Today, not all threats are as obvious as they seem. One such overlooked yet dangerous vulnerability is HTML injection.
While it may sound less harmful than high-profile attacks like SQL injection or cross-site scripting (XSS), HTML injection can silently compromise your application’s integrity, user trust, and even pave the way for more severe exploits.
In this blog, we’ll break down what HTML injection is, how it works, the risks it poses, and—most importantly—how you can protect your web applications from it. Whether you’re a developer, security enthusiast, or tech-savvy business owner, understanding HTML injection is a step toward building safer, more secure websites.
How does HTML injection work?
HTML injection is done using meta-characters. This vulnerability leads to disclosure of a user’s session cookies. It can also allow the attacker to modify the page content seen by the victims (end users). HTML injection occurs due to improper sanitization of user input and improper encoding of output.
This injection vulnerability allows an attacker to inject or send a malicious HTML page to the end users.
As the browser doesn’t know if the page is trusted or not, it will execute and parse all the parts of the page. If the page had any malicious codes in it, the codes would be executed on behalf of the end user. This vulnerability will, in turn, make the end user vulnerable to many more attacks.
This attack can be executed using a wide range of methods and attributes that could be used to render HTML content. If this method is provided with untrusted input, then there will be a massive chance for HTML injection attack.
Example of HTML injection
The below code has unvalidated input. This code is used to create dynamic HTML in the page context:
var userposition=location.href.indexOf("user=");
var user=location.href.substring(userposition+5);
document.getElementById("Welcome").innerHTML=" Hello, "+user;
If the code is like this, then an attacker can use the URL below.
https://www.example.beaglesecurity.com/page.html?user=<img%20src='aaa'%20onerror=alert(1)>
The above URL will add the page into the image tag. The application will execute a JavaScript code inserted by the malicious user in the HTML context.
What are the impacts of HTML injection?
The impacts of an HTML Injection attack can vary depending on how it’s exploited and the structure of the web application.
Major impacts of HTML injection attacks
1. Website defacement
Attackers can modify how a web page appears by injecting unauthorized HTML content such as fake headlines, altered images, or misleading messages which can damage the credibility and professionalism of your brand.
2. Phishing attacks
By injecting forms or links that mimic legitimate ones, attackers can trick users into entering sensitive information like login credentials, credit card numbers, or personal details.
3. Malicious redirects
HTML injection can be used to insert links or meta tags that redirect users to malicious websites. This can lead to malware infections or scams.
4. Data leakage
In some cases, attackers can craft payloads that manipulate how user data is displayed, leaking information meant to remain hidden.
5. Session hijacking
If the injection point allows script tags (blurring into XSS territory), attackers could execute malicious scripts that steal session cookies or tokens.
How can you prevent the impact of HTML injection?
HTML injection vulnerabilities often occur when user input is directly inserted into a web page without proper validation or sanitization. This creates an opportunity for attackers to inject malicious HTML code.
1. Filter meta-characters from user inputs
Meta-characters such as <, >, “, ‘, and & have special meanings in HTML. If these characters are not properly handled, attackers can use them to break out of the expected input context and inject their own code.
Use input sanitization libraries or frameworks that automatically filter or escape these characters.
For example, in PHP, you can use:
htmlspecialchars($user_input, ENT_QUOTES, 'UTF-8');
In JavaScript, use client-side libraries like DOMPurify to sanitize user-generated content before rendering.
2. Apply strong input validation
Validate all user inputs against a strict set of rules. This means only allowing the characters and formats that are necessary, based on the field’s purpose.
Examples:
For a name field: Only allow alphabets and a few special characters.
For an email field: Use regex to allow only valid email formats.
Server-side validation is critical. Even if you use client-side validation, never rely on it alone, as it can be bypassed.
Wrapping up
HTML injection may seem like a minor vulnerability, but it can lead to serious security risks if left unaddressed.
By filtering user input, validating data effectively, and using proper output encoding, developers can significantly reduce the chances of an injection attack. Prioritizing these secure coding practices ensures a safer and more reliable web application for all users.
