Update verify_webhook_signature to match more recent security guideli…

…nes (github-developer#9)

template_server.rb

@@ -118,29 +118,28 @@ def authenticate_installation(payload)
       @installation_client = Octokit::Client.new(bearer_token: @installation_token)
     end
 
-    # Check X-Hub-Signature to confirm that this webhook was generated by
+    # Check X-Hub-Signature-256 to confirm that this webhook was generated by
     # GitHub, and not a malicious third party.
     #
     # GitHub uses the WEBHOOK_SECRET, registered to the GitHub App, to
-    # create the hash signature sent in the `X-HUB-Signature` header of each
+    # create the hash signature sent in the `X-HUB-Signature-256` header of each
     # webhook. This code computes the expected hash signature and compares it to
-    # the signature sent in the `X-HUB-Signature` header. If they don't match,
+    # the signature sent in the `X-HUB-Signature-256` header. If they don't match,
     # this request is an attack, and you should reject it. GitHub uses the HMAC
-    # hexdigest to compute the signature. The `X-HUB-Signature` looks something
-    # like this: "sha1=123456".
-    # See https://developer.github.com/webhooks/securing/ for details.
+    # hexdigest to compute the signature. The `X-HUB-Signature-256` looks something
+    # like this: "sha256=123456".
+    # See https://docs.github.com/en/developers/webhooks-and-events/webhooks/securing-your-webhooks for details.
     def verify_webhook_signature
-      their_signature_header = request.env['HTTP_X_HUB_SIGNATURE'] || 'sha1='
+      their_signature_header = request.env['HTTP_X_HUB_SIGNATURE_256'] || 'sha256='
       method, their_digest = their_signature_header.split('=')
-      our_digest = OpenSSL::HMAC.hexdigest(method, WEBHOOK_SECRET, @payload_raw)
-      halt 401 unless their_digest == our_digest
+      our_digest = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), WEBHOOK_SECRET, @payload_raw)
+      halt 401 unless Rack::Utils.secure_compare(their_digest, our_digest)
 
       # The X-GITHUB-EVENT header provides the name of the event.
       # The action value indicates the which action triggered the event.
       logger.debug "---- received event #{request.env['HTTP_X_GITHUB_EVENT']}"
       logger.debug "----    action #{@payload['action']}" unless @payload['action'].nil?
     end
-
   end
 
   # Finally some logic to let us run this server directly from the command line,