CVE-2026-31549: Linux Kernel NULL-Deref Vulnerability

CVE-2026-31549 Overview

A NULL pointer dereference vulnerability has been identified in the Linux kernel's i2c cp2615 driver. The vulnerability exists because the driver uses the USB device serial string as the i2c adapter name without verifying that the string actually exists. When a malicious or malformed USB device is connected that lacks a serial number string, the driver attempts to access a NULL pointer, leading to a kernel crash and denial of service condition.

This vulnerability affects systems that use the cp2615 i2c driver module, which provides support for Silicon Labs CP2615 USB-to-I2C bridge devices. An attacker with physical access to the system or the ability to connect USB devices could exploit this vulnerability to cause system instability or denial of service.

Critical Impact

Local attackers can trigger a kernel NULL pointer dereference via malicious USB devices, causing system crashes and denial of service.

Affected Products

• Linux Kernel version 5.13 and later
• Linux Kernel 7.0 release candidates (rc1 through rc7)
• Linux Kernel stable versions with the cp2615 driver module enabled

Discovery Timeline

• 2026-04-24 - CVE-2026-31549 published to NVD
• 2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31549

Vulnerability Analysis

The vulnerability resides in the cp2615 i2c driver within the Linux kernel's USB subsystem. During device probe initialization, the driver retrieves the USB device's serial number string to use as the i2c adapter name. The fundamental issue is that the driver assumes this serial string will always be present and valid, without performing any NULL pointer validation before accessing it.

When a USB device is connected that does not contain a serial number descriptor, the USB core returns a NULL pointer for the serial string. The cp2615 driver then attempts to dereference this NULL pointer when setting up the i2c adapter name, resulting in a kernel oops or panic depending on kernel configuration.

This flaw is classified as CWE-476 (NULL Pointer Dereference). The vulnerability requires local access to the system, as an attacker would need the ability to connect a USB device or emulate USB device connections. While the vulnerability does not directly allow code execution or data exfiltration, it can be reliably triggered to cause availability impacts through system crashes.

Root Cause

The root cause is a missing NULL pointer check in the cp2615 driver's probe function. The driver directly accesses usb_dev->serial without first verifying that the pointer is non-NULL. USB devices are not required to provide serial number descriptors, making this an oversight in the driver's input validation.

The fix involves adding a validation check to verify that the USB device has a valid serial number string before attempting to use it. If no serial number exists, the driver should either reject the device or use an alternative naming scheme for the i2c adapter.

Attack Vector

The attack vector requires local access to the target system. An attacker could exploit this vulnerability by:

1. Physically connecting a malicious USB device that has been crafted without a serial number descriptor
2. Using a USB device emulator or programmable USB hardware to present a device without a serial string
3. Exploiting another vulnerability that allows USB device injection or manipulation

When the vulnerable system attempts to initialize the malicious device, the cp2615 driver triggers the NULL pointer dereference during the probe phase, causing a kernel crash. This attack does not require any user interaction beyond the initial device connection and does not require elevated privileges if the user has physical access to USB ports.

The attack can be described through the following sequence:

1. Attacker prepares a USB device without a serial number string in its device descriptor
2. The device is connected to a system running a vulnerable Linux kernel with the cp2615 module loaded or available for auto-loading
3. The kernel's USB subsystem detects the device and matches it to the cp2615 driver
4. The cp2615 probe function is called, which attempts to read the serial string
5. The driver dereferences the NULL pointer, triggering a kernel oops/panic
6. System becomes unresponsive or reboots, achieving denial of service

Detection Methods for CVE-2026-31549

Indicators of Compromise

• Kernel oops or panic messages referencing the cp2615 driver module in system logs
• Unexpected system reboots or crashes following USB device connection events
• Dmesg entries showing NULL pointer dereference in the i2c or USB subsystem code paths
• USB device connection events from unknown or unrecognized hardware immediately preceding system instability

Detection Strategies

• Monitor kernel logs (/var/log/kern.log, dmesg) for NULL pointer dereference errors related to the cp2615 driver
• Implement USB device whitelisting to restrict connections from untrusted or unauthorized USB devices
• Deploy endpoint detection and response (EDR) solutions capable of monitoring USB device connection events and correlating them with system stability issues
• Use Linux audit framework to log USB device connections and module loading events

Monitoring Recommendations

• Configure syslog forwarding to centralized SIEM for kernel panic and oops message aggregation
• Enable USB device connection logging through udev rules for forensic analysis
• Set up automated alerts for unexpected cp2615 module loading on systems that should not have this hardware
• Monitor for repeated system crashes that may indicate active exploitation attempts

How to Mitigate CVE-2026-31549

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the NULL pointer check fix
• If updating is not immediately possible, blacklist the cp2615 module on systems that do not require CP2615 USB-to-I2C bridge functionality
• Restrict physical access to USB ports on critical systems
• Implement USB device authorization policies using USBGuard or similar tools

Patch Information

Multiple patches have been released across different kernel branches to address this vulnerability. The fix adds a verification check to ensure the USB device has a serial number before accessing it during driver probe.

Patched commits are available from the following kernel git repositories:

• Kernel Git Commit 13ccf9b
• Kernel Git Commit 4a22af8
• Kernel Git Commit 69aece6
• Kernel Git Commit a977829
• Kernel Git Commit aa79f99
• Kernel Git Commit e68c267
• Kernel Git Commit efe996b

Workarounds

• Blacklist the cp2615 module by adding blacklist cp2615 to /etc/modprobe.d/blacklist.conf on systems that do not require this driver
• Use physical USB port locks or disable unused USB ports in BIOS/UEFI where possible
• Implement USBGuard policies to block unauthorized USB devices from being enumerated by the system
• For containerized or virtualized environments, restrict USB passthrough to only trusted and necessary devices

# Blacklist the vulnerable cp2615 module
echo "blacklist cp2615" | sudo tee /etc/modprobe.d/blacklist-cp2615.conf

# Unload the module if currently loaded
sudo modprobe -r cp2615

# Verify module is not loaded
lsmod | grep cp2615

# Update initramfs to persist the blacklist
sudo update-initramfs -u