On the Laravel News site they've posted an announcement recommending all Laravel 5.6.x and 5.5.x users upgrade to the latest release (5.6.30 & 5.5.42) due to a security issue dealing with the
Laravel 5.6.30 and Laravel 5.5.42 have both been released to fix a security issue and is recommended that all users upgrade as soon as possible.
This update also includes changes to cookie encryption and serialization logic. In addition to the upgrade, they also recommend rotating the key if you believe any malicious user (or former developer/employee) had access to it. The upgrade guide has the information you need to make the update to your application.