This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in CUPS Web Interface.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when handling a malicious HTTP request. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the user's browser, via a crafted HTTP request.
Apple Computer Common UNIX Printing System (CUPS) prior to 2.0.3
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser.
Apply the most recent upgrade or patch from the vendor.