CVE-2025-37891: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-37891 Overview

A buffer overflow vulnerability has been discovered in the Linux kernel's ALSA (Advanced Linux Sound Architecture) subsystem within the UMP (Universal MIDI Packet) SysEx message conversion functionality. The vulnerability exists in the MIDI 1.0 to UMP packet conversion function, which contains an undersized internal buffer that can be overflowed when processing SysEx messages.

The conversion function was designed with an internal buffer size of 4 bytes, based on the assumption that this would be the maximum size for MIDI1 UMP packet data. However, the implementation failed to account for SysEx messages, which are handled differently and can contain up to 6 bytes as processed in the do_convert_to_ump() function. This oversight leads to a buffer overflow condition when longer SysEx messages are received, potentially corrupting kernel memory.

Critical Impact

Local attackers with low privileges can trigger a buffer overflow in the kernel's ALSA UMP subsystem, potentially leading to memory corruption, privilege escalation, or system instability.

Affected Products

• Linux Kernel versions prior to the security patches
• Linux Kernel 6.15-rc1 through 6.15-rc4
• Systems using ALSA with UMP/MIDI functionality

Discovery Timeline

• 2025-05-19 - CVE CVE-2025-37891 published to NVD
• 2025-11-17 - Last updated in NVD database

Technical Details for CVE-2025-37891

Vulnerability Analysis

This vulnerability represents a classic buffer overflow condition within the Linux kernel's sound subsystem. The root cause stems from an incorrect buffer size allocation in the MIDI to UMP conversion pathway. When the kernel processes incoming MIDI 1.0 data and converts it to the Universal MIDI Packet format, it utilizes an internal temporary buffer. The original implementation sized this buffer at 4 bytes, which is adequate for standard MIDI1 UMP packet data.

However, SysEx (System Exclusive) messages follow a different format specification within the UMP protocol. These messages can carry up to 6 bytes of data per packet, as evidenced in the do_convert_to_ump() function implementation. When a SysEx message exceeding 4 bytes is received, the conversion function writes beyond the allocated buffer boundary, resulting in a heap or stack buffer overflow depending on the buffer's memory allocation context.

The vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input), highlighting the fundamental input validation failure in the conversion logic.

Root Cause

The vulnerability originates from an undersized buffer allocation in the ALSA UMP subsystem. The developers assumed the maximum MIDI1 UMP packet data size would be 4 bytes but overlooked that SysEx messages within the UMP specification can contain up to 6 bytes. This design oversight in the do_convert_to_ump() function means that when longer SysEx data is processed, the write operations exceed the buffer boundary, corrupting adjacent memory regions.

Attack Vector

Exploitation of this vulnerability requires local access to the system with low privileges. An attacker would need the ability to send crafted MIDI SysEx messages to a system running the vulnerable kernel code. The attack does not require user interaction and could be triggered through:

• Connecting a malicious MIDI device that sends specially crafted SysEx messages
• Using software MIDI applications to generate oversized SysEx packets
• Exploiting applications that process untrusted MIDI files or streams

The memory corruption resulting from the buffer overflow could potentially be leveraged to achieve arbitrary code execution in kernel context, leading to complete system compromise. Successful exploitation could result in confidentiality, integrity, and availability impacts to the affected system.

Detection Methods for CVE-2025-37891

Indicators of Compromise

• Kernel panic or system crashes related to ALSA or sound subsystem components
• Unexpected memory corruption errors in kernel logs referencing UMP or MIDI functions
• Anomalous MIDI device activity or unexplained SysEx message processing
• Stack traces in dmesg pointing to do_convert_to_ump() or related ALSA functions

Detection Strategies

• Monitor kernel logs for ALSA subsystem crashes or memory corruption warnings
• Implement kernel-level monitoring for buffer overflow conditions in the sound subsystem
• Deploy endpoint detection and response (EDR) solutions capable of detecting kernel-level anomalies
• Use SentinelOne's Singularity Platform to detect suspicious kernel behavior patterns

Monitoring Recommendations

• Enable KASAN (Kernel Address Sanitizer) in development environments to detect buffer overflows
• Monitor for unusual MIDI device connections or activity on systems where MIDI is not expected
• Implement logging for ALSA subsystem events and audit MIDI-related kernel module activity
• Review system logs for signs of exploitation attempts targeting the sound subsystem

How to Mitigate CVE-2025-37891

Immediate Actions Required

• Apply the latest kernel security patches that address this buffer overflow vulnerability
• Prioritize patching for systems that handle MIDI data from untrusted sources
• Consider temporarily disabling the ALSA UMP/MIDI functionality if not required and patching is delayed
• Review and restrict physical access to MIDI ports on affected systems

Patch Information

The Linux kernel maintainers have released security patches to address this vulnerability. The fix extends the internal buffer size from 4 bytes to 6 bytes to properly accommodate SysEx UMP messages. Multiple patch commits are available for different kernel branches:

• Kernel Git Commit 226beac5605a
• Kernel Git Commit 42ef48dd4ebb
• Kernel Git Commit 56f1f30e6795
• Kernel Git Commit ce4f77bef276

System administrators should update to the latest stable kernel version that includes these patches.

Workarounds

• Disable or blacklist the snd-ump kernel module if UMP/MIDI functionality is not required
• Restrict access to MIDI devices using udev rules to limit which users can interact with MIDI hardware
• Implement network segmentation to isolate systems processing MIDI data from untrusted sources
• Monitor for exploitation attempts while awaiting patch deployment

# Temporarily blacklist the UMP module if not needed
echo "blacklist snd_ump" | sudo tee /etc/modprobe.d/disable-ump.conf
sudo update-initramfs -u

# Verify the module is not loaded
lsmod | grep snd_ump