The data can be exploited by both authenticated as well as unauthenticated attackers.
An unauthenticated attacker can inject arbitrary input at one point in vulnerable PHP file, while authenticated attacker can inject at multiple points.
To exploit this vulnerability, the attacker has to find the appropriate classes with beneficial effects, if there is no classes with beneficial effects, it is not exploitable.
"One way to exploit this vulnerability is by utilizing the Tracer class. It is used to write stack trace information to a log using a Logger instance, which wraps an instance of PEAR's Log class. By using a Log_file instance as an instance of Log, it is possible to write the arbitrary data to the arbitrary file." The researcher wrote in his blog post.
