How to Prevent Replay Attacks on Your Website

Fight against replay attacks: an easy way token-based approach

Replay attacks, in which attackers intercept and resend network packets that do not belong to them, are extremely dangerous and sometimes even cause serious damage. Even on encrypted communication channels, an attacker may launch such attacks without access to the decryption key. An attacker simply eavesdrops on your lines and roughly understands the tasks performed by a particular set of packets, and then by resending those packets or requests, can interrupt your communication or cause more serious damage.

This article will introduce a simple and easy-to-use basic method to prevent websites from being subjected to replay attacks. It also prevents upset users from repetitively repeating the last POST request during the wrong time.

This is far from a complete solution. It has flaws and issues to be resolved, but it gives you a rough idea of ​​how tokens and simple protocols can enhance website security. Sample code and implementation are done using ASP.NET and C#, but the concept can be deployed to any other platform or programming language.

Concept of one-time token

The core idea of ​​the solution provided in this article is to bind each HTTP response to a token string that is only valid for the next POST request. Here is a simple breakdown of the steps involved:

1. The client makes a GET request by typing the URL or page or clicking a link.
2. The server generates a random token. It then stores a copy of the token in the session and embeds the copy of the token into the <input type="hidden"> tag of the response sent to the client.
3. The client processes the content and sends a POST request to the server when the user clicks a button containing the randomly generated token.
4. The server receives the request and continues to process it only when the attached token is equal to the token stored in the user session.
5. The server invalidates the token and returns to step 2, in which it formulates the response using the new random token.

In this way, even if a malicious user intercepts a critical request sent to the server, the request cannot be repeated because the token contained in the request will no longer be valid after being sent to the server. The same is true for careless users who mistakenly press the F5 key and resend the request after posting information to the server.

Test environment

To implement the concept of a one-time token, we will create a sample page with a simple text box and a submit button. We will also add a tag control to display the test output.

The background code will be a simple snippet showing the submission time plus the data contained in the text box.

This is the output of the page after the initial GET request:

After submitting the page, the output will look like this:

The problem is that if you refresh the page, it will republish your data and repeat the last request, the server will handle it without any problem. Now imagine if you just made a key deal of $1,000,000 and accidentally pressed the F5 key on your keyboard. Or worse, some malicious users intercept your request, find it is a payment transaction, and repeat it to steal your funds and retaliate against you.

Solution

To prevent duplicate POST requests, we update the tag to add a hidden field that will store the token.

Next, we will create a function that generates a random token and embeds it into both the hidden fields and the session collection.

After

, we will change the Page_Load() function so that the published data will be displayed only if the published token is equal to the token stored in the session.

Finally, we will override the OnPreRender() function to generate a new token before the final output is sent to the client. This is what makes it a one-time token, as it updates every time a new request is sent.

Now when you click the button to submit the form, it works the same way as before. However, if you try to simulate a replay attack by refreshing the page, you will receive the following error because the token sent with the form is no longer equal to the token stored on the server:

In this way, we can distinguish between valid button click submissions and error-repeated requests.

Improve code

Although this code solves the page's replay attack problem, it still has some issues that need to be solved:

• It must be repeated on all pages
• If you have multiple tabs on the same website, it won't work because the tokens are shared between requests
• It's very ugly

As an avid object-oriented programming (OOP) enthusiast, I've been looking for opportunities to leverage the power of this best programming paradigm to refactor and improve code.

To solve the above problem, the first thing we need to do is define a class that will encapsulate the token generation function. We name the class TokenizedPage and derive it from System.Web.UI.Page so that we can use it for pages in the future.

Next, to make the code easier to read and manage, we encapsulate the page token and the session token into two different properties added to the TokenizedPage class. To make the code easy to port in web pages, we will use the ViewState collection instead of hiding the input field to store the page token. We also use the Page.Title property as the key to store the token in the session. This will improve our code and partially solve the second issue that will limit the use of our website to a single tab of the browser. By applying this change, we will be able to open different pages of the website in different tabs, but we will not be able to open multiple instances of the same page in separate tabs, as they still share the token. This issue will be resolved later.

Next, we add a read-only boolean property called IsTokenValid, which follows examples of other Page properties such as IsPostBack and IsValid. The purpose of this property is to ensure that the page token is equal to the session token.

Finally, we add the override of the GenerateRandomToken() function and the OnPreRender() event, just like it does in the test environment.

Now, in order to use the one-time token mode, we just need to create a new page, derive it from the TokenizedPage, and use IsTokenValid when a one-time token is needed.

It's much better.

Make it better

One problem with this code is that if you have two tabs in your browser pointing to the same page, publishing one tab will invalidate the tokens of the other tab because they use the same session token key. This can be solved by adding the token ID, which will ensure that each request-response sequence that occurs in one tab uses its own set of unique tokens and does not interfere with other requests on the same page. The first thing to do is return the TokenizedPage class and add the TokenID property. This property generates a random ID when called in the initial GET request for the first time and stores it in the ViewState collection for future reuse.

Next, we will change the SessionHiddenToken property to use the TokenId property instead of using the Page.Title property.

The wonderful thing is that since we use the abstraction and encapsulation principles (thanks again for the benefits of OOP), we don't need to make any other changes, and the new mechanism will work with all the pages we derived from TokenizedPage.

Remaining questions

This is what it is about the one-time token mode. There are two questions left:

• For each session (more precisely, the number of GET requests sent to each session), an unlimited number of token IDs will be generated. This can be solved by implementing a stack or cache mechanism that pops up older IDs when they exceed a quantity limit or are not used for a specific duration. I will leave the implementation to you.
• The default random number generator is not the safest and reliable source of randomness you call it, and a savvy hacker may be able to predict token sequences. However, if you use SSL encryption, they cannot get the token anyway.

Do you have any enhancements to add or would like to share their implementation in other platforms and programming languages? Please leave a message in the comment section below.

FAQ on Preventing Website Replay Attacks (FAQ)

What is a replay attack and how does it work?

Replay attack is a form of cyberattack in which effective data transmission is repeated or delayed maliciously or fraudulently. An attacker does this by intercepting and retransmitting the data, which may be part of a masquerading attack via IP packet replacement. This poses a serious threat to the security of communications and data.

How to detect replay attacks?

Detection of replay attacks can be challenging because the data being transmitted is valid and initially sent by an authorized user. However, there are some signs that replay attacks can be shown. These include noting the delay of duplicate transmission or data transmission. In addition, implementing security measures such as timestamps and serial numbers can help detect replay attacks.

What are the consequences of the replay attack?

The consequences of a replay attack can be serious, depending on the nature of the intercepted data. It can lead to unauthorized access to sensitive information, fraudulent transactions, and even violations of security systems. This can lead to financial losses, reputational damage and potential legal impact.

How to prevent my website from being replayed?

There are several strategies to prevent replay attacks. These include the use of secure communication protocols (such as SSL or TLS), the implementation of timestamps or serial numbers, and the use of one-time passwords or random numbers. Additionally, regularly monitoring and auditing your network traffic can help detect and prevent replay attacks.

What is a random number and how does it prevent replay attacks?

Random number is a random or pseudo-random number that is only used once in a communication session. It prevents playback attacks by ensuring that each data transfer is unique, even if the same data is sent multiple times. This prevents the attacker from replaying data transmission successfully.

How to prevent replay attacks from SSL or TLS?

SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are encryption protocols that provide network secure communication. They prevent replay attacks by encrypting the data being transmitted and using a combination of sequence numbers and timestamps to ensure uniqueness of each data transmission.

What role does timestamp play in preventing replay attacks?

Timestamps can play a key role in preventing replay attacks. By adding a timestamp to each data transfer, you can ensure that each transfer is unique and cannot be played successfully. If a playback transmission is detected, it can be easily identified and discarded based on the timestamp.

Can the replay attack be carried out on any website?

Yes, replay attacks may be conducted on any website without proper security measures. However, websites that transmit sensitive information, such as financial data or personal information, are particularly vulnerable to attacks.

Is replay attack common?

While not as common as some other types of cyberattacks, replay attacks do occur and can have serious consequences. Therefore, it is very important to take measures to prevent them.

What are some best practices for preventing replay attacks?

Some best practices to prevent replay attacks include the use of secure communication protocols, enforcing timestamps or serial numbers, using one-time passwords or random numbers, and periodic monitoring and auditing of your network traffic. Additionally, educating your users about the importance of security and encouraging them to use secure passwords can also help prevent replay attacks.

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