CVE-2025-68313 in Linux
Summary
by MITRE • 12/16/2025
In the Linux kernel, the following vulnerability has been resolved:
x86/CPU/AMD: Add RDSEED fix for Zen5
There's an issue with RDSEED's 16-bit and 32-bit register output variants on Zen5 which return a random value of 0 "at a rate inconsistent with randomness while incorrectly signaling success (CF=1)". Search the web for AMD-SB-7055 for more detail.
Add a fix glue which checks microcode revisions.
[ bp: Add microcode revisions checking, rewrite. ]
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Analysis
by VulDB Data Team • 02/22/2026
The vulnerability CVE-2025-68313 addresses a critical flaw in the Linux kernel's handling of the RDSEED instruction on AMD Zen5 processors. This issue specifically impacts the hardware random number generation capabilities of modern AMD processors, where the RDSEED instruction is designed to provide cryptographically secure random numbers through hardware entropy sources. The flaw manifests in both 16-bit and 32-bit output variants of the RDSEED instruction, creating a significant security concern for systems relying on hardware-based random number generation for cryptographic operations. The vulnerability is particularly concerning because it produces random values of zero at rates that are inconsistent with proper randomness characteristics, while simultaneously incorrectly indicating success through the carry flag (CF=1) which should only be set when valid random data is generated.
The technical implementation of this vulnerability stems from a microcode issue in AMD Zen5 processors where the RDSEED instruction fails to properly generate random data. According to AMD-SB-7055 documentation referenced in the patch, this flaw affects the underlying hardware implementation of the random number generator circuitry, causing it to return zero values with patterns that violate statistical randomness requirements. The issue is not merely a software bug but a fundamental flaw in the processor's microcode that affects how the RDSEED instruction operates at the hardware level. The Linux kernel's x86 CPU subsystem, specifically the AMD processor handling code, must account for this hardware inconsistency through microcode revision checking to prevent systems from incorrectly trusting the output of the RDSEED instruction. The patch implements a fix glue mechanism that performs microcode revision checks before enabling or using the RDSEED instruction, effectively preventing the system from utilizing the faulty hardware implementation.
The operational impact of this vulnerability extends beyond simple random number generation failures to potentially compromise cryptographic security measures that depend on hardware random number generators. Systems using RDSEED for generating cryptographic keys, nonces, or other security-critical random values may be at risk of generating predictable or compromised outputs, particularly when the hardware returns zero values at inconsistent rates. This vulnerability affects Linux systems running on AMD Zen5 processors and could impact security-sensitive applications including TLS/SSL implementations, cryptographic key generation, secure communications, and any software relying on high-quality random number generation. The inconsistency in the zero-value generation rate means that attackers could potentially predict or exploit patterns in the random number generation, undermining the security assumptions of cryptographic protocols that depend on true randomness. The vulnerability affects the broader security posture of Linux systems by potentially weakening the entropy sources available for cryptographic operations, which could lead to key recovery attacks or other cryptographic failures.
Mitigation strategies for CVE-2025-68313 involve implementing microcode revision checking mechanisms within the Linux kernel's CPU detection and handling code, specifically for AMD Zen5 processors. The fix glue approach requires the kernel to verify microcode revisions before enabling RDSEED instruction usage, effectively disabling the faulty implementation until appropriate microcode updates are available. System administrators should ensure their systems are running patched kernel versions that include this microcode checking logic and monitor for AMD microcode updates that address the underlying hardware issue. The solution aligns with industry best practices for handling hardware-level security flaws, particularly those affecting cryptographic primitives, and follows the principle of least privilege by disabling potentially compromised functionality until it can be verified as safe. This vulnerability demonstrates the importance of maintaining up-to-date microcode and kernel versions, as well as the need for comprehensive hardware-software integration testing in security-critical environments. The fix addresses the issue through a defensive programming approach that prevents the use of known-bad hardware implementations rather than attempting to work around them, which is consistent with security frameworks that emphasize robust error handling and validation of hardware capabilities.
This vulnerability maps to CWE-330 (Use of Insufficiently Random Values) and CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) within the Common Weakness Enumeration framework, highlighting the fundamental flaw in random number generation quality. The remediation approach aligns with ATT&CK technique T1552.004 (File and Directory Permissions Modification) and T1068 (Exploitation for Privilege Escalation) as systems may need to be patched and updated to maintain proper security boundaries. The issue also relates to T1499.004 (Encryption for Data Destruction) in scenarios where compromised random number generation could affect encryption key quality and overall system security posture.