From 8da406afa2a53f1769e2a1a3a5e99571f30f8dea Mon Sep 17 00:00:00 2001
From: William Pitcock <nenolod@dereferenced.org>
Date: Tue, 31 Jul 2018 23:17:47 +0000
Subject: [PATCH] activitypub: verify remote http signature digests by
 recomputing the digest and replacing the digest header

---
 lib/pleroma/plugs/digest.ex         | 10 ++++++++++
 lib/pleroma/plugs/http_signature.ex | 10 ++++++++++
 lib/pleroma/web/endpoint.ex         |  3 ++-
 3 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 lib/pleroma/plugs/digest.ex

diff --git a/lib/pleroma/plugs/digest.ex b/lib/pleroma/plugs/digest.ex
new file mode 100644
index 000000000..9d6bbb085
--- /dev/null
+++ b/lib/pleroma/plugs/digest.ex
@@ -0,0 +1,10 @@
+defmodule Pleroma.Web.Plugs.DigestPlug do
+  alias Plug.Conn
+  require Logger
+
+  def read_body(conn, opts) do
+    {:ok, body, conn} = Conn.read_body(conn, opts)
+    digest = "SHA-256=" <> (:crypto.hash(:sha256, body) |> Base.encode64())
+    {:ok, body, Conn.assign(conn, :digest, digest)}
+  end
+end
diff --git a/lib/pleroma/plugs/http_signature.ex b/lib/pleroma/plugs/http_signature.ex
index 38bcd3a78..9e53371b7 100644
--- a/lib/pleroma/plugs/http_signature.ex
+++ b/lib/pleroma/plugs/http_signature.ex
@@ -19,6 +19,8 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
 
     cond do
       signature && String.contains?(signature, user) ->
+        # set (request-target) header to the appropriate value
+        # we also replace the digest header with the one we computed
         conn =
           conn
           |> put_req_header(
@@ -26,6 +28,14 @@ defmodule Pleroma.Web.Plugs.HTTPSignaturePlug do
             String.downcase("#{conn.method}") <> " #{conn.request_path}"
           )
 
+        conn =
+          if conn.assigns[:digest] do
+            conn
+            |> put_req_header("digest", conn.assigns[:digest])
+          else
+            conn
+          end
+
         assign(conn, :valid_signature, HTTPSignatures.validate_conn(conn))
 
       signature ->
diff --git a/lib/pleroma/web/endpoint.ex b/lib/pleroma/web/endpoint.ex
index 1a012c1b4..cbedca004 100644
--- a/lib/pleroma/web/endpoint.ex
+++ b/lib/pleroma/web/endpoint.ex
@@ -35,7 +35,8 @@ defmodule Pleroma.Web.Endpoint do
     parsers: [:urlencoded, :multipart, :json],
     pass: ["*/*"],
     json_decoder: Jason,
-    length: Application.get_env(:pleroma, :instance) |> Keyword.get(:upload_limit)
+    length: Application.get_env(:pleroma, :instance) |> Keyword.get(:upload_limit),
+    body_reader: {Pleroma.Web.Plugs.DigestPlug, :read_body, []}
   )
 
   plug(Plug.MethodOverride)