CVE-2025-38249: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-38249 Overview

CVE-2025-38249 is an out-of-bounds read vulnerability [CWE-125] in the Linux kernel's ALSA USB audio driver. The flaw exists in the snd_usb_get_audioformat_uac3() function within the USB Audio Class 3 (UAC3) handling code. The function passes the length value returned by snd_usb_ctl_msg() directly to memory allocation without validating that the buffer is large enough for a uac3_cluster_header_descriptor. A malicious or malformed USB device can return a shorter-than-expected length, causing the driver to read past the end of the allocated buffer.

Critical Impact
A local attacker with the ability to attach a crafted USB audio device can trigger kernel memory disclosure or a kernel crash, resulting in denial of service on affected Linux systems.

Affected Products

Linux kernel (multiple stable branches, including 6.16-rc1, 6.16-rc2, and 6.16-rc3)
Debian Linux 11.0 (addressed via Debian LTS advisories)
Any Linux distribution shipping the vulnerable ALSA usb-audio driver

Discovery Timeline

2025-07-09 - CVE-2025-38249 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-38249

Vulnerability Analysis

The vulnerability resides in the Linux kernel's Advanced Linux Sound Architecture (ALSA) USB audio driver, specifically in the UAC3 cluster descriptor parsing path. When the kernel enumerates a USB audio device, snd_usb_get_audioformat_uac3() issues a control message via snd_usb_ctl_msg() to retrieve cluster header information from the device. The returned length value is USB device-controlled and is used to allocate a buffer without any lower-bound validation.

The allocated buffer is then cast to a uac3_cluster_header_descriptor structure, and its fields are dereferenced directly. If the device returns a length smaller than sizeof(struct uac3_cluster_header_descriptor), the field access reads memory beyond the allocation boundary. Because ALSA USB audio device probing occurs automatically on device attachment, this parsing runs in kernel context without user interaction beyond plugging in the device.

Root Cause

The root cause is missing input validation on attacker-controlled data crossing the USB trust boundary. The driver trusts the device-reported descriptor length and skips the size check required before treating the buffer as a fixed-layout structure. The upstream fix adds an explicit length check to ensure the returned buffer is at least as large as uac3_cluster_header_descriptor before any field is accessed.

Attack Vector

Exploitation requires local access, either physical USB access or an already-compromised low-privilege account capable of instantiating a USB gadget. The attacker presents a crafted USB device that responds to the UAC3 cluster header request with a short length value. When the kernel probes the device, the out-of-bounds read is triggered, potentially disclosing adjacent kernel heap memory or causing a kernel oops. The impact is limited to confidentiality of kernel memory and availability of the host; the flaw does not directly enable code execution or integrity loss. Verified patch commits are published in the mainline and stable trees, including Kernel Git Commit 0ee87c2 and Kernel Git Commit fb4e2a6.

Detection Methods for CVE-2025-38249

Indicators of Compromise

Kernel log entries from the snd-usb-audio module referencing UAC3 parsing errors, unexpected descriptor lengths, or KASAN out-of-bounds read reports
Unexpected kernel oops, panics, or dmesg traces originating in snd_usb_get_audioformat_uac3 after a USB device is attached
Repeated USB enumeration events from the same device VID/PID that trigger audio driver faults

Detection Strategies

Enable KASAN or KFENCE on test systems to catch the out-of-bounds read at runtime during USB device probing
Monitor /var/log/kern.log and journalctl -k for warnings from the ALSA USB audio subsystem tied to descriptor validation
Correlate USB device attachment events (via udev or auditdUSB_* events) with subsequent kernel faults

Monitoring Recommendations

Ingest kernel and USB subsystem logs into a centralized logging platform and alert on ALSA driver stack traces
Track kernel package versions across the fleet to identify hosts still running vulnerable builds
Alert on unauthorized USB device connections on servers and workstations that do not require external audio hardware

How to Mitigate CVE-2025-38249

Immediate Actions Required

Update the Linux kernel to a version that includes the upstream fix on all affected stable branches
Apply the Debian security update referenced in the Debian LTS Announcement #00007 and Debian LTS Announcement #00008
Restrict physical USB access to servers and critical endpoints where audio functionality is not required
Inventory systems running the snd-usb-audio driver and prioritize patching hosts with exposed USB ports

Patch Information

The fix adds a length check in snd_usb_get_audioformat_uac3() to verify that the buffer returned by snd_usb_ctl_msg() is large enough to contain a uac3_cluster_header_descriptor before any field is dereferenced. Patch commits are available across stable branches, including Kernel Git Commit 24ff7d4, Kernel Git Commit 2dc1c3e, Kernel Git Commit 6eb2117, Kernel Git Commit 74fcb38, Kernel Git Commit c3fb926, and Kernel Git Commit 11e740d.

Workarounds

Blacklist the snd-usb-audio kernel module on systems that do not require USB audio devices
Enforce USB device allow-lists through udev rules or USBGuard to block unknown audio-class devices
Disable USB ports at the firmware or physical level on high-value servers where USB peripherals are not needed

bash

# Blacklist the vulnerable driver until the kernel is patched
echo "blacklist snd-usb-audio" | sudo tee /etc/modprobe.d/blacklist-snd-usb-audio.conf
sudo update-initramfs -u
sudo reboot

# Verify current kernel version after patching
uname -r