what is a replay attack? A Complete Guide

what is a replay attack? A Complete Guide

What if the inaccessible security measures protecting your digital transactions could be misguided, allowing unauthorized third-party access to sensitive information? This problem gives rise to the concept known as a “Replay Attack.” Well, the main question is, what exactly is a replay attack, and how does it affect and threaten secure communications?

In this guide, I am going to explore in detail the concept of a replay attack, its working mechanism, and how you can avoid and prevent this network attack, which can otherwise be a severe issue. Therefore, keep reading to enlighten yourself and find tips to prevent this issue.

What Is a Replay Attack?

Understanding what a replay attack is really crucial in the world of cybersecurity to safeguard yourself against any attack from malicious individuals. So, the main question is, what is a replay attack? A replay attack refers to a network attack in which the cybercriminal secretly listens in on secure network communication, illegally captures the transmitted data, and then re-transmit it later.

The main goal of a replay attack is to deceive the system into treating the re-transmitted data as if it were authentic and valid. In simple terms, a replay attack is when a sneaky individual snatches and reuses the previously captured messages. The most dangerous thing about this attack is that the attackers do not have to decipher the message.

However, despite not deciphering the message, they can still trick the recipient into believing that the message they receive is authentic. Furthermore, replay attacks pose a threat because they are significantly difficult to detect. In addition, replay attacks can be successful even if the original transmission is protected.

How Does It Work?

Since you know what is a replay attack, now let me tell you how this attack works and the procedures involved in it. To thoroughly understand the concept of a replay attack, it is essential to understand the basics of computer networking.

What are Data Packets?

Every computer communicates with each other by sharing data packets and interpreting the packets they receive. Data packets, in simple words, refer to the chunks of data that a computer system uses to communicate with each other. At the center of the widely used Transmission Control Protocol (TCP) protocol, one of the main protocols used in the computer network lies a crucial procedure called the three-way handshake.

This process, as the name suggests, consists of three steps, which is like a special routine that any computer wanting to communicate with another computer has to go through before they exchange conversation. Initially, the computer that wants to start the conversation (also known as the client) sends a SYN packet to the computer it wants to communicate with, also known as the server.

In return, the computer that the client wants to talk to sends a SYN/ACK packet back to it, acknowledging that it received the message. In the final step, the client replies with an affirmative message or ACK packet to the server, indicating they are ready to communicate. Moreover, all of these packets transmitted during these steps have their own sequence number, which helps the computer keep track of their conversation.

Furthermore, the amount of useful information a packet sniffer seizes relies on how tough the network’s security measures are. It can collect various important details, including the website someone visits, the content viewed, and more.  In worse cases, it might also seize sensitive information, such as password hash.

Unencrypted Data

Unencrypted data, in simple language, refers to a message that is not locked, so if another party interprets it, they can effortlessly understand the information without needing a special key. Such data is vulnerable to attack and manipulation from malicious third parties. In simple words, just imagine that if person A asks for some money from person B in a message.

Now, a cybercriminal takes control of this message, modifies the bank details, and sends it again. Person A, not doubting the message received, might send the money without any idea that this money is being transferred to the cybercriminal bank account.

Encrypted Data

Encrypted data means a message or information that is protected and can only be understood by someone with the right ‘key’ to decode it. Although seizing the encrypted data might not appear quite useful, it is still worth a lot to the attacker. The only thing they need to do is interpret and send the complete package (message and key) to the server and deceive them into recognizing the information.

Key Features of Replay Attack

Breaking down a replay attack involves several steps and elements. The entire process is described below:

Stage 1 (Interception or Packet Sniffing)

The first stage is referred to as “packet sniffing,” which involves secretly monitoring a network, observing the information or packets exchanged among its connected computers, and afterward gathering, storing, and recording these packets for future hacking attempts. In most cases, a packet sniffer tool is used by the hacker for this purpose. The most commonly employed packet sniffer is ‘Wireshark.’

Stage 2 (Snatching )

Now, moving on to the next step, which involves stealing a network’s user session ID. A session ID is a distinctive number usually assigned by the server to the client for a specific duration. This ID helps the server recognize and manage the user’s session.

Session IDs are mostly produced by using random number generators, making them quite challenging to guess. Web servers mostly use these numbers to URLs, creating unique strings that help manage and identify user sessions.

Furthermore, when session IDs are not added to URLs, servers commonly link them to either cookies or concealed form fields. Cookies are tiny files that manage user activities on a network and locally store specific users’ information on their browser or computer. Therefore, it is a common practice to include session IDs in cookies.

When session IDs are included in the cookies, the client sends these cookies to the server in each packet exchange. This whole process allows the server to recognize the client and maintain their session. To carry out the session ID theft, a cybercriminal must manipulate or eavesdrop on network traffic and intercept the relevant data containing the session ID or cookies.

Stage 3 (Resending The Data)

Moving on to the third step, where the hacker takes the real action. Lastly, after gathering the data, the hacker utilizes the session ID to manipulate the server to satisfy their demands. The hacker stores the authentication packet that the hacker has seized and sends it to the receiver.

Example of a Replay Attack

Let me give you a simple example to clarify the concept of a replay attack. Suppose there is a person named A who wants to borrow some money from his friend named B. Since they are friends, B transfers money to A.

However, A’s original request for money was interrupted by a third party or a hacker who resends the same request to B. B, who sees the message, does not feel suspicious, and decides to transfer the same amount again. Unfortunately, this time, the money is transferred to the hacker’s account.

Stopping a Replay Attack

A replay attack can create quite an issue; hence, it is necessary to know safety measures to safeguard against such attacks. Resolving such issues revolves around employing effective encryption methods.

Generating Random Session Keys

As I said above, addressing this issue is all about the right encryption. In encrypted messages, there are “keys” included in them. When these messages are deciphered, these keys unlock and reveal the content.

To avoid this problem, the sender, as well as the receiver, should create completely random session keys. This is only valid for just one transaction and cannot be reused. However, since it’s only valid for a limited time, its usefulness to a hacker is restricted.


Another thing you can do to prevent a replay attack is to timestamp messages. This can guarantee that a specific message or request is only utilized once. If the server receives messages that are either too old or early, maybe by just a few hundred milliseconds, they will refuse them.

One-Time Passwords

You can also use a one-time password as a security measure. OTPs are like session IDs in that they expire after the first try or after a set period. Banks frequently use them to confirm the identity of clients.

Sequencing & Repeat Messages

By assigning sequential numbers to valid messages, the receiving server can reject packets that arrive out of order. Lastly, another thing you can do is to guide the server to reject any repeated messages. Moreover, the Window Communication Foundation (WCF), a framework for creating service-related practices, also uses this preventive mechanism.

Replay Attack FAQs

How are replay attacks prevented?

Replay attacks can be protected by employing different safety mechanisms to tackle this issue. This issue cannot be resolved by using only one method; instead, you need to use them collectively. You can employ safety mechanisms such as using One-Time-Password (OTP), timeframe messages, sequencing, and repeating messages.

Concluding Statement

To sum up, a replay attack is a network attack in which the hacker intercepts a message to deceive the receiver into thinking it is a legitimate request. To address this issue, you can follow the instructions provided in this guide.


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