This signature indicates a DNS protocol anomaly. It indicates detection of an undersized Domain Name Service (DNS) message.
DNS is a system that translates between human-readable host or domain names (e.g. www.fortinet.com) and machine-understandable Internet Protocol addresses. According to RFC 1035 each DNS message consists of header and body parts. The header has a fixed size of 12 bytes, which is the minimum length of a valid DNS message.
Any unprotected DNS server may be vulnerable.
This is an anomaly, which may indicate potential attack attempts.