CVE-2026-56109: ALSA Library Use-After-Free Vulnerability

CVE-2026-56109 Overview

CVE-2026-56109 is a double-free vulnerability [CWE-415] in the Advanced Linux Sound Architecture (ALSA) library before version 1.2.16.1. The flaw resides in the parse_def() function within src/conf.c. When the parser processes nested compound or array configuration blocks, it fails to validate the return value of get_nonwhite() before continuing execution. This causes snd_config_delete() to be invoked twice on the same already-freed node. Attackers supplying maliciously crafted ALSA configuration text can corrupt memory, trigger NULL-pointer writes, or cause invalid memory reads.

Critical Impact
A local attacker can corrupt memory in any process linked against alsa-lib by supplying a malicious configuration file, leading to denial of service and potential undefined behavior.

Affected Products

ALSA library (alsa-lib) versions prior to 1.2.16.1
Linux distributions bundling vulnerable alsa-lib releases
Applications and audio frameworks linked against affected alsa-lib versions

Discovery Timeline

2026-06-22 - CVE-2026-56109 published to NVD
2026-06-23 - Last updated in NVD database

Technical Details for CVE-2026-56109

Vulnerability Analysis

The parse_def() function in src/conf.c is responsible for parsing definition blocks within ALSA configuration text. ALSA configuration syntax allows nested compound blocks delimited by {} and array blocks delimited by []. The parser tracks the expected closing character through the endchr variable while traversing nested structures.

During traversal, parse_def() calls get_nonwhite() to read the next non-whitespace character. The function fails to check whether get_nonwhite() returned a negative error code before proceeding to compare the result against endchr. When the helper returns an error, the parser continues into the cleanup branch that invokes snd_config_delete(n) on the partially constructed node n. Subsequent error handling in the calling context invokes snd_config_delete() a second time on the same node, producing a double-free condition.

Exploitation requires the victim process to load attacker-controlled ALSA configuration text. Local users, malicious configuration files placed in user directories, or compromised package payloads can deliver the trigger.

Root Cause

The root cause is a missing return value check in parse_def(). The function treats the result of get_nonwhite() as a valid character without distinguishing the negative error sentinel. This allows the cleanup path and the caller's cleanup path to both free the same snd_config_t node referenced by n.

Attack Vector

The attack vector is local. An attacker supplies crafted ALSA configuration text — through a user-controlled ~/.asoundrc, an embedded configuration string, or any input path that reaches snd_config_load*() routines. Parsing the malicious nested block triggers the double snd_config_delete() call, resulting in memory corruption within the host process.

// Security patch - src/conf.c parse_def()
// conf: add missing return value check in parse_def()
 			endchr = ']';
 		}
 		c = get_nonwhite(input);
+		if (c < 0) {
+			err = c;
+			goto __end;
+		}
 		if (c != endchr) {
 			if (n)
 				snd_config_delete(n);

Source: GitHub Commit 536dd6f8

The patch adds an explicit if (c < 0) check after get_nonwhite() and jumps to the __end label with the error code, preventing the second snd_config_delete() invocation on an already-freed node.

Detection Methods for CVE-2026-56109

Indicators of Compromise

Unexpected crashes or SIGSEGV signals in processes linked against libasound.so when loading configuration files
Presence of alsa-lib packages reporting versions earlier than 1.2.16.1 on Linux endpoints
ALSA configuration files (/etc/asound.conf, ~/.asoundrc) containing deeply nested or malformed compound and array blocks from untrusted sources

Detection Strategies

Inventory installed alsa-lib package versions across Linux endpoints and flag any release prior to 1.2.16.1
Monitor process termination events for audio-handling processes producing double-free or heap corruption diagnostics from glibc (for example, free(): double free detected)
Audit file integrity for system-wide ALSA configuration files and user ~/.asoundrc files for unexpected modifications

Monitoring Recommendations

Forward dmesg, journalctl, and glibc runtime abort messages to a centralized log platform for correlation
Track process crash telemetry for binaries that link libasound.so, including media servers, browsers, and conferencing clients
Enable AddressSanitizer or MALLOC_CHECK_=3 in test and triage environments to surface double-free conditions during validation

How to Mitigate CVE-2026-56109

Immediate Actions Required

Upgrade alsa-lib to version 1.2.16.1 or later on all affected Linux systems
Apply distribution security updates that backport commit 536dd6f8affdf5197c12a63a71c92a70b2833cc0 to maintained alsa-lib branches
Restrict write access to system-wide ALSA configuration paths and audit user-controlled ~/.asoundrc files

Patch Information

The ALSA project released the fix in alsa-lib1.2.16.1. The patch adds a return value check after get_nonwhite() in parse_def() and routes negative results to the unified cleanup label, eliminating the duplicate snd_config_delete() call. See the GitHub Release v1.2.16.1, the upstream commit, and the VulnCheck Advisory for full details.

Workarounds

Remove or replace untrusted ~/.asoundrc and /etc/asound.conf files until packages are upgraded
Run audio-handling processes with reduced privileges and confined file system access to limit blast radius
Disable loading of user-supplied ALSA configuration in multi-tenant systems pending the upgrade

bash

# Verify installed alsa-lib version and upgrade
# Debian / Ubuntu
dpkg -l | grep libasound2
sudo apt update && sudo apt install --only-upgrade libasound2

# Fedora / RHEL
rpm -q alsa-lib
sudo dnf upgrade alsa-lib

# Arch Linux
pacman -Qi alsa-lib
sudo pacman -Syu alsa-lib

# Confirm patched version is 1.2.16.1 or later
strings /usr/lib/x86_64-linux-gnu/libasound.so.2 | grep -i "1\.2\.16"