This indicates an attack attempt to exploit a Code Injection Vulnerability in MantisBT.
The vulnerability is due to an input validation error while parsing a crafted HTTP request. A remote attacker could exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
MantisBT version 1.2.0a3 to 1.2.17
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware of any vendor supplied patch for this issue.