fix: compare HMAC identity hashes securely (#14945)

This switches widget and public inbox HMAC identity checks from direct
string equality to constant-time comparison with a length guard.

Fixes
[https://linear.app/chatwoot/issue/CW-6937](https://linear.app/chatwoot/issue/CW-6937)
Fixes
[https://linear.app/chatwoot/issue/CW-7464](https://linear.app/chatwoot/issue/CW-7464)
Fixes
[https://linear.app/chatwoot/issue/CW-7227](https://linear.app/chatwoot/issue/CW-7227)

app/controllers/api/v1/widget/contacts_controller.rb

@@ -73,11 +73,15 @@ class Api::V1::Widget::ContactsController < Api::V1::Widget::BaseController
   end
 
   def valid_hmac?
-    params[:identifier_hash] == OpenSSL::HMAC.hexdigest(
+    expected_hash = OpenSSL::HMAC.hexdigest(
       'sha256',
       @web_widget.hmac_token,
       params[:identifier].to_s
     )
+    identifier_hash = params[:identifier_hash].to_s
+    return false unless identifier_hash.bytesize == expected_hash.bytesize
+
+    ActiveSupport::SecurityUtils.secure_compare(identifier_hash, expected_hash)
   end
 
   def permitted_params

app/controllers/public/api/v1/inboxes/contacts_controller.rb

@@ -35,11 +35,15 @@ class Public::Api::V1::Inboxes::ContactsController < Public::Api::V1::InboxesCon
   end
 
   def valid_hmac?
-    params[:identifier_hash] == OpenSSL::HMAC.hexdigest(
+    expected_hash = OpenSSL::HMAC.hexdigest(
       'sha256',
       @inbox_channel.hmac_token,
       params[:identifier].to_s
     )
+    identifier_hash = params[:identifier_hash].to_s
+    return false unless identifier_hash.bytesize == expected_hash.bytesize
+
+    ActiveSupport::SecurityUtils.secure_compare(identifier_hash, expected_hash)
   end
 
   def permitted_params