoof_stage2.json"} ->
+        %Tesla.Env{
+          status: 200,
+          url: "https://patch.cx/media/spoof_stage2.json",
+          headers: [{"content-type", "application/activity+json"}],
+          body: spoofed_object_with_ids("https://patch.cx/media/unpredictable.json")
+        }
+
       # Spoof: cross-domain redirect with original domain id
       %{method: :get, url: "https://patch.cx/objects/spoof_media_redirect1"} ->
         %Tesla.Env{
@@ -79,7 +119,7 @@ defp spoofed_object_with_ids(
       %{method: :get, url: "https://patch.cx/objects/spoof_redirect"} ->
         %Tesla.Env{
           status: 200,
-          url: "https://patch.cx/objects/spoof",
+          url: "https://patch.cx/objects/spoof_redirect",
           headers: [{"content-type", "application/activity+json"}],
           body: spoofed_object_with_ids("https://patch.cx/objects/spoof_redirect")
         }
@@ -110,6 +150,7 @@ defp spoofed_object_with_ids(
         %{method: :get, url: "https://social.sakamoto.gq/notice/9wTkLEnuq47B25EehM"} ->
           %Tesla.Env{
             status: 200,
+            url: "https://social.sakamoto.gq/objects/f20f2497-66d9-4a52-a2e1-1be2a39c32c1",
             body: File.read!("test/fixtures/fetch_mocks/9wTkLEnuq47B25EehM.json"),
             headers: HttpRequestMock.activitypub_object_headers()
           }
@@ -208,6 +249,18 @@ test "it does not fetch a spoofed object with wrong content type" do
                )
     end
 
+    test "it does not fetch a spoofed object with id different from URL" do
+      assert {:error, "Object's ActivityPub id/url does not match final fetch URL"} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://patch.cx/media/03ca3c8b4ac3ddd08bf0f84be7885f2f88de0f709112131a22d83650819e36c2.json"
+               )
+
+      assert {:error, "Object's ActivityPub id/url does not match final fetch URL"} =
+               Fetcher.fetch_and_contain_remote_object_from_id(
+                 "https://patch.cx/media/spoof_stage1.json"
+               )
+    end
+
     test "it does not fetch an object via cross-domain redirects (initial id)" do
       assert {:error, {:cross_domain_redirect, true}} =
                Fetcher.fetch_and_contain_remote_object_from_id(

test/pleroma/web/activity_pub/activity_pub_test.exs

@@ -312,7 +312,7 @@ test "fetches user featured collection" do
   end
 
   test "fetches user featured collection using the first property" do
-    featured_url = "https://friendica.example.com/raha/collections/featured"
+    featured_url = "https://friendica.example.com/featured/raha"
     first_url = "https://friendica.example.com/featured/raha?page=1"
 
     featured_data =
@@ -350,7 +350,7 @@ test "fetches user featured collection using the first property" do
   end
 
   test "fetches user featured when it has string IDs" do
-    featured_url = "https://example.com/alisaie/collections/featured"
+    featured_url = "https://example.com/users/alisaie/collections/featured"
     dead_url = "https://example.com/users/alisaie/statuses/108311386746229284"
 
     featured_data =
@@ -1720,7 +1720,7 @@ test "doesn't crash when follower and following counters are hidden" do
             json(
               %{
                 "@context" => "https://www.w3.org/ns/activitystreams",
-                "id" => "http://remote.org/users/masto_hidden_counters/followers"
+                "id" => "http://remote.org/users/masto_hidden_counters/following"
               },
               headers: HttpRequestMock.activitypub_object_headers()
             )
@@ -1732,7 +1732,7 @@ test "doesn't crash when follower and following counters are hidden" do
             json(
               %{
                 "@context" => "https://www.w3.org/ns/activitystreams",
-                "id" => "http://remote.org/users/masto_hidden_counters/following"
+                "id" => "http://remote.org/users/masto_hidden_counters/followers"
               },
               headers: HttpRequestMock.activitypub_object_headers()
             )

test/support/http_request_mock.ex

@@ -572,6 +572,7 @@ def get("https://social.stopwatchingus-heidelberg.de/.well-known/host-meta", _,
      }}
   end
 
+  # Mastodon status via display URL
   def get(
         "http://mastodon.example.org/@admin/99541947525187367",
         _,
@@ -581,6 +582,23 @@ def get(
     {:ok,
      %Tesla.Env{
        status: 200,
+       url: "http://mastodon.example.org/@admin/99541947525187367",
+       body: File.read!("test/fixtures/mastodon-note-object.json"),
+       headers: activitypub_object_headers()
+     }}
+  end
+
+  # same status via its canonical ActivityPub id
+  def get(
+        "http://mastodon.example.org/users/admin/statuses/99541947525187367",
+        _,
+        _,
+        _
+      ) do
+    {:ok,
+     %Tesla.Env{
+       status: 200,
+       url: "http://mastodon.example.org/users/admin/statuses/99541947525187367",
        body: File.read!("test/fixtures/mastodon-note-object.json"),
        headers: activitypub_object_headers()
      }}