CVE-2026-31615: Linux Kernel Privilege Escalation Flaw

CVE-2026-31615 Overview

CVE-2026-31615 is an Input Validation Error vulnerability in the Linux kernel's USB gadget subsystem, specifically affecting the Renesas USB3 driver (renesas_usb3). The vulnerability exists in the standard request handlers for GET_STATUS and SET/CLEAR_FEATURE USB control requests, where endpoint numbers extracted from the host-supplied wIndex field are not properly validated before use.

When processing USB control requests, the driver directly extracts the endpoint number from untrusted host input without verifying that the endpoint number falls within the valid range of endpoints configured for the device. This lack of validation can result in out-of-bounds memory access when the driver attempts to dereference a pointer calculated from the invalid endpoint index.

Critical Impact
Local attackers with physical USB access or control over a malicious USB host can trigger a Null Pointer Dereference leading to kernel crash and denial of service.

Affected Products

Linux Kernel (multiple versions with Renesas USB3 gadget driver)
Systems using USB gadget functionality with renesas_usb3 driver
Embedded devices and platforms utilizing Renesas USB3 controllers

Discovery Timeline

April 24, 2026 - CVE-2026-31615 published to NVD
April 28, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31615

Vulnerability Analysis

The vulnerability resides in the renesas_usb3 driver's handling of USB standard requests. USB control transfers include a setup packet with a wIndex field that specifies the target endpoint for requests like GET_STATUS, SET_FEATURE, and CLEAR_FEATURE. The driver extracts this endpoint number directly from the host-supplied data without bounds checking.

This vulnerability pattern is similar to a previously addressed issue in the Aspeed USB Device Controller driver (Kernel Commit ee0d382feb44), where the same lack of endpoint index validation existed. The fix involves adding validation logic to ensure the requested endpoint number matches an actual endpoint configured on the device before proceeding with pointer arithmetic or memory access operations.

Root Cause

The root cause is improper input validation (CWE-476) in the USB control request handlers. The code path trusts the wIndex field from USB setup packets without verifying that the endpoint number is within the bounds of the device's actual endpoint array. When an invalid endpoint index is provided, subsequent pointer dereference operations can access uninitialized or invalid memory locations, resulting in a Null Pointer Dereference.

Attack Vector

The attack requires local access to the system, typically through physical USB connectivity or control over a malicious USB host device. An attacker can craft malicious USB control requests containing out-of-bounds endpoint numbers in the wIndex field. When the vulnerable driver processes these requests during GET_STATUS or SET/CLEAR_FEATURE operations, it performs invalid pointer arithmetic leading to memory corruption or kernel panic.

The vulnerability mechanism involves the following sequence:

A malicious USB host sends a standard request (GET_STATUS, SET_FEATURE, or CLEAR_FEATURE) to the USB gadget device
The request includes an invalid endpoint number in the wIndex field that exceeds the actual number of endpoints configured
The renesas_usb3 driver extracts this endpoint number without validation
The driver attempts to access an endpoint structure using this invalid index
This results in an out-of-bounds memory access or Null Pointer Dereference, causing system instability or kernel crash

Detection Methods for CVE-2026-31615

Indicators of Compromise

Unexpected kernel panics or system crashes during USB device operations
Kernel log messages indicating Null Pointer Dereference in USB gadget subsystem
Stack traces referencing renesas_usb3 driver functions in crash dumps
Unusual USB activity patterns or malformed USB control requests in USB protocol analyzers

Detection Strategies

Monitor kernel logs (dmesg) for Null Pointer Dereference errors associated with USB gadget operations
Deploy kernel debugging tools (KASAN, UBSAN) to detect out-of-bounds memory access in USB subsystem
Implement USB protocol monitoring to detect anomalous endpoint numbers in control requests
Use hardware USB analyzers to capture and inspect control transfer setup packets

Monitoring Recommendations

Enable kernel oops and panic logging to capture crash information for forensic analysis
Configure crash dump collection (kdump) to preserve kernel state during crashes
Monitor system stability metrics on devices using Renesas USB3 controllers
Set up alerts for repeated kernel crashes in USB-related code paths

How to Mitigate CVE-2026-31615

Immediate Actions Required

Apply the latest Linux kernel security patches that include endpoint index validation
Restrict physical USB access to trusted devices and personnel
Consider disabling USB gadget functionality if not required for operations
Monitor for unusual USB activity on affected systems

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability across multiple kernel branches. The fix adds validation logic to verify that endpoint numbers in USB control requests match the actual endpoints configured on the device before processing.

The following kernel commits contain the security fix:

Workarounds

Disable the renesas_usb3 gadget driver module if USB gadget functionality is not required
Implement physical access controls to prevent unauthorized USB connections
Use USB port blockers or disable USB ports at the BIOS/UEFI level on sensitive systems
Deploy endpoint security solutions that can monitor and restrict USB device connections

bash

# Disable renesas_usb3 module if not required
echo "blacklist renesas_usb3" >> /etc/modprobe.d/blacklist-usb-gadget.conf

# Verify module is not loaded
lsmod | grep renesas_usb3

# If loaded, remove the module (requires no active USB gadget connections)
modprobe -r renesas_usb3