HTML injection attacks: Understanding the threat and how to protect your website

html injection

Attacks carried out through malicious codes can be significantly detrimental to the integrity of your website and the information you store. One of them is HTML injection attacks where the cybercriminal inserts malicious codes to change the website and its content. If the website admins do not carefully validate the users, hackers can use this vulnerability to inject their HTML codes into the website.

HTML injection attacks can harm your website and the people visiting it in different ways. Depending on the type of attack, hackers may redirect your users to an unfriendly website or exploit the website to retrieve sensitive data. Here are a full guide on HTML injection attacks and how to protect your website against them.

Types of HTML injection attacks

There are two main types of HTML injection attacks; non-persistent (reflective) and persistent (stored) attacks. Both of these attacks can harm a website and lead to data theft, stolen personal information, or brand reputation loss.

Non-persistent HTML injection

This is also known as “reflective attacks” since the HTML input of the unauthorized user is almost instantly reflected on the website. This is a serious vulnerability due to the lack of validation and sanitation of the user input via website admins before reflecting the content on their website.

These attacks are non-persistent because the code is not stored on the servers, it is just reflected on the end-user’s screen when they visit the website. The main reason behind these attacks is the failure to properly verify any user input before showing it on the website.

Persistent HTML injection

These attacks are also known as “stored attacks” because simply the malicious HTML input is stored on the website servers, its back-end database. These malicious codes are first stored and then displayed on the website for the end-user again, due to lack of validation.

Persistent HTML injection is usually a result of a vulnerability in the web application, paving the way for criminals to sneak into the system and inject their malicious code into the server. It can indeed be much simpler than that; through retrieving the login credentials of an authorized user and injecting the code from their account.

Identifying an HTML injection attack

Identifying an HTML injection attack can be challenging since the attack is not always so obvious until it's too late. On that note, early identification of these attacks can save your website’s reputation and the information of your users. Below you’ll see some indicators that your website might have been a victim of HTML injection.

1-) Changes in the web page content

If your page is attacked by an HTML injection attack, you are likely to see unknown pop-up messages, directing links, unexpected images or text, and possibly strange content. To look into this, you need to carefully go over your website in the eyes of an end-user to make sure there is no unknown content of any type on your website.

2-) Overwhelming traffic

If you saw a sudden and unprecedented increase in your website’s traffic, you can be suspicious of an HTML attack. If you are wondering where all that traffic came from, you need to look to see if there are any signs of connections to suspicious IP addresses.

3-) Error messages on pages

HTML injection attacks are capable of damaging the integrity of your pages, so the page may display error messages such as “404 Not Found”. If you have any pages with such error messages, it might be a good idea to review them on the back end. There might be unknown codes on the website needed to be removed.

4-) Suspicious URLs

URLs are used by HTML injection attackers to redirect users to another website, and you can tell whether a URL is good or not by simply looking at it. Check all the URLs on your websites to see if you have any unknown scripts or unusual characters. If you do, promptly remove them and correct the URL since it may be injected by attackers to retrieve the personal information of the users.

Protecting against HTML injection attacks

Securing a website to prevent HTML injection attacks is not as complicated as you might think. These attacks are definitely detrimental to the integrity of your website, but you can easily get all the tools you need to make your website more robust against them. Here are some tips on how to do that.

Securing remote access

It is common for HTML injection attackers to benefit from a vulnerability of remote connections, or get the user credentials of an employee. They can then inject their malicious codes and harm your users.

But how to make use of secure remote access? One of the first things you need to do is to increase the validation of your remote users before granting them the ability to input HTML codes and encrypt your communication to prevent stolen credentials.

Implement backups and a recovery plan

HTML injection attacks can happen no matter what you do, and if it does, the question is how to eliminate them from your systems. That’s why having a comprehensive backup of your system and a strong recovery plan matters.

Even in the case of an HTML injection attack, if you can detect it in the early stages, you can just use your backup and recovery plan to terminate the malicious codes without losing any data or pages.

Input validation and sanitization

Users should not be able to process their inputs before they are validated. Lack of proper validation and sanitation is one of the most common causes of these attacks, so using a whitelist of valid inputs and reviewing all of them before processing will be a big help.

It is also important to note that all special characters or scripts should be removed before the input is processed. Unchecked user inputs may put your users’ information at risk.

Monitor and review logs

Stored (or persistent) attacks are carried out by injecting the code into the back end, so it usually requires a sign-in process. Whether this is due to stolen credentials or a vulnerability in the system, you need to regularly check the logs to see any suspicious activity. By doing so, you can tell whether an HTML attack might have happened or not, and respond quickly before the damage is done.


HTML injection attacks are hard to tell and highly detrimental to websites. They directly target the visitors of a website to steal personal data such as financial information, passwords, or emails.

As the website owner, you are responsible for detecting these attacks and protecting your website against them using the tools and tips above. Although HTML injection can be challenging to detect, these tips will strengthen your arsenal against them.

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