Linux Kernel CVE-2026-53121

Exploitation requires a Linux system with an AMD processor where the amd-pstate driver is active in EPP mode (the default on supported AMD CPUs running Linux 5.17+). The memory leak is triggered specifically when the hardware EPP register set operation fails during CPU initialization - a condition determined by AMD firmware and ACPI CPPC interface behavior, not directly controllable by a normal local user. The CVSS vector confirms local access with low-privilege execution context is sufficient, but the EPP failure condition itself is hardware/firmware dependent. Limiting factors: this cannot be triggered remotely; it cannot be reliably triggered on hardware where EPP initialization succeeds (the common case); exploitation to a meaningful denial-of-service requires sustained, repeated CPU hotplug cycles or abnormal firmware behavior. Systems where amd-pstate is explicitly disabled (boot parameter amd_pstate=disable) or running non-AMD CPUs are entirely unaffected.

Real-world risk is low-to-moderate and primarily operational rather than adversarial. The CVSS 3.1 base score of 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) reflects local availability impact only - no confidentiality or integrity exposure. EPSS at 0.15% (5th percentile) confirms that exploitation by threat actors is nearly absent in observed telemetry. There is no CISA KEV listing and no public exploit code identified at time of analysis. The availability impact (A:H) is technically accurate in that a complete kernel heap exhaustion from repeated leaked allocations could crash or destabilize the system, but achieving this in practice requires either a sustained CPU hotplug loop or a very long-running system with frequent EPP init failures - both uncommon in production. The AC:L assignment in the provided NVD vector may overstate exploitability: the error condition requires the EPP hardware-set to fail, which depends on AMD firmware/ACPI behavior not universally controllable by a local user. This is a defect to patch on schedule during routine kernel updates rather than an emergency response priority. Ubuntu USN-8568-1 and USN-8566-1 confirm vendor acknowledgment.

An AMD-based Linux server performing frequent CPU hotplug operations - such as a cloud hypervisor dynamically onlining and offlining vCPUs, or a power-management daemon cycling CPUs - encounters a firmware-side EPP register failure on each amd_pstate_epp_cpu_init() call. Each failed initialization leaks one cpudata allocation into the kernel heap. Over thousands of hotplug cycles, cumulative memory pressure causes kernel OOM conditions or system instability, effectively achieving a local denial of service. No public exploit code is available; this scenario requires specific AMD hardware behavior that causes EPP set to fail.