CVE-2025-38086: Linux Kernel Use-After-Free Vulnerability

CVE-2025-38086 Overview

CVE-2025-38086 is an uninitialized memory access vulnerability in the Linux kernel's ch9200 USB Ethernet driver. The vulnerability exists in the ch9200_mdio_read() function, where a local buffer called "buff" may remain uninitialized when control_read() fails to meet a specific condition. When this occurs, the uninitialized buffer contents are accessed and returned, potentially leading to information disclosure or system instability.

Critical Impact

Local attackers with low privileges can trigger a denial of service condition by exploiting the uninitialized memory access in the ch9200 network driver during MII network interface restart operations.

Affected Products

• Linux Kernel versions 4.3 and later (multiple stable branches affected)
• Debian Linux 11.0
• Systems using CH9200 USB Ethernet adapters

Discovery Timeline

• June 28, 2025 - CVE-2025-38086 published to NVD
• December 17, 2025 - Last updated in NVD database

Technical Details for CVE-2025-38086

Vulnerability Analysis

This vulnerability stems from improper error handling in the ch9200 USB Ethernet driver's MDIO read function. When mii_nway_restart() is called to restart the network interface autonegotiation, it invokes ch9200_mdio_read() to read MII register values. Inside ch9200_mdio_read(), a local buffer named "buff" is intended to be populated by control_read(). However, control_read() only initializes this buffer under a specific condition—when the error value equals the expected size.

The problematic code flow occurs because ch9200_mdio_read() ignores the return value from control_read(). When the condition err == size is not satisfied within control_read(), the memcpy() that would populate the buffer is skipped, leaving "buff" containing stack garbage. Despite this failure state, the function proceeds to return the uninitialized buffer contents via buff[0] | buff[1] << 8, exposing stack memory.

This pattern represents a classic case of missing return value validation, which is particularly dangerous in kernel drivers where uninitialized memory access can lead to unpredictable behavior, kernel crashes, or potential information leakage from kernel stack memory.

Root Cause

The root cause is the absence of return value checking for control_read() in ch9200_mdio_read(). The control_read() function conditionally initializes its output buffer only when the USB control transfer succeeds with the expected data size. Without validating this return value, the calling function has no way to know whether the buffer was properly initialized before accessing its contents.

Attack Vector

An attacker with local access and low privileges can potentially trigger this vulnerability by:

1. Connecting a CH9200-based USB Ethernet adapter to the system
2. Triggering a network interface restart operation that invokes mii_nway_restart()
3. Creating conditions where the USB control transfer fails or returns an unexpected size
4. The uninitialized buffer access occurs, potentially causing a kernel panic or leaking stack information

The attack requires local access as indicated by the local attack vector classification, and the primary impact is availability degradation through denial of service.

Detection Methods for CVE-2025-38086

Indicators of Compromise

• Unexpected kernel panics or oops messages referencing ch9200_mdio_read or related MII functions
• System crashes occurring during network interface operations on CH9200 USB Ethernet devices
• Kernel log entries showing uninitialized memory warnings in the ch9200 driver

Detection Strategies

• Monitor kernel logs for oops, panics, or warnings originating from the drivers/net/usb/ch9200.c driver
• Use kernel address sanitizer (KASAN) or memory debugging tools to detect uninitialized memory access patterns
• Implement endpoint detection rules that flag repeated kernel crashes tied to USB network driver activity

Monitoring Recommendations

• Enable kernel crash dump collection to capture diagnostic information for post-incident analysis
• Monitor for unusual USB device connection patterns, particularly CH9200 chipset-based adapters
• Deploy SentinelOne Singularity Platform to detect anomalous kernel behavior and potential exploitation attempts

How to Mitigate CVE-2025-38086

Immediate Actions Required

• Apply the official kernel patches from the Linux kernel stable branches immediately
• If patching is not immediately possible, consider disabling or blacklisting the ch9200 kernel module
• Review systems for CH9200 USB Ethernet adapter usage and prioritize patching those systems

Patch Information

The fix involves checking the return value of control_read() and returning early with an error if the control transfer fails. This prevents the uninitialized buffer from being accessed. Official patches have been committed to multiple kernel stable branches:

• Kernel Git Commit 119766d
• Kernel Git Commit 33163c6
• Kernel Git Commit 4da7fcc
• Kernel Git Commit 6bd2569
• Kernel Git Commit 9a350f3
• Kernel Git Commit cdaa6d1

Debian users should refer to the Debian LTS Announcements for distribution-specific patched packages.

Workarounds

• Blacklist the ch9200 kernel module if the CH9200 USB Ethernet adapter is not required on the system
• Use alternative USB Ethernet adapters with different chipsets until patching is completed
• Restrict physical access to systems to prevent unauthorized USB device connections

# Blacklist the ch9200 module as a temporary workaround
echo "blacklist ch9200" | sudo tee /etc/modprobe.d/blacklist-ch9200.conf
sudo update-initramfs -u
# Verify the module is blacklisted after reboot
lsmod | grep ch9200