Laravel 5.6.15 and 5.5.40 is released with a security patch

The Laravel team has just released v5.6.15 and v5.5.40 and both include a security update so it’s recommended that everyone upgrade. In Taylor’s announcement he mentions the root cause:

Yesterday, Felix Wiedemann alerted me to a potential exploit of the Laravel Encrypter component that may cause the Encrypter to fail on decryption and unexpectedly return false.

To exploit this, the attacker must be able to modify the encrypted payload before it is decrypted. Depending on the code within your application, this could lead to unexpected behavior when combined with weak type comparisons, for example:

$decyptedValue = decrypt($secret);
if ($decryptedValue == '') {
    // Code is run even though decrypted value is false...
}

In these new releases, the underlying code has been changed to force the Encrypter to throw an exception when this occurs.