CVE-2026-31520: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31520 Overview

A memory leak vulnerability has been identified in the Linux kernel's HID (Human Interface Device) driver for Apple devices. The apple_report_fixup() function was found to allocate memory using kmemdup() but never properly freeing it, leading to a kernel memory leak condition that could impact system stability over time.

Critical Impact

Continuous memory leakage in the kernel HID subsystem can lead to resource exhaustion and potential denial of service conditions on systems using Apple HID devices.

Affected Products

• Linux kernel (HID Apple driver module)
• Systems utilizing Apple HID devices (keyboards, trackpads, mice)
• Linux distributions with affected kernel versions

Discovery Timeline

• April 22, 2026 - CVE-2026-31520 published to NVD
• April 23, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31520

Vulnerability Analysis

The vulnerability exists in the HID Apple driver's report fixup mechanism within the Linux kernel. The apple_report_fixup() function is responsible for modifying HID report descriptors for Apple devices to ensure proper compatibility with the Linux HID subsystem.

The core issue stems from improper memory management where the function allocates a new buffer using kmemdup() to create a modified copy of the report descriptor. However, the allocated memory is never freed after use. The caller of report_fixup() does not take ownership of the returned pointer, creating an ownership ambiguity that results in the memory leak.

According to the proper API contract, the report_fixup() function is permitted to return a sub-portion of the input rdesc parameter, whose lifetime is managed by the caller. The implementation incorrectly deviated from this pattern by returning newly allocated memory without establishing a proper deallocation path.

Root Cause

The root cause is a memory management oversight in the apple_report_fixup() function. The function used kmemdup() to allocate kernel memory for a modified report descriptor buffer, but failed to implement corresponding memory deallocation logic. This violates the expected behavior where either:

1. The returned buffer should be a portion of the caller-managed input buffer, or
2. The callee should maintain responsibility for freeing any newly allocated memory

The lack of clear memory ownership semantics between the caller and callee functions contributed to this vulnerability.

Attack Vector

The memory leak is triggered each time an Apple HID device initializes or reconnects to the system. While this vulnerability does not directly allow code execution, repeated triggering of the memory leak (through device reconnection or during normal system operation with multiple HID devices) can lead to:

1. Gradual kernel memory exhaustion
2. System performance degradation
3. Potential denial of service if memory resources become critically depleted
4. System instability requiring a reboot to reclaim leaked memory

The vulnerability requires local access to systems with Apple HID devices, though the impact affects the entire system's stability.

Detection Methods for CVE-2026-31520

Indicators of Compromise

• Gradually increasing kernel memory usage over time, particularly in the slab allocator
• Memory allocation warnings in kernel logs related to the HID subsystem
• System slowdown or instability on machines with Apple HID devices connected
• Kernel memory debugging tools (kmemleak) reporting unreferenced memory allocations in the HID Apple module

Detection Strategies

• Monitor kernel memory utilization trends using tools like /proc/meminfo and slabtop
• Enable kernel memory leak detection (CONFIG_DEBUG_KMEMLEAK) to identify unreferenced allocations
• Track HID device initialization events in system logs for correlation with memory growth
• Use SentinelOne's kernel-level monitoring to detect anomalous memory consumption patterns

Monitoring Recommendations

• Implement periodic kernel memory audits on systems with Apple HID peripherals
• Configure alerts for sustained kernel memory growth without corresponding user-space activity
• Monitor dmesg output for memory allocation failures or warnings in the HID subsystem
• Deploy endpoint detection solutions capable of kernel-level resource monitoring

How to Mitigate CVE-2026-31520

Immediate Actions Required

• Update the Linux kernel to a patched version that addresses the memory leak
• Apply vendor-specific kernel patches from your Linux distribution
• Consider temporarily disabling the HID Apple module (hid-apple) if Apple HID devices are not critical
• Monitor system memory usage and plan for reboots if memory exhaustion symptoms appear

Patch Information

The vulnerability has been addressed through multiple kernel commits across different stable branches. The patches modify the apple_report_fixup() function to properly manage memory allocation and deallocation, or to use the caller-provided buffer appropriately.

Relevant kernel patches are available at:

• Kernel Git Commit 239c151
• Kernel Git Commit 2635d0c
• Kernel Git Commit 31860c3
• Kernel Git Commit be1a341
• Kernel Git Commit e2f090a
• Kernel Git Commit e652ebd

Workarounds

• Unload the hid-apple kernel module if Apple HID devices are not required: modprobe -r hid_apple
• Blacklist the module to prevent automatic loading by adding blacklist hid_apple to /etc/modprobe.d/blacklist.conf
• Implement scheduled system reboots to reclaim leaked memory until patching is complete
• Use generic HID drivers as an alternative for basic Apple device functionality

# Temporarily unload the vulnerable HID Apple module
modprobe -r hid_apple

# Blacklist the module to prevent automatic loading
echo "blacklist hid_apple" >> /etc/modprobe.d/blacklist.conf

# Verify the module is not loaded
lsmod | grep hid_apple