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

In mid-October 2025, engineers identified an architectural flaw in the AMD Zen 5 series processors related to the RDSEED instruction, with a Linux kernel patch soon disabling RDSEED functionality. AMD issued an official statement acknowledging the issue and said a microcode update will be released to correct the flaw, which primarily affects the confidentiality and integrity of RDSEED operations and has been classified as a high-severity security issue.

According to AMD’s advisory, the bug in Zen 5 may cause the RDSEED instruction to return 0 at a rate inconsistent with randomness while incorrectly signalling success, and it was first publicly reported via the Linux Kernel Mailing List rather than through AMD’s coordinated vulnerability disclosure process; the 16-bit and 32-bit RDSEED instructions are affected, while the 64-bit RDSEED remains unaffected.

For now, developers are advised to use the 64-bit RDSEED instruction, disable RDSEED entirely, or revise its implementation on Zen 5 systems. According to AMD’s roadmap, an AGESA firmware update addressing the microcode issue for AMD EPYC 9005 will be released on 14 November, with updated Family 1Ah microcode also made available through linux-firmware[.]git; updates for Ryzen 9000 and Ryzen AI 300 series are expected in late November, and EPYC Embedded 9000 and 4005 fixes in the January 2026 AGESA firmware release.