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Introduction:

HTML Injection is a deceptively simple yet potent web vulnerability that allows attackers to inject malicious HTML code into a vulnerable web page. Unlike its more notorious cousin, Cross-Site Scripting (XSS), HTML Injection often flies under the radar, manipulating the user interface to erode trust, phish for credentials, or deface a brand, all without triggering a single security alert. This stealthy attack vector underscores the critical importance of rigorous input validation and contextual output encoding in all web applications.

Learning Objectives:

• Understand the fundamental mechanics and potential impacts of HTML Injection attacks.
• Learn to identify common vulnerabilities that lead to HTML Injection during security assessments.
• Master the implementation of robust defensive coding practices to mitigate this risk.

You Should Know:

1. Identifying Reflected HTML Injection Points

The first step is to probe all user-input endpoints. A common method is to use a simple payload that alters the page’s structure without executing script.

`

test

`

Step-by-step guide:

Identify a parameter in a URL or a form field that is reflected in the page’s response. Common examples include search queries, URL redirect parameters (?next=), or form feedback fields.

Submit the payload `

test

`.

Observe the HTTP response. If the page renders a large “test” heading, the application is vulnerable. The application is accepting raw HTML and not encoding it before output.
This confirms a lack of proper output encoding on the server side.

2. Testing for Stored HTML Injection

Stored attacks persist within the application and affect every user who views the compromised page, such as a comment or profile field.
``

Step-by-step guide:

Locate any functionality where your input is stored and displayed later (e.g., user profiles, support tickets, comment sections).
Submit the above image tag payload. The `src` attribute points to a server you control. The `onerror` attribute ensures the broken image is hidden, making the injection stealthier.
If the image tag is executed, your server will receive an HTTP request to log.php, confirming the vulnerability. This proves the application does not sanitize stored data upon output.

3. Exploiting Injection for Phishing

Once injection is confirmed, attackers can craft credible phishing forms.
`

Your session is expired, please re-login:

`

Step-by-step guide:

Identify a vulnerable parameter that is reflected in a context where users would expect a form (e.g., a login page’s `?message=` parameter or a user settings page).
Inject the above form payload. This creates a fake login form seamlessly blended into the legitimate website.
When submitted, the credentials are sent directly to the attacker’s server. This demonstrates the severe trust-breaking impact of a seemingly low-risk vulnerability.

4. Bypassing Basic Blacklist Filters

Applications may attempt to filter dangerous tags. Simple obfuscation can often bypass these filters.

`ipt>alert(1)` or ``

Step-by-step guide:

If a payload like `` which then executes.
Alternatively, use an entirely different tag with an event handler attribute, like the `` tag with an `onerror` attribute. This does not require the `