Self XSS Vulnerability in a Rich Text Editor

During my recent security testing, I identified a Self Cross-Site Scripting (Self-XSS) issue in a web-based ticketing platform. This write-up focuses on the technical details and learning aspects while keeping the target fully anonymized.

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What is the Issue

The application uses a rich text editor for user input, commonly found in ticket systems, comment sections, and dashboards.

While testing the editor features, I discovered that the insert link functionality was not properly handling certain types of input. This allowed crafted payloads to be injected and executed in the browser.

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Root Cause

The core issue lies in improper handling of user-controlled input inside the editor.

Specifically:

• The URL field in the insert link feature accepted complex input
• The input was not fully sanitized before being processed
• The editor allowed rendering of embedded HTML through data URIs

This created a situation where browser-executable content could be introduced.

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Technical Breakdown

Payload Used

<object data='data:text/html;base64,PHN2Zy9vbmxvYWQ9YWxlcnQoMSk+'></object>

What this does

• data:text/html;base64,... allows embedding HTML content directly
• The Base64 string decodes to an SVG element with an onload event
• When rendered, the browser executes JavaScript

This is a common technique to bypass basic filters that only block <script> tags.

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Execution Flow

1. User opens the editor
2. Clicks on insert link option
3. Enters crafted payload in URL field
4. Saves the content
5. When the content is rendered:

• The object tag loads the data URI
• The embedded SVG executes JavaScript via onload

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Observed Behavior

• JavaScript executed successfully in the browser
• The execution was restricted to the same user session
• The payload did not impact other users or administrators

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Why It is Self XSS

This case was classified as Self-XSS because:

• The attack requires the user to inject the payload themselves
• No automatic execution for other users
• No cross-user data exposure
• No privilege escalation

From a risk perspective, this is considered low impact in most bug bounty programs.

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Why It Still Matters

Even though this is labeled as low severity, it is still important from a security standpoint.

1. Indicator of Weak Input Handling

It shows that the application does not fully sanitize complex inputs.

2. Potential for Chaining

If combined with other issues like:

• Clickjacking
• Social engineering
• Stored input reuse

It could lead to more serious exploitation.

3. Editor Attack Surface

Rich text editors are historically prone to XSS-related issues due to their flexibility.

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Recommendations for Developers

1. Strict Input Validation

Do not allow raw HTML or dangerous tags in user input fields.

2. Sanitize Editor Output

Use well-tested sanitization libraries to clean content before rendering.

3. Block Dangerous Schemes

Restrict usage of:

• data: URIs
• javascript: protocols

4. Apply Content Security Policy (CSP)

Limit execution of inline scripts and restrict resource loading.

5. Context-Aware Encoding

Ensure proper encoding based on where the data is rendered.

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Key Takeaways

• Not all XSS issues are high impact
• Understanding context is critical in vulnerability assessment
• Rich text editors require deep testing beyond basic payloads
• Always think in terms of exploitation possibilities, not just execution

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About Me

I am focused on web application security and VAPT.
I am open to remote opportunities and interested in working with startups and small teams where I can contribute and grow.