This indicates an attempt to exploit a SQL Injection vulnerability through HTTP headers.
The vulnerability is a result of the application's failure to check user supplied input before using it in an SQL query. As a result, a remote attacker can send a crafted query to execute SQL commands on a vulnerable server.
This is a generic signature against web-based SQL injections.
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the latest patch to the vulnerable software.