Zenbleed – AMD’s Zen2 Processor Flaw Allows Attackers to Steal Sensitive Data

The CPUs that are based on x86-64 architecture feature XMM registers (128-bit), recently extended to 256-bit (YMM) and 512-bit (ZMM) for greater capacity.

Beyond number crunching, the large registers (YMM and ZMM) are employed in various scenarios, including standard C library functions such as:- 

• strcmp
• memcpy
• strlen

Tavis Ormandy, a Google security researcher, found a fresh AMD Zen2 CPU vulnerability enabling data theft at 30KB/sec per core, risking passwords and encryption keys.

Zenbleed Flaw

The AMD’s Zen2 processor vulnerability has been tracked as “CVE-2023-20593,” this results from mishandling ‘vzeroupper’ instruction, impacting modern processors’ speculative execution. 

Ormandy found hardware events with fuzzing and performance counters, confirmed via the “Oracle Serialization” approach.

Using this technique, the author exposed CVE-2023-20593 in Zen2 CPUs by spotting discrepancies between randomly generated programs and serialized oracles. 

The flaw allowed optimized data leakage from various system operations, even in virtual machines, isolated sandboxes, and containers.

On May 15, 2023, the researcher informed AMD of the flaw and now released a PoC exploit for CVE-2023-20593. Though written for Linux, the bug impacts all Zen 2 CPUs regardless of the operating system.

Products affected

Here below, we have mentioned all the products that are affected:-

• AMD Ryzen 3000 Series Processors
• AMD Ryzen PRO 3000 Series Processors
• AMD Ryzen Threadripper 3000 Series Processors
• AMD Ryzen 4000 Series Processors with Radeon Graphics
• AMD Ryzen PRO 4000 Series Processors
• AMD Ryzen 5000 Series Processors with Radeon Graphics
• AMD Ryzen 7020 Series Processors with Radeon Graphics
• AMD EPYC “Rome” Processors

For ‘Zenbleed’-affected CPUs, make sure to apply AMD’s microcode update immediately or await a future BIOS fix from your vendor.

It’s been recommended users set up the “chicken bit” to DE_CFG[9] as a mitigation method, but it causes a CPU performance drop. Detecting Zenbleed exploitation is likely impossible since it’s stealthy, needing no elevated privileges or special system calls.

However, the practical impact of Zenbleed is limited to users with local access and specialized knowledge to exploit it.

But, experts have strongly recommended that users keep their systems updated with the latest security patches and BIOS updates.

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