A significant increase this weekend in TCP traffic with source port zero detected could be part of reconnaissance efforts in preparation for more serious attacks, according to security researchers from Cisco Systems.
There's usually no service on a system that listens on port 0 (zero) because, according to the Transport Protocol port assignments by the Internet Assigned Numbers Authority (IANA), port zero is a "reserved port" and should not be used. Therefore receiving TCP (Transmission Control Protocol) traffic with a source port of zero is unusual.
"Generally speaking port zero traffic can be indicative of a possible reconnaissance attack, and may be a precursor to more serious penetration attempts," Craig Williams, technical lead in the threat research team at Cisco, said Monday in a blog post.
Attackers can use such abnormal traffic to fingerprint operating systems and network security devices, because different OSes and network equipment can respond differently to port zero traffic, the researcher said. "This can enable the attacker to make a more precise attempt to compromise a network."
Cisco saw a spike in TCP port zero traffic Saturday, with an increase in both the traffic volume and the number of sensors that detected such activity. The magnitude observed by the sensors was five times higher than usual, Williams said.
The highest volume of traffic came from eight Internet Protocol (IP) addresses assigned to Ecatel, a Dutch provider of dedicated server hosting and content delivery services, Williams said.
"We do not know the intent of these packets," the researcher said. "This could be a simple research project to map portions of the Internet by operating system and service pack or it could be an attacker preparing to do something nefarious."
As a general rule, users who see port zero activity on their networks should treat it suspicious and should investigate the source, Williams said, adding that this port should be blocked in firewalls because it doesn't serve a legitimate purpose.
It's hard to speculate what the goal of the traffic might be since Cisco only revealed the source port for it and not the destination ports, HD Moore, the chief research officer at security firm Rapid7, said Tuesday via email. The scans performed as part of Project Sonar, a community effort supported by Rapid7 that involves scanning Internet-facing systems to collect data and improve security, use non-zero source ports, he said.
According to Moore, many intrusion detection systems and firewalls already drop port zero traffic so there isn't much benefit to using this source port. "If I had to guess, I would say the traffic is the result of a broken embedded device (SIP phone, etc.) or a new piece of malware with a bug in its scanning code," he said.
Even if the traffic picked up by Cisco's sensors is part of a reconnaissance effort by attackers, the scanned systems might not be attacked in the immediate future. According to the Cisco researchers, reconnaissance may take place months before an actual attack begins.