Hackers abuse single bit change in Intel CPU register to evade detection

Palo Alto Networks discovers that Trap Flag is being abused to notify malware it is being analyzed

Security researchers have discovered a specific single bit (Trap Flag) in the Intel CPU register that malware can abuse to evade sandbox detection.

According to researchers at Palo Alto Networks’ Unit 42 threat research group, malware can detect whether it is executing in a physical or virtual machine (VM) by monitoring the response of the CPU after setting this single bit.

Malware usually avoids detection by checking if it is being run in a virtualized “sandbox” environment set up to safely analyze potential malware. When the malware finds out it is executing in a virtual machine, it will terminate its execution or provide fake outputs to hide its real intentions.

In this instance, to detect VM use in a sandbox, malware could check the CPU’s behavior after enabling the trap flag. This is the eighth single bit in the EFLAGs register of the Intel x86 CPU architecture. 

If the trap flag is enabled before a single instruction is executed, the CPU will raise an exception (single-step mode) after the instruction is completed. This exception stops the CPU execution to allow the exception handler to examine the contents of the registers and memory location. Before allowing code execution to continue, the CPU must also clear the trap flag.

“To determine whether a VM is used, malware can check whether the single-step exception was delivered to the correct CPU instruction, after executing specific instructions (e.g. CPUID, RDTSC, IN) that cause the VM to exit with the TF enabled. During VM exits, the hypervisor – also known as Virtual Machine Monitor (VMM) – will emulate the effects of the physical CPU it encounters,” said researchers.

Researchers also said there was an ongoing cat-and-mouse game between malware authors crafting evasion techniques to prevent effective analysis and sandbox authors researching novel ways to defeat those evasions.

Related Resource

2021 IBM Security X-Force Insider Threat Report

Top discovery methods and recommendations for insider attacks

White background with a black border on side - whitepaper from IBMDownload now

“This is one of the main drivers that led us at Palo Alto Networks to build our own custom hypervisor for malware analysis. Since we have full control over the software stack, including the virtualization layer, we can react to new and emerging threats,” said researchers. 

“In this particular case, once we had identified the issue with the incorrect emulation of the trap flag, our hypervisor team was able to test and deploy a fix.”

Researchers have since been able to fix this evasion problem for any malware sample by deploying this technique.

Featured Resources

B2B under quarantine

Key B2C e-commerce features B2B need to adopt to survive

Download now

The top three IT pains of the new reality and how to solve them

Driving more resiliency with unified operations and service management

Download now

The five essentials from your endpoint security partner

Empower your MSP business to operate efficiently

Download now

How fashion retailers are redesigning their digital future

Fashion retail guide

Download now

Recommended

Intel unveils Xeon W-3300 Ice Lake CPUs for workstations
components

Intel unveils Xeon W-3300 Ice Lake CPUs for workstations

30 Jul 2021
Most employees put their workplace at risk by taking cyber security shortcuts
cyber security

Most employees put their workplace at risk by taking cyber security shortcuts

27 Jul 2021
Intel warns global chip shortage could last until 2023
components

Intel warns global chip shortage could last until 2023

26 Jul 2021
What is Maze Ransomware?
ransomware

What is Maze Ransomware?

22 Jul 2021

Most Popular

RMIT to be first Australian university to implement AWS supercomputing facility
high-performance computing (HPC)

RMIT to be first Australian university to implement AWS supercomputing facility

28 Jul 2021
Samsung Galaxy S21 5G review: A rose-tinted experience
Mobile Phones

Samsung Galaxy S21 5G review: A rose-tinted experience

14 Jul 2021
The benefits of workload optimisation
Sponsored

The benefits of workload optimisation

16 Jul 2021