How does a TCP FIN Scan attack work?

It typically unfolds over 2 phases. It begins with: An adversary sends TCP packets with the FIN flag but not associated with an existing connection to target ports.

How do you prevent CAPEC-302?

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.

What weaknesses does CAPEC-302 target?

CAPEC-302 exploits 1 CWE weakness, including CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor).

How severe is CAPEC-302?

MITRE rates CAPEC-302 as Low severity.

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Detailed attack pattern

Stable

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.

Last updated June 1, 2026

Low

Typical severity

Per MITRE

Not rated

Likelihood of attack

Per MITRE

1

Related weaknesses

CWEs this targets

Detailed

Abstraction

MITRE classification

Overview

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.

How the attack works

The phases an attacker typically follows to carry out this attack.

Step 1
Experiment
An adversary sends TCP packets with the FIN flag but not associated with an existing connection to target ports.
Step 2
Experiment
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.

What the attacker needs

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.

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.

Consequences

What a successful CAPEC-302 attack can achieve.

Other

Affects: Confidentiality

Bypass Protection Mechanism, Hide Activities

Affects: Confidentiality, Access Control, Authorization

How to mitigate it

Defenses that reduce the risk of CAPEC-302.

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.

Frequently asked questions

Common questions about CAPEC-302.

What is CAPEC-302?: 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.
How does a TCP FIN Scan attack work?: It typically unfolds over 2 phases. It begins with: An adversary sends TCP packets with the FIN flag but not associated with an existing connection to target ports.
How do you prevent CAPEC-302?: 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.
What weaknesses does CAPEC-302 target?: CAPEC-302 exploits 1 CWE weakness, including CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor).
How severe is CAPEC-302?: MITRE rates CAPEC-302 as Low severity.

References

Attack-pattern data is sourced from the MITRE CAPEC catalog (v3.9). Weakness associations link to the corresponding CWE entries on RadicalNotion.AI.

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