11.3. Gray-Box Testing
In the third and final phase of security assessment, the black-box testing procedures are executed again but this time using the knowledge acquired in the white-box testing phase. This is similar to the type of testing an attacker might do when he has access to the source code, but here you have a slight advantage because you know the layout of the files on disk, the configuration, and changes made to the original source code (if any). This time you are also allowed to have access to the target system while you are testing it from the outside. For example, you can look at the application logs to discover why some of your attacks are failing.
The gray-box testing phase is the time to confirm or deny the assumptions about vulnerabilities you made in the black-box phase. For example, maybe you thought Apache was vulnerable to a particular problem but you did not want to try to exploit it at that time. Looking at it from the inside, it is much easier and quicker to determine if your assumption was correct.