This indicates an attempt to exploit an HTTP Response Splitting vulnerability in cPanel.
This is caused by the application's failure to sanitize user supplied input to the "failurl" parameter. A successful attack may allow a remote attacker to steal cookie-based authentication credentials and control the web content. Other attacks are also possible.
cPanel 11.25 build 42174
WebHost Manager 11.25 build 42174
Previous versions may also be affected.
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply the most recent upgrade or patch from the vendor.