feat(signature): add a freshness and coverage policy for incoming signatures

Signature.check_signature_policy/2 requires date or (created) to be
signed and within signature_max_age (2 h) / signature_max_clock_skew
(1 h), and requires a signed, hashed digest for requests with a body.

Based on Akkoma's http_signatures fork (required headers, max_sig_age)
and Mastodon's SignedRequest (clock skew margin).

config/config.exs

@@ -373,6 +373,9 @@ config :pleroma, :activitypub,
   note_replies_output_limit: 5,
   sign_object_fetches: true,
   authorized_fetch_mode: false,
+  signature_policy: :enforce,
+  signature_max_age: 7200,
+  signature_max_clock_skew: 3600,
   client_api_enabled: false,
   anonymize_reporter: false,
   anonymize_reporter_local_nickname: ""

config/description.exs

@@ -1788,6 +1788,27 @@ config :pleroma, :config_description, [
         description:
           "List of IPs (CIDR format accepted) to exempt from HTTP Signatures requirement (for example to allow debugging, you shouldn't otherwise need this)"
       },
+      %{
+        key: :signature_policy,
+        type: {:dropdown, :atom},
+        description:
+          "What to do with incoming HTTP signatures that do not sign the date (or `(created)`), are outside the allowed time window, or do not sign the digest of a request body. `:enforce` treats them as invalid, `:log` only logs them",
+        suggestions: [:enforce, :log]
+      },
+      %{
+        key: :signature_max_age,
+        type: :integer,
+        description:
+          "How old, in seconds, the signed date of an incoming HTTP signature can be. Default: 7200 (2 hours)",
+        suggestions: [7200]
+      },
+      %{
+        key: :signature_max_clock_skew,
+        type: :integer,
+        description:
+          "How far, in seconds, the signed date of an incoming HTTP signature can be in the future. Default: 3600 (1 hour). A `(created)` time in the future is always rejected",
+        suggestions: [3600]
+      },
       %{
         key: :note_replies_output_limit,
         type: :integer,

docs/configuration/cheatsheet.md

@@ -286,6 +286,9 @@ Notes:
 * `sign_object_fetches`: Sign object fetches with HTTP signatures
 * `authorized_fetch_mode`: Require HTTP signatures for AP fetches
 * `authorized_fetch_mode_exceptions`: List of IPs (CIDR format accepted) to exempt from HTTP Signatures requirement (for example to allow debugging, you shouldn't otherwise need this)
+* `signature_policy`: What to do with incoming HTTP signatures that do not sign the `date` header (or `(created)`), whose signed date is outside the window below, or that do not sign the `digest` header of a request with a body (or whose body Pleroma did not hash). `:enforce` (default) treats them as invalid, `:log` only logs a warning.
+* `signature_max_age`: How old, in seconds, the signed date of an incoming HTTP signature can be. Default: `7200` (2 hours).
+* `signature_max_clock_skew`: How far, in seconds, the signed `date` of an incoming HTTP signature can be in the future. Default: `3600` (1 hour). A `(created)` time in the future is always rejected.
 
 ## Pleroma.User
 

lib/pleroma/signature.ex

@@ -95,6 +95,84 @@ defmodule Pleroma.Signature do
     Timex.format!(date, "{WDshort}, {0D} {Mshort} {YYYY} {h24}:{m}:{s} GMT")
   end
 
+  @doc """
+  Checks the parts of an incoming signature that the cryptographic check does
+  not cover, so that a captured request cannot be replayed later or with a
+  different body.
+
+  The signature must cover `date` or `(created)`, and the signed time must be
+  within the configured window. Requests other than GET and HEAD must also
+  sign `digest`, and DigestPlug must have hashed their body. Based on Akkoma's http_signatures fork and Mastodon's
+  SignedRequest.
+  """
+  @spec check_signature_policy(Plug.Conn.t(), DateTime.t()) :: :ok | {:error, atom()}
+  def check_signature_policy(%Plug.Conn{} = conn, now \\ DateTime.utc_now()) do
+    signature = HTTPSignatures.signature_for_conn(conn)
+    signed = Map.get(signature, "headers", [])
+
+    cond do
+      "date" not in signed and "(created)" not in signed ->
+        {:error, :date_not_signed}
+
+      conn.method not in ["GET", "HEAD"] and "digest" not in signed ->
+        {:error, :digest_not_signed}
+
+      # The plug puts the digest computed by DigestPlug into the header. With
+      # no computed digest (e.g. multipart), nothing checked the body.
+      conn.method not in ["GET", "HEAD"] and is_nil(conn.assigns[:digest]) ->
+        {:error, :body_not_hashed}
+
+      true ->
+        with :ok <- check_signed_date(conn, signed, now) do
+          check_signed_created(signature, signed, now)
+        end
+    end
+  end
+
+  defp check_signed_date(conn, signed, now) do
+    if "date" in signed do
+      # Exactly one header: a second one could make this check and the
+      # signature check see different dates.
+      with [date] <- Plug.Conn.get_req_header(conn, "date"),
+           {:ok, %DateTime{} = date} <- Timex.parse(date, "{RFC1123}") do
+        check_time_window(date, now)
+      else
+        _ -> {:error, :invalid_date}
+      end
+    else
+      :ok
+    end
+  end
+
+  defp check_signed_created(signature, signed, now) do
+    if "(created)" in signed do
+      with created when is_binary(created) <- signature["created"],
+           {created, ""} <- Integer.parse(created),
+           {:ok, created} <- DateTime.from_unix(created) do
+        check_time_window(created, now)
+      else
+        _ -> {:error, :invalid_date}
+      end
+    else
+      :ok
+    end
+  end
+
+  defp check_time_window(signed_at, now) do
+    age = DateTime.diff(now, signed_at)
+
+    cond do
+      age > Pleroma.Config.get([:activitypub, :signature_max_age], 7200) ->
+        {:error, :date_too_old}
+
+      -age > Pleroma.Config.get([:activitypub, :signature_max_clock_skew], 3600) ->
+        {:error, :date_in_future}
+
+      true ->
+        :ok
+    end
+  end
+
   @spec validate_signature(Plug.Conn.t(), String.t()) :: boolean()
   def validate_signature(%Plug.Conn{} = conn, request_target) do
     # Newer drafts for HTTP signatures now use @request-target instead of the

test/pleroma/signature_test.exs

@@ -116,6 +116,135 @@ defmodule Pleroma.SignatureTest do
     end
   end
 
+  describe "check_signature_policy/2" do
+    @now ~U[2026-10-27 12:00:00Z]
+
+    defp policy_conn(method, signed_headers, extra_headers, extra_params \\ "") do
+      signature =
+        "keyId=\"https://remote.example/users/a#main-key\",headers=\"#{signed_headers}\"" <>
+          extra_params <> ",signature=\"fake\""
+
+      %Plug.Conn{
+        method: method,
+        req_headers: [{"signature", signature}, {"host", "pleroma.example"} | extra_headers],
+        assigns: %{digest: "SHA-256=abc"}
+      }
+    end
+
+    defp date_header(seconds_ago) do
+      date = DateTime.add(@now, -seconds_ago) |> DateTime.to_naive()
+      {"date", Signature.signed_date(date)}
+    end
+
+    test "accepts a fresh signed GET (Pleroma, Mastodon, Misskey style)" do
+      for headers <- ["(request-target) host date", "(request-target) host date accept"] do
+        conn = policy_conn("GET", headers, [date_header(10)])
+        assert Signature.check_signature_policy(conn, @now) == :ok
+      end
+    end
+
+    test "accepts a fresh signed POST covering the digest" do
+      conn =
+        policy_conn("POST", "(request-target) host content-length digest date", [
+          date_header(10),
+          {"digest", "SHA-256=abc"}
+        ])
+
+      assert Signature.check_signature_policy(conn, @now) == :ok
+    end
+
+    test "accepts (created) instead of date (GoToSocial style)" do
+      created = DateTime.to_unix(@now) - 10
+
+      conn =
+        policy_conn(
+          "POST",
+          "(request-target) (created) host digest",
+          [{"digest", "SHA-256=abc"}],
+          ",algorithm=\"hs2019\",created=#{created}"
+        )
+
+      assert Signature.check_signature_policy(conn, @now) == :ok
+    end
+
+    test "rejects signatures that cover neither date nor (created)" do
+      conn = policy_conn("GET", "(request-target) host", [date_header(10)])
+      assert Signature.check_signature_policy(conn, @now) == {:error, :date_not_signed}
+    end
+
+    test "rejects a body request whose digest is not signed" do
+      conn =
+        policy_conn("POST", "(request-target) host date", [
+          date_header(10),
+          {"digest", "SHA-256=abc"}
+        ])
+
+      assert Signature.check_signature_policy(conn, @now) == {:error, :digest_not_signed}
+    end
+
+    test "rejects a signed date that is missing or unparsable" do
+      conn = policy_conn("GET", "(request-target) host date", [])
+      assert Signature.check_signature_policy(conn, @now) == {:error, :invalid_date}
+
+      conn = policy_conn("GET", "(request-target) host date", [{"date", "yesterday"}])
+      assert Signature.check_signature_policy(conn, @now) == {:error, :invalid_date}
+    end
+
+    test "rejects more than one date header" do
+      conn =
+        policy_conn("GET", "(request-target) host date", [date_header(10), date_header(9000)])
+
+      assert Signature.check_signature_policy(conn, @now) == {:error, :invalid_date}
+    end
+
+    test "rejects a signed digest when the body was not hashed" do
+      conn =
+        policy_conn("POST", "(request-target) host date digest", [
+          date_header(10),
+          {"digest", "SHA-256=abc"}
+        ])
+        |> Map.put(:assigns, %{})
+
+      assert Signature.check_signature_policy(conn, @now) == {:error, :body_not_hashed}
+    end
+
+    test "rejects a signed date that is too old or too far in the future" do
+      clear_config([:activitypub, :signature_max_age], 7200)
+      clear_config([:activitypub, :signature_max_clock_skew], 3600)
+
+      conn = policy_conn("GET", "(request-target) host date", [date_header(7200)])
+      assert Signature.check_signature_policy(conn, @now) == :ok
+
+      conn = policy_conn("GET", "(request-target) host date", [date_header(7201)])
+      assert Signature.check_signature_policy(conn, @now) == {:error, :date_too_old}
+
+      conn = policy_conn("GET", "(request-target) host date", [date_header(-3600)])
+      assert Signature.check_signature_policy(conn, @now) == :ok
+
+      conn = policy_conn("GET", "(request-target) host date", [date_header(-3601)])
+      assert Signature.check_signature_policy(conn, @now) == {:error, :date_in_future}
+    end
+
+    test "rejects a (created) timestamp that is too old" do
+      clear_config([:activitypub, :signature_max_age], 60)
+      created = DateTime.to_unix(@now) - 61
+
+      conn =
+        policy_conn("GET", "(request-target) (created) host", [], ",created=#{created}")
+
+      assert Signature.check_signature_policy(conn, @now) == {:error, :date_too_old}
+    end
+
+    test "uses the cavage default of a signed date when headers are not given" do
+      conn = %Plug.Conn{
+        method: "GET",
+        req_headers: [{"signature", "keyId=\"https://remote.example/users/a#main-key\""}]
+      }
+
+      assert Signature.check_signature_policy(conn, @now) == {:error, :invalid_date}
+    end
+  end
+
   describe "key_id_to_actor_id/1" do
     test "it properly deduces the actor id for misskey" do
       assert Signature.key_id_to_actor_id("https://example.com/users/1234/publickey") ==