AMD ‘Zenbleed’ bug allows Meltdown-like data leakage

AMD has started issuing some patches for its processors affected by a serious silicon-level bug dubbed Zenbleed that can be exploited by rogue users and malware to steal passwords, cryptographic keys, and other secrets from software running on a vulnerable system.

Zenbleed affects Ryzen and Epyc Zen 2 chips, and can be abused to swipe information at a rate of at least 30Kb per core per second. That’s practical enough for someone on a shared server, such as a cloud-hosted box, to spy on other tenants. Exploiting Zenbleed involves abusing speculative execution, though unlike the related Spectre family of design flaws, the bug is pretty easy to exploit. It is more on a par with Meltdown.

Malware already running on a system, or a rogue logged-in user, can exploit Zenbleed without any special privileges and inspect data as it is being processed by applications and the operating system, which can include sensitive secrets, such as passwords. It’s understood a malicious webpage, running some carefully crafted JavaScript, could quietly exploit Zenbleed on a personal computer to snoop on this information.

The vulnerability was highlighted today by Google infosec guru Tavis Ormandy, who discovered the data-leaking vulnerability while fuzzing hardware for flaws, and reported it to AMD in May. Ormandy, who acknowledged some of his colleagues for their help in investigating the security hole, said AMD intends to address the flaw with microcode upgrades, and urged users to “please update” their vulnerable machines as soon as they are able to.

Proof-of-concept exploit code, produced by Ormandy, is available here, and we’ve confirmed it works on a Zen 2 Epyc server system when running on the bare metal. While the exploit runs, it shows off the sensitive data being processed by the box, which can appear in fragments or in whole depending on the code running at the time.

If you stick any emulation layer in between, such as Qemu, then the exploit understandably fails.

What’s hit?

The bug affects all AMD Zen 2 processors including the following series: Ryzen 3000; Ryzen Pro 3000; Ryzen Threadripper 3000; Ryzen 4000 Pro; Ryzen 4000, 5000, and 7020 with Radeon Graphics; and Epyc Rome datacenter processors.

AMD today issued a security advisory here, using the identifiers AMD-SB-7008 and CVE-2023-20593 to track the vulnerability. The chip giant scored the flaw as a medium severity one, describing it as a “cross-process information leak.”

A microcode patch for Epyc 7002 processors is available now. As for the rest of its affected silicon: AMD is targeting December 2023 for updates for desktop systems (eg, Ryzen 3000 and Ryzen 4000 with Radeon); October for high-end desktops (eg, Threadripper 3000); November and December for workstations (eg, Threadripper Pro 3000); and November to December for mobile (laptop-grade) Ryzens. Shared systems are the priority, it would seem, which makes sense given the nature of the design blunder.


Source: AMD ‘Zenbleed’ bug allows Meltdown-like data leakage

Robin Edgar

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