CVE-2026-53370: Linux Kernel Privilege Escalation Flaw

CVE-2026-53370 Overview

CVE-2026-53370 is a Linux kernel vulnerability in the Intel x86 performance monitoring subsystem (perf/x86/intel). The flaw affects Auto-Counter Reload (ACR) mask validation and configuration logic used for performance event sampling. User-space supplied ACR masks (attr.config2) receive incomplete validation, allowing bits that reference indices belonging to a different ACR events group. An early return path on invalid masks skips subsequent ACR groups entirely, and stale hardware ACR masks (hw.config1) persist across configuration writes. The upstream kernel patch tightens validation, drops only invalid mask bits instead of aborting, and clears stale state before writing new values.

Critical Impact
Improper validation in the kernel performance subsystem can lead to inconsistent hardware state, incorrect performance counter behavior, and potential local reliability issues when user-space processes configure ACR events.

Affected Products

Linux kernel — perf/x86/intel subsystem
Intel x86 platforms exposing Auto-Counter Reload (ACR) performance events
Distributions shipping affected upstream kernel revisions prior to the referenced stable commits

Discovery Timeline

2026-07-19 - CVE-2026-53370 published to NVD
2026-07-19 - Last updated in NVD database

Technical Details for CVE-2026-53370

Vulnerability Analysis

The defect resides in how the Intel performance monitoring code validates and applies ACR masks supplied through the perf_event_open interface. Three distinct problems interact. First, the validation logic accepts mask bits that reference event indices outside the current ACR group. Second, when the kernel encounters an invalid mask bit, it returns early rather than sanitizing the affected event and continuing. That early return prevents remaining ACR events in the group from being configured. Third, the hardware mask register mirrored by hw.config1 is not cleared before the new mask is written, so residual bits from a previous configuration remain active.

The fix computes the event index group for each ACR group and strips any user-supplied bits that fall outside that group. The kernel now drops invalid mask portions and continues iterating through remaining events. Each event's stale hw.config1 is cleared before the new mask is written. A FIXME comment marks a related theoretical issue where disabling a non-leader event member causes bit-shifting errors in the surviving group members' acr_mask.

Root Cause

Insufficient input validation of a user-controlled bitmask combined with non-idempotent configuration writes. The kernel trusted portions of attr.config2 that should have been constrained to the calling event's index group, and it failed to reset hardware state between reconfigurations.

Attack Vector

Exploitation requires local access to invoke perf_event_open with a crafted attr.config2 value referencing ACR events. The malformed mask produces inconsistent counter state and can suppress configuration of sibling ACR events. No verified proof-of-concept has been published, and no evidence of exploitation in the wild exists. The vulnerability manifests entirely within kernel performance instrumentation and is described in the upstream commits referenced below.

No verified exploit code is available. Refer to Kernel Git Commit 5ad732a, Kernel Git Commit aab56b9, and Kernel Git Commit c05e01c for the authoritative patch details.

Detection Methods for CVE-2026-53370

Indicators of Compromise

Unexpected user-space processes issuing perf_event_open syscalls with populated attr.config2 fields referencing ACR events.
Anomalous performance counter output or inconsistent sampling results on Intel hosts running unpatched kernels.
Kernel log entries relating to perf/x86/intel ACR event configuration on hosts where performance profiling is not part of normal operations.

Detection Strategies

Audit installed kernel versions against distribution advisories to identify hosts missing the upstream ACR validation fix.
Enable auditd rules on the perf_event_open syscall to record processes configuring ACR event groups.
Correlate performance subsystem telemetry with process execution data to identify unprivileged callers manipulating ACR masks.

Monitoring Recommendations

Track kernel package versions across the fleet and flag hosts running builds preceding the referenced stable commits.
Monitor /sys/kernel/debug/tracing and perf subsystem activity for unexpected use of ACR events by non-administrative accounts.
Alert on repeated failed perf_event_open invocations, which may indicate mask fuzzing.

How to Mitigate CVE-2026-53370

Immediate Actions Required

Apply the upstream Linux kernel patch containing the ACR mask validation improvements as soon as vendor builds become available.
Inventory Intel x86 systems using performance monitoring counters and prioritize patching hosts that expose perf_event_open to untrusted local users.
Restrict perf_event_paranoid settings to limit access to performance events by unprivileged users.

Patch Information

The fix is delivered through the following upstream commits: Kernel Git Commit 5ad732a, Kernel Git Commit aab56b9, and Kernel Git Commit c05e01c. Distribution vendors will backport these commits into their supported kernel branches. Consult the appropriate vendor advisory before rebuilding or upgrading production kernels.

Workarounds

Raise the kernel.perf_event_paranoid sysctl to 3 where supported to require CAP_PERFMON or CAP_SYS_ADMIN for perf_event_open.
Limit local shell access on multi-tenant Intel hosts until patched kernels are deployed.
Disable performance monitoring for workloads that do not require it, reducing exposure of the ACR configuration path.

bash

# Restrict perf_event_open to privileged users until patched kernels are deployed
sudo sysctl -w kernel.perf_event_paranoid=3
echo 'kernel.perf_event_paranoid=3' | sudo tee /etc/sysctl.d/90-perf-cve-2026-53370.conf

# Verify installed kernel version against vendor advisory
uname -r