fix(federation): enforce the signature policy on incoming requests

HTTPSignaturePlug checks the policy before the cryptographic check. A
signed GET that fails it counts as unsigned; the inbox answers 401 to a
POST that fails it instead of queueing a failed-signature retry, which
only re-checks the signature. signature_policy: :log only logs.

Based on Akkoma's HTTP signature hardening.

changelog.d/signature-freshness.security

@@ -0,0 +1 @@
+Reject incoming HTTP signatures that do not sign the date, are older than 2 hours (or more than 1 hour in the future), or do not sign the digest of a request body, so captured requests cannot be replayed. Set `config :pleroma, :activitypub, signature_policy: :log` to only log these

lib/pleroma/web/activity_pub/activity_pub_controller.ex

@@ -386,18 +386,25 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubController do
   end
 
   def inbox(%{assigns: %{valid_signature: false}} = conn, params) do
-    if unsupported_http_message_signature?(conn) do
+    cond do
-      reject_unsupported_http_message_signature(conn)
+      unsupported_http_message_signature?(conn) ->
-    else
+        reject_unsupported_http_message_signature(conn)
-      Federator.incoming_failed_signature_ap_doc(%{
-        method: conn.method,
-        req_headers: conn.req_headers,
-        request_path: conn.request_path,
-        params: params,
-        query_string: conn.query_string
-      })
 
-      json(conn, "ok")
+      reason = conn.assigns[:signature_error] ->
+        conn
+        |> put_status(:unauthorized)
+        |> json("error, invalid HTTP signature (#{reason})")
+
+      true ->
+        Federator.incoming_failed_signature_ap_doc(%{
+          method: conn.method,
+          req_headers: conn.req_headers,
+          request_path: conn.request_path,
+          params: params,
+          query_string: conn.query_string
+        })
+
+        json(conn, "ok")
     end
   end
 

lib/pleroma/web/plugs/http_signature_plug.ex

@@ -45,20 +45,52 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
 
   defp maybe_assign_valid_signature(conn) do
     if has_signature_header?(conn) do
-      # we replace the digest header with the one we computed in DigestPlug
+      conn = put_computed_digest(conn)
-      conn =
-        case conn do
-          %{assigns: %{digest: digest}} = conn -> put_req_header(conn, "digest", digest)
-          conn -> conn
-        end
 
-      assign(conn, :valid_signature, Signature.validate_signature(conn))
+      case check_signature_policy(conn) do
+        :ok ->
+          assign(conn, :valid_signature, Signature.validate_signature(conn))
+
+        {:error, reason} ->
+          reject_signature(conn, reason)
+      end
     else
       Logger.debug("No signature header!")
       conn
     end
   end
 
+  # we replace the digest header with the one we computed in DigestPlug
+  defp put_computed_digest(%{assigns: %{digest: digest}} = conn),
+    do: put_req_header(conn, "digest", digest)
+
+  defp put_computed_digest(conn), do: conn
+
+  defp check_signature_policy(conn) do
+    with {:error, reason} <- Signature.check_signature_policy(conn) do
+      if Pleroma.Config.get([:activitypub, :signature_policy], :enforce) == :log do
+        Logger.warning(
+          "HTTP signature policy violation (#{reason}) for #{conn.method} #{conn.request_path}, allowed by config"
+        )
+
+        :ok
+      else
+        {:error, reason}
+      end
+    end
+  end
+
+  # The request is treated as badly signed. The reason is kept so the inbox
+  # can refuse it instead of queueing a failed-signature retry, which only
+  # checks the cryptographic signature.
+  defp reject_signature(conn, reason) do
+    Logger.debug("Rejecting HTTP signature: #{reason}")
+
+    conn
+    |> assign(:valid_signature, false)
+    |> assign(:signature_error, reason)
+  end
+
   defp maybe_assign_actor_id(%{assigns: %{valid_signature: true}} = conn) do
     adapter = Application.get_env(:http_signatures, :adapter)
 

test/pleroma/web/activity_pub/activity_pub_controller_test.exs

@@ -9,6 +9,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
   alias Pleroma.Activity
   alias Pleroma.Delivery
   alias Pleroma.Object
+  alias Pleroma.Signature
   alias Pleroma.Tests.ObanHelpers
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.ActivityPub
@@ -49,6 +50,16 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
     |> put_req_header("signature", "keyId=\"#{actor_id}#main-key\"")
   end
 
+  defp put_fake_signature(conn, actor_id, date \\ Signature.signed_date()) do
+    conn
+    |> put_req_header("date", date)
+    |> put_req_header("digest", "SHA-256=fake-digest")
+    |> put_req_header(
+      "signature",
+      "keyId=\"#{actor_id}#main-key\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest content-type\",signature=\"fake-signature\""
+    )
+  end
+
   defp expect_signature_retry_from(%User{} = signer) do
     signer_json = UserView.render("user.json", %{user: signer}) |> Map.delete("featured")
 
@@ -792,12 +803,12 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
         }
       }
 
-      Mox.expect(Pleroma.StubbedHTTPSignaturesMock, :validate_conn, fn _conn -> false end)
+      Mox.stub(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("date", Signature.signed_date())
         |> put_req_header("content-digest", "sha-256=:fake-digest:")
         |> put_req_header(
           "signature-input",
@@ -832,12 +843,12 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       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)
+      Mox.stub(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("date", Signature.signed_date())
         |> put_req_header("digest", "SHA-256=fake-digest")
         |> put_req_header("signature", "sig1=:fake-signature:")
         |> Map.update!(:req_headers, &[{"signature", legacy_signature} | &1])
@@ -871,7 +882,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       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("date", Signature.signed_date())
         |> put_req_header("digest", "SHA-256=fake-digest")
         |> put_req_header(
           "signature-input",
@@ -881,12 +892,44 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
           "signature",
           "keyId=\"https://activitypubbot.example/user/ok#main-key\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest content-type\",signature=\"fake-signature\""
         )
-        |> post("/inbox", data)
+        |> post("/inbox", Jason.encode!(data))
 
       assert "ok" == json_response(conn, 200)
       assert [_] = all_enqueued(worker: SignatureRetryWorker)
     end
 
+    test "refuses a replayed signed POST instead of queueing a retry", %{conn: conn} do
+      data = File.read!("test/fixtures/mastodon-post-activity.json") |> Jason.decode!()
+      stale = NaiveDateTime.add(NaiveDateTime.utc_now(), -3 * 3600)
+
+      conn =
+        conn
+        |> put_req_header("content-type", "application/activity+json")
+        |> put_fake_signature(data["actor"], Signature.signed_date(stale))
+        |> post("/inbox", Jason.encode!(data))
+
+      assert "error, invalid HTTP signature (date_too_old)" == json_response(conn, 401)
+      assert [] == all_enqueued(worker: SignatureRetryWorker)
+      assert [] == all_enqueued(worker: ReceiverWorker)
+    end
+
+    test "refuses a signed POST that does not cover the digest", %{conn: conn} do
+      data = File.read!("test/fixtures/mastodon-post-activity.json") |> Jason.decode!()
+
+      conn =
+        conn
+        |> put_req_header("content-type", "application/activity+json")
+        |> put_req_header("date", Signature.signed_date())
+        |> put_req_header(
+          "signature",
+          "keyId=\"#{data["actor"]}#main-key\",headers=\"(request-target) host date\",signature=\"fake\""
+        )
+        |> post("/inbox", Jason.encode!(data))
+
+      assert "error, invalid HTTP signature (digest_not_signed)" == json_response(conn, 401)
+      assert [] == all_enqueued(worker: SignatureRetryWorker)
+    end
+
     test "does not create a forged post after failed signature retry", %{conn: conn} do
       alice = insert(:user, local: false, ap_id: "https://one.com/users/alice")
       bob = insert(:user, local: false, ap_id: "https://two.com/users/bob")
@@ -918,8 +961,8 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
         conn
         |> assign(:valid_signature, false)
         |> put_req_header("content-type", "application/activity+json")
-        |> put_req_header("signature", "keyId=\"https://one.com/users/alice#main-key\"")
+        |> put_fake_signature(alice.ap_id)
-        |> post("/inbox", data)
+        |> post("/inbox", Jason.encode!(data))
 
       assert "ok" == json_response(conn, 200)
 
@@ -950,8 +993,8 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
         conn
         |> assign(:valid_signature, false)
         |> put_req_header("content-type", "application/activity+json")
-        |> put_req_header("signature", "keyId=\"https://one.com/users/alice#main-key\"")
+        |> put_fake_signature(alice.ap_id)
-        |> post("/inbox", data)
+        |> post("/inbox", Jason.encode!(data))
 
       assert "ok" == json_response(conn, 200)
 
@@ -982,8 +1025,8 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
         conn
         |> assign(:valid_signature, false)
         |> put_req_header("content-type", "application/activity+json")
-        |> put_req_header("signature", "keyId=\"https://one.com/users/alice#main-key\"")
+        |> put_fake_signature(alice.ap_id)
-        |> post("/inbox", data)
+        |> post("/inbox", Jason.encode!(data))
 
       assert "ok" == json_response(conn, 200)
 
@@ -1025,13 +1068,13 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       conn =
         conn
         |> put_req_header("content-type", "application/activity+json")
-        |> put_req_header("date", "Thu, 25 Jul 2024 13:33:31 GMT")
+        |> put_req_header("date", Signature.signed_date())
         |> put_req_header("digest", "SHA-256=fake-digest")
         |> put_req_header(
           "signature",
           "keyId=\"#{alice.ap_id}#main-key\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest content-type\",signature=\"fake-signature\""
         )
-        |> post("/inbox", data)
+        |> post("/inbox", Jason.encode!(data))
 
       assert conn.assigns.valid_signature == false
       assert "ok" == json_response(conn, 200)
@@ -1063,13 +1106,13 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubControllerTest do
       conn =
         conn
         |> put_req_header("content-type", "application/activity+json")
-        |> put_req_header("date", "Thu, 25 Jul 2024 13:33:31 GMT")
+        |> put_req_header("date", Signature.signed_date())
         |> put_req_header("digest", "SHA-256=fake-digest")
         |> put_req_header(
           "signature",
           "keyId=\"#{alice.ap_id}#main-key\",algorithm=\"rsa-sha256\",headers=\"(request-target) host date digest content-type\",signature=\"fake-signature\""
         )
-        |> post("/inbox", data)
+        |> post("/inbox", Jason.encode!(data))
 
       assert conn.assigns.valid_signature == false
       assert "ok" == json_response(conn, 200)

test/pleroma/web/plugs/http_signature_plug_mrf_test.exs

@@ -26,7 +26,11 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugMRFTest do
     |> expect(:validate_conn, fn _ -> true end)
 
     build_conn(:get, "/doesntmattter", %{"actor" => actor})
-    |> put_req_header("signature", "keyId=\"#{actor}#main-key")
+    |> put_req_header("date", Pleroma.Signature.signed_date())
+    |> put_req_header(
+      "signature",
+      "keyId=\"#{actor}#main-key\",headers=\"(request-target) host date\",signature=\"fake\""
+    )
     |> put_format("activity+json")
     |> HTTPSignaturePlug.call(%{})
   end

test/pleroma/web/plugs/http_signature_plug_policy_test.exs

@@ -0,0 +1,119 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.HTTPSignaturePlugPolicyTest do
+  # Not async: changes global config and uses global HTTP mocks.
+  use Pleroma.Web.ConnCase
+
+  alias Pleroma.Keys
+  alias Pleroma.Object.Fetcher
+  alias Pleroma.Repo
+  alias Pleroma.StubbedHTTPSignaturesMock, as: HTTPSignaturesMock
+  alias Pleroma.User
+  alias Pleroma.Web.ActivityPub.InternalFetchActor
+  alias Pleroma.Web.ActivityPub.Publisher
+  alias Pleroma.Web.Plugs.DigestPlug
+  alias Pleroma.Web.Plugs.HTTPSignaturePlug
+
+  import ExUnit.CaptureLog
+  import Mox
+  import Phoenix.Controller, only: [put_format: 2]
+  import Pleroma.Factory
+  import Plug.Conn
+
+  defp incoming_conn(method, url, headers, body \\ "") do
+    %URI{host: host, path: path} = URI.parse(url)
+
+    headers = Enum.map(headers, fn {name, value} -> {String.downcase(name), value} end)
+
+    Plug.Test.conn(method, path, body)
+    |> Map.put(:req_headers, [{"host", host} | headers])
+    |> put_format("activity+json")
+  end
+
+  # Local users have no public_key column set; publish it like a remote copy would.
+  defp store_public_key(%User{} = user) do
+    {:ok, _, public_key} = Keys.keys_from_pem(user.keys)
+    entry = :public_key.pem_entry_encode(:SubjectPublicKeyInfo, public_key)
+
+    user
+    |> Ecto.Changeset.change(public_key: :public_key.pem_encode([entry]))
+    |> Repo.update!()
+    |> User.set_cache()
+  end
+
+  defp capture_request(method) do
+    test_pid = self()
+
+    Tesla.Mock.mock_global(fn %{method: ^method, headers: headers, body: body} = env ->
+      send(test_pid, {:request, headers, body})
+      %{env | status: 200, body: ""}
+    end)
+  end
+
+  test "accepts deliveries signed by Pleroma's publisher" do
+    stub_with(Pleroma.Web.ActivityPub.TransmogrifierMock, Pleroma.Web.ActivityPub.Transmogrifier)
+    capture_request(:post)
+
+    activity = insert(:note_activity)
+    store_public_key(User.get_by_ap_id(activity.data["actor"]))
+    inbox = "https://remote.example/inbox"
+
+    {:ok, _} =
+      %{inbox: inbox, activity_id: activity.id}
+      |> Publisher.prepare_one()
+      |> Publisher.publish_one()
+
+    assert_received {:request, headers, body}
+
+    {:ok, _, conn} =
+      incoming_conn(:post, inbox, [{"content-length", "#{byte_size(body)}"} | headers], body)
+      |> DigestPlug.read_body([])
+
+    conn = HTTPSignaturePlug.call(conn, %{})
+
+    assert conn.assigns.valid_signature == true
+    refute conn.halted
+  end
+
+  test "accepts object fetches signed by Pleroma's fetcher" do
+    clear_config([:activitypub, :sign_object_fetches], true)
+    capture_request(:get)
+    store_public_key(InternalFetchActor.get_actor())
+
+    url = "https://remote.example/objects/1"
+    Fetcher.fetch_and_contain_remote_object_from_id(url)
+
+    assert_received {:request, headers, _body}
+
+    conn =
+      incoming_conn(:get, url, headers)
+      |> HTTPSignaturePlug.call(%{})
+
+    assert conn.assigns.valid_signature == true
+  end
+
+  test "only logs policy violations when the policy is set to :log" do
+    clear_config([:activitypub, :signature_policy], :log)
+
+    HTTPSignaturesMock
+    |> expect(:validate_conn, fn _ -> true end)
+
+    log =
+      capture_log(fn ->
+        conn =
+          incoming_conn(:post, "https://pleroma.example/inbox", [
+            {"date", Pleroma.Signature.signed_date()},
+            {"signature",
+             "keyId=\"https://remote.example/users/a#main-key\",headers=\"(request-target) host date\",signature=\"fake\""}
+          ])
+          |> HTTPSignaturePlug.call(%{})
+
+        assert conn.assigns.valid_signature == true
+        refute conn.halted
+      end)
+
+    assert log =~ "digest_not_signed"
+  end
+end

test/pleroma/web/plugs/http_signature_plug_test.exs

@@ -13,6 +13,15 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
   import Phoenix.Controller, only: [put_format: 2]
   import Plug.Conn
 
+  defp put_signature(conn, actor, signed_headers \\ "(request-target) host date") do
+    conn
+    |> put_req_header("date", Pleroma.Signature.signed_date())
+    |> put_req_header(
+      "signature",
+      "keyId=\"#{actor}#main-key\",headers=\"#{signed_headers}\",signature=\"fake\""
+    )
+  end
+
   test "it calls HTTPSignatures to check validity if the actor signed it" do
     params = %{"actor" => "http://mastodon.example.org/users/admin"}
     conn = build_conn(:get, "/doesntmattter", params)
@@ -22,10 +31,7 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
 
     conn =
       conn
-      |> put_req_header(
+      |> put_signature("http://mastodon.example.org/users/admin")
-        "signature",
-        "keyId=\"http://mastodon.example.org/users/admin#main-key"
-      )
       |> put_format("activity+json")
       |> HTTPSignaturePlug.call(%{})
 
@@ -51,10 +57,7 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
 
       conn =
         orig_conn
-        |> put_req_header(
+        |> put_signature("http://mastodon.example.org/users/admin")
-          "signature",
-          "keyId=\"http://mastodon.example.org/users/admin#main-key"
-        )
         |> HTTPSignaturePlug.call(%{})
 
       assert conn.assigns.valid_signature == false
@@ -71,10 +74,7 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
 
       conn =
         orig_conn
-        |> put_req_header(
+        |> put_signature("http://mastodon.example.org/users/admin")
-          "signature",
-          "keyId=\"http://mastodon.example.org/users/admin#main-key"
-        )
         |> HTTPSignaturePlug.call(%{})
 
       assert conn.assigns.valid_signature == true
@@ -108,10 +108,7 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
       conn =
         conn
         |> Map.put(:remote_ip, {192, 168, 0, 1})
-        |> put_req_header(
+        |> put_signature("http://mastodon.example.org/users/admin")
-          "signature",
-          "keyId=\"http://mastodon.example.org/users/admin#main-key"
-        )
         |> HTTPSignaturePlug.call(%{})
 
       assert conn.remote_ip == {192, 168, 0, 1}
@@ -131,10 +128,7 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
 
     conn =
       build_conn(:get, "/doesntmattter", %{"actor" => "http://mastodon.example.org/users/admin"})
-      |> put_req_header(
+      |> put_signature("http://mastodon.example.org/users/admin")
-        "signature",
-        "keyId=\"http://mastodon.example.org/users/admin#main-key"
-      )
       |> put_format("activity+json")
       |> HTTPSignaturePlug.call(%{})
 
@@ -152,14 +146,84 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlugTest do
 
     conn =
       build_conn(:get, "/doesntmattter", %{"actor" => "http://allowed.example.org/users/admin"})
-      |> put_req_header(
+      |> put_signature("http://allowed.example.org/users/admin")
-        "signature",
-        "keyId=\"http://allowed.example.org/users/admin#main-key"
-      )
       |> put_format("activity+json")
       |> HTTPSignaturePlug.call(%{})
 
     assert conn.assigns.valid_signature == true
     assert conn.halted == false
   end
+
+  describe "signature policy" do
+    test "treats a signed GET with a stale date as unsigned" do
+      HTTPSignaturesMock
+      |> expect(:validate_conn, 0, fn _ -> true end)
+
+      stale = NaiveDateTime.add(NaiveDateTime.utc_now(), -3 * 3600)
+
+      conn =
+        build_conn(:get, "/doesntmattter")
+        |> put_signature("http://mastodon.example.org/users/admin")
+        |> put_req_header("date", Pleroma.Signature.signed_date(stale))
+        |> put_format("activity+json")
+        |> HTTPSignaturePlug.call(%{})
+
+      assert conn.assigns.valid_signature == false
+      assert conn.assigns.signature_error == :date_too_old
+      refute conn.halted
+    end
+
+    test "marks a signed POST that does not cover the digest as invalid" do
+      HTTPSignaturesMock
+      |> expect(:validate_conn, 0, fn _ -> true end)
+
+      conn =
+        build_conn(:post, "/inbox")
+        |> put_signature("http://mastodon.example.org/users/admin")
+        |> put_format("activity+json")
+        |> HTTPSignaturePlug.call(%{})
+
+      assert conn.assigns.valid_signature == false
+      assert conn.assigns.signature_error == :digest_not_signed
+      refute conn.halted
+    end
+
+    test "uses the digest computed from the body for signed POSTs" do
+      HTTPSignaturesMock
+      |> expect(:validate_conn, fn conn ->
+        get_req_header(conn, "digest") == ["SHA-256=computed"]
+      end)
+
+      conn =
+        build_conn(:post, "/inbox")
+        |> put_signature(
+          "http://mastodon.example.org/users/admin",
+          "(request-target) host date digest"
+        )
+        |> put_req_header("digest", "SHA-256=claimed")
+        |> assign(:digest, "SHA-256=computed")
+        |> put_format("activity+json")
+        |> HTTPSignaturePlug.call(%{})
+
+      assert conn.assigns.valid_signature == true
+    end
+
+    test "marks a signed POST whose body was not hashed as invalid" do
+      HTTPSignaturesMock
+      |> expect(:validate_conn, 0, fn _ -> true end)
+
+      conn =
+        build_conn(:post, "/inbox")
+        |> put_signature(
+          "http://mastodon.example.org/users/admin",
+          "(request-target) host date digest"
+        )
+        |> put_req_header("digest", "SHA-256=claimed")
+        |> put_format("activity+json")
+        |> HTTPSignaturePlug.call(%{})
+
+      assert conn.assigns.valid_signature == false
+      assert conn.assigns.signature_error == :body_not_hashed
+    end
+  end
 end