diff --git a/BREAKING_CHANGES_FOR_V16.md b/BREAKING_CHANGES_FOR_V16.md
new file mode 100644
index 00000000..3259869c
--- /dev/null
+++ b/BREAKING_CHANGES_FOR_V16.md
@@ -0,0 +1,63 @@
+# Breaking change notice for version 16.0.0
+
+## Removal of `Session#serialize` and `Session.deserialize` methods
+
+The `Session#serialize` and `Session.deserialize` methods have been removed due to a security vulnerability. The `deserialize` method used `Oj.load` without safe mode, which allows instantiation of arbitrary Ruby objects.
+
+These methods were originally created for session persistence when the library handled session storage. After session storage was deprecated in v12.3.0, applications became responsible for their own session persistence, making these methods unnecessary for their original purpose.
+
+### Why this change?
+
+**No impact on most applications:** The `shopify_app gem` stores individual session attributes in database columns and reconstructs sessions using `Session.new()`, which is the recommended pattern.
+
+## Migration Guide
+
+If your application was using `Session#serialize` and `Session.deserialize` for session persistence, you'll need to refactor to store individual session attributes and reconstruct sessions using `Session.new()`.
+
+### Previous implementation (removed in v16.0.0)
+
+```ruby
+# Storing a session
+session = ShopifyAPI::Auth::Session.new(
+  shop: "example.myshopify.com",
+  access_token: "shpat_xxxxx",
+  scope: "read_products,write_orders"
+)
+
+serialized_data = session.serialize
+# Store serialized_data in Redis, database, etc.
+redis.set("session:#{session.id}", serialized_data)
+
+# Retrieving a session
+serialized_data = redis.get("session:#{session_id}")
+session = ShopifyAPI::Auth::Session.deserialize(serialized_data)
+```
+
+### New implementation (required in v16.0.0)
+
+Store individual session attributes and reconstruct using `Session.new()`:
+
+## Reference: shopify_app gem implementation
+
+The [shopify_app gem](https://github.com/Shopify/shopify_app) provides a reference implementation of session storage that follows these best practices:
+
+**Shop Session Storage** ([source](https://github.com/Shopify/shopify_app/blob/main/lib/shopify_app/session/shop_session_storage.rb)):
+```ruby
+# Stores attributes in database columns
+def store(auth_session)
+  shop = find_or_initialize_by(shopify_domain: auth_session.shop)
+  shop.shopify_token = auth_session.access_token
+  shop.save!
+end
+
+# Reconstructs using Session.new()
+def retrieve(id)
+  shop = find_by(id: id)
+  return unless shop
+
+  ShopifyAPI::Auth::Session.new(
+    shop: shop.shopify_domain,
+    access_token: shop.shopify_token
+  )
+end
+```
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 166152d1..3e8e88f9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 Note: For changes to the API, see https://shopify.dev/changelog?filter=api
 ## Unreleased
+- ⚠️ [Breaking] Removed `Session#serialize` and `Session.deserialize` methods due to security concerns (RCE vulnerability via `Oj.load`). These methods were not used internally by the library. If your application relies on session serialization, use `Session.new()` to reconstruct sessions from stored attributes instead.
 
 ### 15.0.0
 
diff --git a/lib/shopify_api/auth/session.rb b/lib/shopify_api/auth/session.rb
index 68cef277..3642f4de 100644
--- a/lib/shopify_api/auth/session.rb
+++ b/lib/shopify_api/auth/session.rb
@@ -117,29 +117,22 @@ def from(shop:, access_token_response:)
             shopify_session_id: access_token_response.session,
           )
         end
-
-        sig { params(str: String).returns(Session) }
-        def deserialize(str)
-          Oj.load(str)
-        end
       end
 
       sig { params(other: Session).returns(Session) }
       def copy_attributes_from(other)
-        JSON.parse(other.serialize).keys.each do |key|
-          next if key.include?("^")
-
-          variable_name = "@#{key}"
-          instance_variable_set(variable_name, other.instance_variable_get(variable_name))
-        end
+        @shop = other.shop
+        @state = other.state
+        @access_token = other.access_token
+        @scope = other.scope
+        @associated_user_scope = other.associated_user_scope
+        @expires = other.expires
+        @associated_user = other.associated_user
+        @is_online = other.online?
+        @shopify_session_id = other.shopify_session_id
         self
       end
 
-      sig { returns(String) }
-      def serialize
-        Oj.dump(self)
-      end
-
       alias_method :eql?, :==
       sig { params(other: T.nilable(Session)).returns(T::Boolean) }
       def ==(other)