Protecting VMWare from CPUID hypervisor detection

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One of the less-known anti-sandbox detection tricks relies on the instruction CPUID that is executed with EAX=1 as an input parameter. When executed the values returned by general purpose registers describe the various Processor Info and Feature Bits. Some malware samples analyse the bit 31th of the value returned in the register ECX; the bit is equal to zero on the real CPU/hypervisor and 1 on the guest VM.

You can test this trick by running the following program.

When executed on a real CPU it should give you the following output:

cpuid_novm

 

On the guest OS it should (not surprisingly) show the following output:

cpuid_vm

Depending on your hardware set up and the way your Virtual Platform is executing code you may get different results (modification of virtualization and dynamic translation settings may affect the results).

Bypassing this trick on the code level (e.g. during debugging session or via patching) is not easy as you need to find a sequence of code that is responsible for the detection first. And if the code is metamorphic that won’t be easy (CPUID is just 2 bytes long i.e. 0F A2 and the input values may be initiated using random sequence of code).

It turns out VMWare (tested on 10.0.3) supports a mechanism that allows to modify the CPUID instruction results. All you have to do is add the following line to your .vmx file:

cpuid.1.ecx="0---:----:----:----:----:----:----:----"

It instructs the VMWare to mask the 31st bit of ECX when the CPUID instruction is executed. As such, running the same program on the re-configured VM will give you the output as seen on the first screenshot i.e. CPUID returns value observed on the real CPU; malware running on such re-configured VM will get fooled and the sandbox detection can be bypassed.

Note: I am not an expert in VMWare configuration – any modification you do to your VM is on your own risk. If you break anything it’s your own fault 🙂

I encourage you to explore VMWare forums before doing any modification to your setup. Also, remember that you need to fully restart the VM (and the guest system) for the change to take place (it won’t work for a restart from a snapshot).