This indicates an attack attempt to exploit a Buffer Overflow vulnerability in Citrix Access Gateway Plug-in for Windows.
The vulnerability is due to insufficient sanitizing of inputs in the application when handling an overly long "CSEC" header field in HTTP responses. An attacker can exploit this by tricking an unsuspecting user into visiting a malicious webpage and execute arbitrary script code within the context of the targeted user. Failed exploits will likely crash the program, leading to a Denial of Service condition.
Citrix Systems Access Gateway Plug-in for Windows prior to 22.214.171.124
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware of any vendor supplied patch or updates available for this issue.