High-Severity Bug: AMD Zen 5 RDSEED Flaw Risks Randomness Integrity; Microcode Fix Coming

In mid-October 2025, engineers discovered an architectural flaw in the AMD Zen 5 series processors related to the RDSEED instruction. Shortly thereafter, a corresponding Linux kernel patch was submitted to the Kernel Mailing List, disabling the affected processors’ RDSEED functionality entirely.

AMD has now issued an official statement acknowledging the issue and confirming that a microcode update will be released to correct the flaw. The vulnerability primarily affects the confidentiality and integrity of RDSEED operations and has been classified as a high-severity security issue.

In its advisory, AMD stated:

“AMD was notified of a bug in “Zen 5” processors that may cause the RDSEED instruction to return 0 at a rate inconsistent with randomness while incorrectly signaling success (CF=1), indicating a potential misclassification of failure as success. This issue was initially reported publicly via the Linux kernel mailing list and was not submitted through AMD’s Coordinated Vulnerability Disclosure (CVD) process.”

The issue was first publicly reported through the Linux Kernel Mailing List, rather than AMD’s coordinated vulnerability disclosure process. AMD has determined that the 16-bit and 32-bit RDSEED instructions are affected, while the 64-bit RDSEED instruction remains unaffected.

For now, AMD advises developers and engineers to either use the 64-bit RDSEED instruction, disable RDSEED entirely, or revise its implementation on Zen 5 systems to enhance security.

According to AMD’s roadmap, an AGESA firmware update addressing the microcode issue for AMD EPYC 9005 processors will be released on November 14, with updated Family 1Ah microcode also made available through linux-firmware.git. These updates include critical fixes for both Turin and Turin Dense processor cores.

By late November, AMD will issue AGESA updates for the Ryzen 9000 and Ryzen AI 300 series, while EPYC Embedded 9000 and 4005 processors are expected to receive their fixes in the January 2026 AGESA firmware release.

Related Posts:

• RIP Twitter.com: X Retires Old Domain, Forcing Passkey Reset by Nov 10
• Important wolfSSL Update: Critical Apple Trust Store Bypass & Predictable Randomness Flaws Patched
• Canonical has withdrawn Intel’s Spectre Update (20180108) due to hardware issue
• AMD: CPU security patch for Ryzen and EPYC processors come on this week