CVE-2023-28328: Linux Kernel az6027 Driver DoS Flaw

CVE-2023-28328 Overview

A NULL pointer dereference flaw was found in the az6027 driver in drivers/media/usb/dev-usb/az6027.c in the Linux Kernel. The message from user space is not checked properly before transferring into the device. This flaw allows a local user to crash the system or potentially cause a denial of service.

Critical Impact

Local users can exploit improper input validation in the az6027 USB media driver to trigger a NULL pointer dereference, resulting in system crash and denial of service conditions.

Affected Products

• Linux Kernel (all affected versions)
• Red Hat Enterprise Linux 8.0
• Debian Linux (see LTS advisories)

Discovery Timeline

• 2023-04-19 - CVE-2023-28328 published to NVD
• 2025-03-19 - Last updated in NVD database

Technical Details for CVE-2023-28328

Vulnerability Analysis

This vulnerability exists in the az6027 USB media driver, which is used to support certain DVB-T USB devices. The core issue stems from insufficient validation of user-space messages before they are processed and transferred to the device. When a malformed or unexpected message is received from user space, the driver fails to properly check for NULL pointers before dereferencing them, leading to a kernel panic.

The az6027 driver processes USB control messages that originate from user-space applications. Without proper bounds checking and NULL pointer validation, an attacker with local access can craft specific input that triggers the vulnerable code path, causing the kernel to attempt to dereference a NULL pointer and resulting in immediate system crash.

Root Cause

The root cause of CVE-2023-28328 is classified as CWE-476 (NULL Pointer Dereference). The az6027 driver in drivers/media/usb/dev-usb/az6027.c does not adequately validate messages received from user space before processing them. This missing input validation allows NULL pointer values to propagate through the driver code, ultimately resulting in a dereference operation on an invalid memory address.

The vulnerability demonstrates a common pattern in kernel driver development where trust boundaries between user space and kernel space are not properly enforced. User-supplied data must always be treated as untrusted and validated before use in kernel operations.

Attack Vector

The attack vector is local, requiring an authenticated user with access to the affected system. An attacker can exploit this vulnerability by:

1. Gaining local access to a system running a vulnerable Linux kernel with the az6027 driver loaded
2. Crafting malicious input that targets the driver's message handling functionality
3. Sending the crafted message to the driver through normal USB control interfaces
4. Triggering the NULL pointer dereference to crash the system

The vulnerability requires the az6027 USB driver module to be loaded, which typically occurs automatically when compatible USB DVB-T devices are connected to the system. Systems without this driver or without the corresponding hardware are not affected.

Detection Methods for CVE-2023-28328

Indicators of Compromise

• Unexpected kernel panics or system crashes referencing the az6027 driver module
• Kernel oops messages in system logs containing az6027 in the call stack
• Repeated system instability when USB media devices are connected or accessed
• Entries in /var/log/kern.log or dmesg output showing NULL pointer dereference in media/usb subsystem

Detection Strategies

• Monitor kernel logs for panic messages or oops related to the az6027 driver or USB media subsystem
• Implement system monitoring to detect unexpected reboots or crashes on systems with USB DVB-T devices
• Use kernel live patching solutions to detect and alert on vulnerable driver versions
• Deploy endpoint detection solutions capable of monitoring kernel-level events and driver behavior

Monitoring Recommendations

• Configure centralized logging to capture kernel panic events and correlate across affected systems
• Enable kernel crash dump collection (kdump) to facilitate post-incident analysis
• Monitor for unusual user activity involving USB device interactions on sensitive systems
• Review audit logs for local users attempting to interact with media device drivers

How to Mitigate CVE-2023-28328

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the fix for this vulnerability
• Apply vendor-specific patches from Red Hat or Debian as applicable to your distribution
• If the az6027 driver is not required, consider blacklisting the module to prevent automatic loading
• Restrict local system access to trusted users only until patches can be applied

Patch Information

Patches for this vulnerability have been released by multiple Linux distributions. Organizations should prioritize applying updates based on their specific deployment:

• Red Hat Enterprise Linux: Refer to the Red Hat Bug Report #2177389 for patch details and affected package versions
• Debian: Security updates are available as documented in the Debian LTS Advisory May 2023

Consult your distribution's security advisory channels for the specific kernel package versions that contain the fix.

Workarounds

• Blacklist the az6027 kernel module by adding blacklist az6027 to /etc/modprobe.d/blacklist.conf if the driver is not needed
• Unload the az6027 module if currently loaded using modprobe -r az6027 on systems where the hardware is not in use
• Restrict physical access to USB ports on critical systems to prevent unauthorized device connections
• Implement strict access controls to limit which users can interact with USB device drivers

# Blacklist the vulnerable az6027 driver module
echo "blacklist az6027" | sudo tee /etc/modprobe.d/blacklist-az6027.conf

# Unload the module if currently loaded
sudo modprobe -r az6027

# Verify the module is not loaded
lsmod | grep az6027