Rule Category

PROTOCOL-ICMP -- Snort alerted on Internet Control Message Protocol (ICMP) traffic, which allows hosts to send error messages about interruptions in traffic. Administrators can use ICMP to perform diagnostics and troubleshooting, but the protocol can also be used by attackers to gain information on a network. This protocol is vulnerable to several attacks, and many administrators block it altogether, or block selective messages.

Alert Message

PROTOCOL-ICMP Timestamp Reply undefined code

Rule Explanation

This event is generated when a network host generates an ICMP Timestamp Reply with an invalid or undefined ICMP Code. Impact: Information-gathering. An ICMP Timestamp Reply message is sent in response to an ICMP Timestamp Request message. If the ICMP Timestamp Reply message reaches the requesting host it indicates that the replying host is alive. Most OS's (operating systems) will accept an ICMP Timestamp Reply message with an invalid or undefined ICMP code set as a valid ICMP Timestamp Reply. Details: ICMP Type 0 Code 0 is the RFC defined messaging type for ICMP Timestamp Reply datagrams. This type of message is used to determine if a host is active on the network. If ICMP type 8 (echo) traffic is filtered at a firewall, an attacker may try to use type 14 (timestamp) as an alternative. Ease of Attack: Numerous tools and scripts can generate this type of ICMP datagram.

What To Look For

Known Usage

No public information

False Positives

No known false positives


Original rule writer unknown Cisco Talos Matthew Watchinski Nigel Houghton

MITRE ATT&CK Framework



For reference, see the MITRE ATT&CK vulnerability types here:

Rule Vulnerability

CVE Additional Information