CVE-2026-53241: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-53241 Overview

CVE-2026-53241 is a stack out-of-bounds read vulnerability in the Linux kernel's Advanced Linux Sound Architecture (ALSA) sequencer dummy client. The flaw resides in the event forwarding path of the dummy sequencer port, which copies incoming struct snd_seq_event objects into legacy-sized stack storage. When a Universal MIDI Packet (UMP) event reaches the dummy client, the subscriber delivery path reads a UMP-sized packet from the smaller legacy stack buffer, resulting in an over-read past the end of the temporary.

Critical Impact
The vulnerability enables kernel stack memory disclosure through ALSA sequencer subsystem interactions, potentially exposing adjacent stack contents to local actors capable of routing UMP events through the dummy client.

Affected Products

Linux kernel versions containing the ALSA sequencer dummy client with UMP event support
Stable kernel branches referenced by commits 2b5ff4db, 6671a461, 6676b606, and a7ef78a2
Distributions shipping affected upstream kernel versions

Discovery Timeline

2026-06-25 - CVE-2026-53241 published to NVD
2026-06-25 - Last updated in NVD database

Technical Details for CVE-2026-53241

Vulnerability Analysis

The ALSA sequencer dummy client (snd-seq-dummy) provides a loopback port that forwards MIDI events between subscribers. To do this, the dummy port copies each incoming struct snd_seq_event onto a stack-local temporary, rewrites the source and destination routing fields, and dispatches the rewritten copy to all subscribers.

The legacy struct snd_seq_event storage is smaller than struct snd_seq_ump_event. UMP events carry larger packet payloads, and the kernel tracks this through a UMP flag set on the event header. When a UMP event reaches the dummy client, the copy operation preserves the UMP flag but stores the event in legacy-sized stack memory.

Downstream, the subscriber delivery code calls snd_seq_event_packet_size() to determine how many bytes to forward. Because the UMP flag remains set, the function returns the UMP packet size. The delivery path then reads beyond the end of the legacy-sized stack temporary, leaking adjacent kernel stack contents into the forwarded event stream.

Root Cause

The root cause is a type and size mismatch between the declared stack buffer and the event payload size implied by the UMP flag. The dummy client did not use the union __snd_seq_event storage that accommodates both legacy and UMP variants, and it did not capture the correct packet size before rewriting the routing fields.

The upstream fix uses the existing union __snd_seq_event storage and records the packet size reported for the incoming event before mutating common routing fields. This preserves the full UMP packet for UMP events while keeping legacy event handling unchanged. The patches are tracked in commits 2b5ff4db, 6671a461, 6676b606, and a7ef78a2.

Attack Vector

A local user with access to the ALSA sequencer interface (/dev/snd/seq) can subscribe to the dummy client and inject UMP events that traverse the vulnerable forwarding path. No specialized privileges are required beyond standard audio device access. The exploit primitive is an information disclosure of kernel stack memory rather than memory corruption, but the leaked data can include return addresses, function pointers, or sensitive values useful for defeating Kernel Address Space Layout Randomization (KASLR).

Detection Methods for CVE-2026-53241

Indicators of Compromise

Unexpected loading of the snd-seq-dummy kernel module on systems that do not require MIDI sequencing
Unprivileged processes opening /dev/snd/seq and creating dummy client subscriptions on servers or workstations without audio workloads
Anomalous UMP event traffic patterns originating from local processes that do not interact with audio peripherals

Detection Strategies

Monitor kernel module load events for snd_seq_dummy and correlate against expected system roles
Audit process access to ALSA sequencer device nodes using Linux Audit (auditd) rules on /dev/snd/seq
Track ioctl() calls against ALSA sequencer file descriptors that create or subscribe dummy client ports

Monitoring Recommendations

Enable kernel lockdown or module signing to detect unauthorized loading of audio modules in server environments
Forward auditd and kernel ring buffer (dmesg) telemetry to a centralized log platform for retrospective analysis
Alert on kernel patch level drift across the fleet to ensure all hosts receive the stable-tree fix

How to Mitigate CVE-2026-53241

Immediate Actions Required

Apply the stable kernel updates that include commits 2b5ff4db, 6671a461, 6676b606, or a7ef78a2 from your distribution vendor
Inventory hosts running kernels with the snd-seq-dummy module loaded and prioritize them for patching
Restrict access to /dev/snd/seq on systems that do not require user-space MIDI sequencing

Patch Information

The upstream fix modifies the dummy sequencer forwarding routine to use the union __snd_seq_event storage type, which is sized to accommodate both legacy and UMP event variants. The packet size is captured from the incoming event before routing fields are rewritten, ensuring the subsequent copy operation reads only valid bytes. Patches are available in the mainline and stable trees through the kernel.org commits listed in the references.

Workarounds

Blacklist the snd-seq-dummy module on systems where it is not required using /etc/modprobe.d/ configuration
Remove the module from the running kernel with modprobe -r snd_seq_dummy where possible
Tighten permissions on /dev/snd/seq to limit access to trusted audio group members only

bash

# Configuration example
# Prevent snd-seq-dummy from loading on systems that do not require it
echo "blacklist snd_seq_dummy" | sudo tee /etc/modprobe.d/blacklist-snd-seq-dummy.conf
sudo modprobe -r snd_seq_dummy