GHSA-6jvx-8ch9-j2jr: 
PHP vulnerability analysis and mitigation

Laravel experienced a cookie serialization vulnerability affecting versions 5.5.0 through 5.6.30. The vulnerability was discovered and disclosed on August 7, 2018, impacting the Laravel PHP framework. This security issue specifically affected the framework's cookie encryption and serialization logic (Laravel Docs, GitHub Advisory).

The vulnerability relates to the cookie serialization and encryption mechanism in Laravel. The issue could only be exploited if a malicious user gained access to the application's encryption key (APP_KEY environment variable). When exploited, it could allow attackers to craft cookie values using the encryption key and potentially exploit vulnerabilities inherent to PHP object serialization/unserialization, including the ability to call arbitrary class methods within the application (Laravel Docs).

If exploited, this vulnerability could lead to unauthorized access to application sessions and potential execution of arbitrary code within the application context. The impact was significant enough that Laravel 5.6.30 implemented a breaking change to cookie encryption and serialization logic, which invalidated all existing application sessions and required users to re-authenticate (Laravel Docs).

Laravel addressed this vulnerability in version 5.6.30 by disabling all serialization/unserialization of cookie values by default. For applications requiring cookie serialization, developers can re-enable it through the App\Http\Middleware\EncryptCookies middleware by setting the static $serialize property to true. However, this is not recommended if there's any possibility that the encryption key has been compromised. Additionally, Laravel recommends rotating the application encryption key if there's any suspicion it may have been accessed by malicious parties (Laravel Docs).