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. According to the upgrade docs:
This vulnerability may only be exploited if your application encryption key (APP_KEY environment variable) has been accessed by a malicious user. Typically, it is not possible for users of your application to gain access to this value. However, ex-employees that had access to the encryption key may be able to use the key to attack your applications. If you have any reason to believe your encryption key is in the hands of a malicious party, you should always rotate the key to a new value.
Laravel 5.6.30 also contains a breaking change to cookie encryption and serialization logic, so please read the upgrade notes carefully before performing an upgrade.
It’s also worth pointing out again that this is only exploitable if a bad person has your APP_KEY and this shouldn’t ever happen unless a disgruntled ex-employee has it or you’ve left it open in a public git repo or something.
The upgrade guide includes everything you need to know on this release and the security impact, so please read it.