COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-302

TCP FIN Scan

An adversary uses a TCP FIN scan to determine if ports are closed on the target machine. This scan type is accomplished by sending TCP segments with the FIN bit set in the packet header. The RFC 793 expected behavior is that any TCP segment with an out-of-state Flag sent to an open port is discarded, whereas segments with out-of-state flags sent to closed ports should be handled with a RST in response. This behavior should allow the adversary to scan for closed ports by sending certain types of rule-breaking packets (out of sync or disallowed by the TCB) and detect closed ports via RST packets.

Abstraction

Detailed

Typical severity

Low

Likelihood of attack

Medium

Understand the attack

What is this attack pattern?

An adversary uses a TCP FIN scan to determine if ports are closed on the target machine. This scan type is accomplished by sending TCP segments with the FIN bit set in the packet header. The RFC 793 expected behavior is that any TCP segment with an out-of-state Flag sent to an open port is discarded, whereas segments with out-of-state flags sent to closed ports should be handled with a RST in response. This behavior should allow the adversary to scan for closed ports by sending certain types of rule-breaking packets (out of sync or disallowed by the TCB) and detect closed ports via RST packets.

Extended description

In addition to its relative speed in comparison with other types of scans, the major advantage a TCP FIN Scan is its ability to scan through stateless firewall or ACL filters. Such filters are configured to block access to ports usually by preventing SYN packets, thus stopping any attempt to 'build' a connection. FIN packets, like out-of-state ACK packets, tend to pass through such devices undetected. FIN scanning is still relatively stealthy as the packets tend to blend in with the background noise on a network link.

Execution flow

How does the attack proceed, step by step?

1

Experiment

Attack step

An adversary sends TCP packets with the FIN flag but not associated with an existing connection to target ports.

Attack step

An adversary uses the response from the target to determine the port's state. If no response is received the port is open. If a RST packet is received then the port is closed.

Requirements

What does the attack require?

Prerequisites

  • FIN scanning requires the use of raw sockets, and thus cannot be performed from some Windows systems (Windows XP SP 2, for example). On Unix and Linux, raw socket manipulations require root privileges.

Skills required

Not stated in the MITRE record.

Resources required

  • This attack pattern requires the ability to send TCP FIN segments to a host during network reconnaissance. This can be achieved via the use of a network mapper or scanner, or via raw socket programming in a scripting language. Packet injection tools are also useful for this purpose. Depending upon the method used it may be necessary to sniff the network in order to see the response.

Impact

What does a successful attack lead to?

Other

Affected scopes: Confidentiality

Bypass Protection MechanismHide Activities

Affected scopes: Confidentiality, Access Control, Authorization

Defence

How is it prevented and mitigated?

1
FIN scans are detected via heuristic (non-signature) based algorithms, much in the same way as other scan types are detected. An IDS/IPS system with heuristic algorithms is required to detect them.
CAPEC-302: TCP FIN Scan — DayBreach Saldırı Sözlüğü · DayBreach