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Introduction
Cross-Site Scripting (XSS) remains a critical web vulnerability, allowing attackers to inject malicious scripts into web pages viewed by other users. One often-overlooked vector involves unclosed HTML tags, which can bypass filters and enable successful XSS exploitation. This article explores how bug bounty hunters and security professionals can leverage unclosed tags to craft effective payloads.
Learning Objectives
- Understand how unclosed HTML tags can lead to XSS vulnerabilities.
- Learn how to craft XSS payloads that bypass input filters.
- Discover mitigation techniques to prevent such exploits.
You Should Know
1. Identifying Vulnerable HTML Tags
Some HTML tags, if left unclosed, can create injection opportunities. Common examples include:
– ``
– `` or other closing patterns.
3. Exploiting Malformed Attributes
Some filters check for quote-wrapped attributes. Unclosed tags can bypass this.
Example Payload:
<a href=javascript:alert('XSS')>Click
Step-by-Step Explanation:
- The `href` attribute lacks quotes, which some filters miss.
2. The JavaScript pseudo-protocol executes when clicked.
- Works even if strict attribute validation is in place.
4. Using Unclosed Tags in Event Handlers
Event handlers like `onmouseover` can be abused in unclosed elements.
Example Payload:
< div onmouseover=alert(1)
Step-by-Step Explanation:
1. The `` tag is left unclosed.- The `onmouseover` event triggers when a user hovers over the element.
- Effective in cases where full tag parsing is weak.
5. Mitigation: Proper HTML Sanitization
To prevent such attacks, developers should:
- Use strict parsers (e.g., DOMPurify).
- Enforce proper tag closing.
- Implement Content Security Policy (CSP).
Example CSP Header:
Content-Security-Policy: default-src 'self'; script-src 'unsafe-inline'
What Undercode Say
- Key Takeaway 1: Unclosed HTML tags are a powerful XSS vector often missed by automated scanners.
- Key Takeaway 2: Input sanitization must account for malformed markup, not just known payload patterns.
Analysis:
Bug bounty hunters should prioritize testing for unclosed tags, especially in rich text editors and user-generated content. Modern frameworks like React and Angular mitigate some risks, but legacy systems remain vulnerable. As XSS defenses evolve, attackers adapt—making manual testing essential for uncovering overlooked exploits.
Prediction
As web applications increasingly rely on dynamic content, unclosed-tag XSS attacks will persist, particularly in systems with weak sanitization. Future mitigation may involve AI-driven parsing to detect malformed markup in real time, but until then, manual security testing remains crucial.
IT/Security Reporter URL:
Reported By: Therceman Bug – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
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- The `onmouseover` event triggers when a user hovers over the element.
- Effective in cases where full tag parsing is weak.
5. Mitigation: Proper HTML Sanitization
To prevent such attacks, developers should:
- Use strict parsers (e.g., DOMPurify).
- Enforce proper tag closing.
- Implement Content Security Policy (CSP).
Example CSP Header:
Content-Security-Policy: default-src 'self'; script-src 'unsafe-inline'
What Undercode Say
- Key Takeaway 1: Unclosed HTML tags are a powerful XSS vector often missed by automated scanners.
- Key Takeaway 2: Input sanitization must account for malformed markup, not just known payload patterns.
Analysis:
Bug bounty hunters should prioritize testing for unclosed tags, especially in rich text editors and user-generated content. Modern frameworks like React and Angular mitigate some risks, but legacy systems remain vulnerable. As XSS defenses evolve, attackers adapt—making manual testing essential for uncovering overlooked exploits.
Prediction
As web applications increasingly rely on dynamic content, unclosed-tag XSS attacks will persist, particularly in systems with weak sanitization. Future mitigation may involve AI-driven parsing to detect malformed markup in real time, but until then, manual security testing remains crucial.
IT/Security Reporter URL:
Reported By: Therceman Bug – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
