CVE-2025-39757: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-39757 Overview

CVE-2025-39757 is an out-of-bounds read vulnerability [CWE-125] in the Linux kernel's ALSA (Advanced Linux Sound Architecture) USB audio driver. The flaw resides in the handling of USB Audio Class 3 (UAC3) cluster segment descriptors, where the driver failed to validate that descriptor sizes matched their declared lengths and fit within allocated buffers. A malicious or malformed USB device firmware can supply crafted descriptors that trigger unexpected out-of-bounds memory accesses in kernel space.

The issue affects multiple Linux kernel branches and downstream distributions including Debian. Siemens has also published a security advisory covering affected products.

Critical Impact
A crafted USB audio device with malicious firmware can trigger kernel out-of-bounds memory access, leading to information disclosure or denial of service on affected systems.

Affected Products

Linux Kernel (multiple stable branches, including 6.17-rc1)
Debian Linux 11.0
Siemens products referenced in advisory SSA-032379

Discovery Timeline

2025-09-11 - CVE-2025-39757 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-39757

Vulnerability Analysis

The vulnerability exists in the ALSA usb-audio driver's parsing of UAC3 class-specific cluster descriptors. UAC3 (USB Audio Class 3.0) defines cluster segment descriptors that describe channel groupings within an audio function. The driver enumerates these segments based on lengths supplied by the connected device.

Before the fix, the driver did not verify two invariants: whether individual segment sizes matched their declared lengths, and whether the cumulative segment data fit within the allocated buffer. When a USB device supplies descriptors with inflated or inconsistent length fields, the parser reads beyond the boundaries of the allocated buffer, triggering an out-of-bounds read in kernel context.

Exploitation requires physical or logical access to a USB port on the target machine. The vulnerability is triggered during device enumeration when the kernel parses the descriptor tables presented by the plugged-in device.

Root Cause

The root cause is missing input validation on device-supplied descriptor data. The usb-audio driver trusted the length fields advertised by the USB device without cross-checking them against the actual size of the parent descriptor buffer. This falls under [CWE-125] Out-of-Bounds Read, where the parser advances a pointer past buffer boundaries because size constraints are not enforced.

Attack Vector

An attacker requires local access to attach a malicious USB device or reprogram existing device firmware. Once the crafted device is enumerated by the kernel, malformed UAC3 cluster segment descriptors are parsed, causing out-of-bounds memory reads. The impact includes potential kernel memory disclosure and system instability leading to denial of service. The fix, distributed across multiple stable kernel commits, adds size and buffer-boundary checks before dereferencing segment data. See the Linux Kernel patch commit ae17b3b for the reference implementation.

Detection Methods for CVE-2025-39757

Indicators of Compromise

Unexpected kernel oops, panics, or KASAN reports referencing snd-usb-audio or UAC3 descriptor parsing functions
Unusual USB device enumeration events in dmesg immediately preceding kernel faults
Newly attached USB audio-class devices from unknown vendors, particularly those declaring UAC3 (bcdADC 3.0) support

Detection Strategies

Enable Kernel Address Sanitizer (KASAN) on test and development kernels to surface out-of-bounds reads during USB descriptor parsing
Monitor journalctl -k and /var/log/kern.log for ALSA and USB subsystem errors coinciding with device insertion
Deploy audit rules that log USB device connect and disconnect events, correlating device VID/PID with kernel warnings

Monitoring Recommendations

Correlate USB hotplug events with kernel subsystem errors using centralized log ingestion
Track kernel version inventory across the fleet to identify hosts still running unpatched builds
Alert on repeated USB enumeration failures from the same device signature, which may indicate probing for descriptor-parser bugs

How to Mitigate CVE-2025-39757

Immediate Actions Required

Apply the vendor-supplied kernel updates from your Linux distribution as soon as they are available
Restrict physical access to USB ports on servers, kiosks, and shared workstations
Deploy USB device control policies to allowlist only known-good vendor and product IDs
Inventory Siemens products against advisory SSA-032379 and apply firmware updates where indicated

Patch Information

The Linux kernel maintainers released fixes across multiple stable branches. Reference commits include 1034719, 275e375, 47ab3d8, 786571b, 799c06a, 7ef3fd2, ae17b3b, dfdcbcd, and ecfd411. Debian users should apply updates from DLA #7 and DLA #8.

Workarounds

Blacklist the snd-usb-audio module on systems that do not require USB audio functionality
Enforce USB port lockdown through BIOS/UEFI settings or endpoint policy tools
Use USBGuard or similar authorization frameworks to require explicit approval before new USB devices are activated

bash

# Blacklist the snd-usb-audio module until patches are applied
echo "blacklist snd-usb-audio" | sudo tee /etc/modprobe.d/blacklist-snd-usb-audio.conf
sudo update-initramfs -u

# Alternatively, enforce USB device authorization via USBGuard
sudo apt install usbguard
sudo usbguard generate-policy > /etc/usbguard/rules.conf
sudo systemctl enable --now usbguard