What is TCP-ACK Scanning? Last Updated : 23 Jul, 2025 Comments Improve Suggest changes Like Article Like Report TCP-ACK is a third step of the TCP 3-Way Handshake process (SYN, SYN-ACK, ACK). In TCP-ACK, the client acknowledges the response of the Server, and establish a connection to proceed with the data transfer and any other communication processes. TCP-ACK Scanning: In this scan, the ACK packets are sent to the target port in order to know that if that port is filtered or unfiltered. In case of filtered port, the response will be either no response or an ICMP destination unreachable reply packet will be shown. In case of unfiltered port, an RST reply packet will be sent to all the open and closed ports. Procedure:This can is useful when the firewalls are stateless, i.e., they block all the incoming connections by just blocking the first packet of this TCP 3-Way Handshake process, i.e., SYN packets. But the stateless firewall allows ACK packets because of the stateless firewall allows internal hosts to communicate with the rest of the internet. This scan doesn't work for the stateful firewalls. To demonstrate, we will use a simple yet very powerful scanning tool called NMAP. ACK scan command usage: nmap -sA -T4 <anydomain address> Typical Probe Response looks like with their state: TCP RST response - unfilteredNo response received - filteredICMP unreachable error - filtered In the above nmap scan result, you can see in the result about states, services, and ports after a successful TCP ACK scan. PORT STATE SERVICE 80/tcpunfilteredhttp 443/tcpunfilteredhttpsPrevention:First things first, always have a firewall setup for filtering these types of scans.Always configure the server with best practices.Monitor all the scan, that are being performed on the server. Comment 0xshivam Follow 0 Improve 0xshivam Follow 0 Improve Article Tags : Ethical Hacking Ethical Hacking - Hacking Web Servers Explore Ethical Hacking Tutorial 13 min read Introduction to Ethical HackingWhat is Hacktivism ? 11 min read The Hackers Methodology 4 min read Remote Access in Ethical Hacking 5 min read Information Gathering Tools for Kali Linux 5 min read ARIN in Ethical Hacking 3 min read Characteristics of Computer Networks 5 min read Foot Printing and ReconnaissanceWhat is DNS Footprinting? 6 min read Footprinting Through Search Engines 5 min read What is Whois Footprinting? 4 min read Footprinting Using Social Engineering Method 4 min read Scanning NetworksWhat is Credentialed Vulnerability Scan? 4 min read What are Scanning Attacks? 7 min read Malware Scan in Ethical Hacking 5 min read What is Running of a Malware Scan? 5 min read WAScan - web application security scanner in Kali Linux 2 min read What is TCP-ACK Scanning? 2 min read Port Scanning Techniques By Using Nmap 5 min read What is SYN Scanning? 3 min read What is UDP Scanning? 2 min read EnumerationCyber Security - Types of Enumeration 13 min read What is DNS Enumeration? 3 min read SMTP Enumeration 2 min read LDAP Enumeration 6 min read What is NTP Enumeration? 4 min read What is IPsec Enumeration? 4 min read What is NetBIOS Enumeration? 5 min read SNMP Enumeration 7 min read What is Security Testing in Enumeration? 4 min read System HackingWhat is System Hacking in Ethical Hacking? 2 min read What is Windows Hacking ? 6 min read Importance of Physical Security in Ethical Hacking 3 min read What is Non-Electronic Password Attack on a System? 3 min read Password Guessing Attack 5 min read Credential Stuffing in Ethical Hacking 2 min read Reverse Brute Force Attack in System Hacking 4 min read Brute Force Attack 2 min read What is a Default Password Attack Threat? 3 min read USB Drop Attack in System Hacking 5 min read What is Sniffing Attack in System Hacking? 4 min read How to Prevent Man In the Middle Attack? 5 min read How To Generate Rainbow Table Using WinRTGen? 4 min read What is Elcomsoft Distributed Password Recovery? 4 min read pwdump7 in System Hacking 3 min read FGDUMP in System Hacking 5 min read Password Auditing With L0phtcrack 7 Tool 5 min read What is Salted Password Hashing? 4 min read How to Defend Against Password Cracking of Systems? 6 min read How to Defend Against Wi-Fi Pineapple? 6 min read What is DLL Hijacking? 8 min read How to Prevent Privilege Escalation? 4 min read Malware AnalysisMost Popular Methods Used By Hackers to Spread Ransomware 3 min read What is Malvertising? 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