How to Defend Yourself Against A Replay Attack 

If you want to learn how to avoid falling victim to a replay attack, this article was written for you. It contains all of the crucial information you need.

It is an unfortunate side effect of the technological revolution we are all living through that cyber crime is on the rise. While most of us use the technology available to us in constructive and productive ways, there are others who see these technological advances as an opportunity to commit fraud, to steal or to sabotage others. Replay attacks are just one example of this.

What replay attacks are and how they work

Replay attacks, also known as repeat attacks or playback attacks, are a man-in-the-middle type of hacking where a cyber criminal intercepts the communication between you and your browser. 

By inserting themselves in this way, the attacker has placed themselves in a unique position where they are able to gain access to the encrypted information going back and forth between you and the browser.

What this means in practical terms is that the attacker is able to capture and replay or resend the same encrypted message to the browser that you have just put in. And because the message now sent by the hacker is already correctly encrypted and originates from an authentic and original source, the recipient is going to recognise and respond to it as though it were an authentic message. 

A good example of what a replay attack might look like is this. Imagine that an employee at a large company requests a financial transaction by sending an encrypted message to the company’s financial administrator. A cyber criminal who is eavesdropping on the network intercepts the connection and captures the message sent by the employee. Once in possession of the encrypted message, the attacker resends it to the financial administrator, who is most likely going to respond to it as though it were an authentic request.

In some cases, the recipient might find the message suspicious – let us say, for example, that the hacker is requesting that a large sum of money be transferred to an unknown offshore account – but as long as the hacker knows what they are doing and manages to avoid arousing suspicion, the replay attack is likely to be successfully completed.

Although man-in-the-middle replay attacks are the most common, there are other forms of replay attacks where the hacker does not intercept a connection, but instead intentionally resend or delay an encrypted data transmission from their own devices order to impersonate genuine customers or users of any given website and essentially misdirect the page or the recipient into doing what the hacker wants.

All in all, it is easy to see why you should do your best to avoid becoming the victim of a replay attack. But how is this done – is there any way to effectively defend yourself and prevent replay attacks from even happening to you?

Is it possible to spot such an attack?

You might be wondering whether it is possible to spot a replay attack in the making. 

The honest answer is that spotting and preventing a replay attack when you, your business or your website is already being targeted is a rather impossible endeavour. After all, encrypted messages get delayed or resent all of the time due to numerous legitimate and uncontrollable factors – the user’s session expires or their wireless connections drop, and so on and so forth.

Because the vast majority of us are not constantly alert and questioning whether or not our connections are secure, and whether the requests we receive originate from legitimate sources, the best way to prevent an attack is to stop it from happening in the first place, rather than hoping to spot it once it is already happening.

Preventing replay attacks

Replay attack prevention is all about utilising the right method of encryption.

Encrypted messages carry a number of identifiable ‘keys’ within them. It is when these keys are decoded by the recipient device or site that the message becomes readable.

When a cyber criminal carries out a replay or playback attack, they couldn’t care less about the contents of the encrypted message – all they do is capture the encrypted message and resend or delay it as it is, keys and message both together.

Random session keys

To prevent a cyber criminal from being able to capture and resend your encrypted messages, you need to establish a random session key, also known as a random session ID. 

A random session key or session ID is essentially a code that transpires as soon as it has been used to deliver a message just once. In other words, as soon as a random session key has been used for data transmission, it will no longer be of any use to potential hackers, as they will not be able to use it again.


Another preventative measure you can take is to use timestamps on all messages sent and received. 

Timestamping messages is not a fail proof method of preventing replay attacks, but it nevertheless narrows down the window of opportunity that hackers have to access an encrypted message and delay or send it on.

One-time passwords

Yet another authentication process that can help limit suspicious events and attacks is to use one-time passwords. 

A one-time password is a password that is only used once and then discarded, which makes it useless to hackers who might be eavesdropping on the network waiting fir user messages they can capture and use. Using one-time passwords means that even if a prospective attacker records and replays the message, the encryption code will have already expired and thus be completely useless.

Replay attack FAQ

What is replay attack Blockchain?

Replay attacks can happen on the Blockchain whenever tw0 forked crypto currencies both allow transactions to be valid across their chains. 

While this is a mostly a wonderful thing as it permits smooth transactions, it also leaves the Blockchain vulnerable hackers. How a replay attack usually happens on the Blockchain is that the hacker intercepts the connection during the implementation of a hard fork, then resends or delays the encrypted message in order to gain information and access.

What is replay attack biometric?

A biometric replay attack is when the hacker uses the victim’s biometric data and creates a template that they then use to impersonate the victim in hopes of being able to bypass security protocol on various sites.

What is the main ingredient of replay attack?

The prerequisite for a successful replay attack is a cyber criminal being able to eavesdrop on a seemingly secure network communication.

The attacker then intercepts the communication, capturing encrypted messages and resending them to the recipient in order to gain access to the resulting data response or in order to misdirect the recipient into taking whatever course of action the attacker desires.

How are replay attacks prevented?

There are numerous security measures organisations or individuals can take in order to prevent replay attacks. 

The most effective method is to use random session keys, which are a form of code that expires immediately after use. Other effective methods that help limit the ability of attackers to be able to use the sensitive information they capture include using one-time passwords and timestamps. Both of these measures drastically shrinks the window of opportunity that a hacker has to make use of an encrypted message.

How do timestamps prevent replay attacks?

Timestamps do not prevent replay attacks as such, but they lower the chances of a replay attack being successfully completed. 

Timestamps ensure that messages expire quickly after they have been sent, which means that the attacker would have to act extremely fast or else the encrypted message they have captured will be utterly useless to them.


Replay attacks are utterly malicious, require advanced skills, and are difficult to prevent from happening. 

But while there are no fully guaranteed ways of preventing hackers from being able to capture your encrypted messages, there are multiple different security measures you can take to outwit them. 

Using a combination of timestamps, one-time keywords and random session ID keys, you will have taken the steps necessary to drastically reduce the chances of becoming the victim off a replay attack.

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