source: https://www.securityfocus.com/bid/7761/info

A cross-site scripting vulnerability has been discovered in PHP. The problem occurs due to insufficient sanitization of the PHPSESSID URI parameter. An attacker may be capable of exploiting this vulnerability by constructing a malicious link containing script code embedded within this variable.

Successful exploitation of this issue would allow an attacker to execute arbitrary script code in a victim's browser within the context of the visited website. This may allow for the theft of sensitive information or other attacks. 

http://www.example.org/index.php?PHPSESSID="><script>...</script>