Which of the following is an issue with signature-based intrusion detection systems?

An issue with signature-based ID is that only attack signatures that are stored in their database are detected.

New attacks without a signature would not be reported. They do require constant updates in order to maintain their effectiveness.

Reference used for this question:

KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 49.