GEN:SID 1:453
Message ICMP Timestamp Request
Summary This event is generated when an ICMP Timestamp request is made.
Impact Information gathering.  An ICMP Timestamp request can determine if a host is active.
Detailed Information An ICMP Timestamp request is used by the ping command to elicit an ICMP Timestamp reply from a listening live host.  This rule alerts on a generic ICMP request where no payload is included in the message or the payload does not match more specific rules.

If ICMP type 8 (echo) traffic is filtered at a firewall, and attacker may try to use type 13 (timestamp) as an alternative.
Affected Systems All
Attack Scenarios An attacker may attempt to determine live hosts in a network prior to launching an attack.
Ease of Attack Simple
Corrective Action Block inbound ICMP Timestamp requests.
Additional References