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Re: Crash IE with 11 bytes ;)
From: "The Central Scroutinizer" <scroutinizer () beeb net>
Date: Wed, 28 Jul 2004 23:13:59 +0100

Here's a detailed description of what's going wrong with [STYLE]@;/*

The problem is the unterminated comment "/*"; IE computes the length of the comment for a memcpy opperation by substracting the end pointer form the start pointer. The comment starts behind "/*" and should end at "*/", but since there is no terminator, the start of the string is used. IE there for calculates the string to be -2 unicode characters long. The subsequent memcpy will try to copy 0xFFFFFFFE bytes untill it gets a read or write exception. (You will see the offending instruction is a REP MOVSD)

Unfortunately for us hackers, I believe you cannot control the length value for the memcpy other then setting it to -2. So you will always cause a read or write exception. You will only overwrite a small part of the heap before the exception is caused so overwriting the SEH to controlling execution is also ruled out.

Conclusion: lame DoS

I did find another way to use this to cause an exception at a different location:
[SCRIPT]
 d = window.open().document;
 d.write("x");
 d.body.innerHTML = "<STYLE>@;/*";
[/SCRIPT]
This will crash because of a null pointer in a CMP [ESI], 0.
It didn't look interesting to me, so no detailed investigation.

Cheers,

Cheers, nice analysis, nasty bug, I bet the guy who wrote the code is looking very sheepish :o)

TCS


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