CVE-2026-90220 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ALSA: seq: Don't leak the extension cell pointer in the bounce payload
The bounce_error_event() embeds the failed event in the bounce payload by pointing data.ext.ptr at it. When that event is a queued variable-length event, its own data.ext.ptr holds the address of its first extension cell, put there by snd_seq_event_dup(). The payload goes out verbatim through snd_seq_expand_var_event(), so the address reaches userspace.
That is the same address commit 705dd6dcbc0e ("ALSA: seq: Clear variable event pointer on read") removed from the event header. The read path still clears it there, just above the call that expands the payload.
Embed a sanitised copy instead, treated exactly as snd_seq_read() treats the header. A stack copy is enough because delivery is synchronous and snd_seq_event_dup() copies before returning.
An unprivileged client reaches this by setting SNDRV_SEQ_FILTER_BOUNCE, queueing a variable-length event to a port that does not exist and reading the bounce back. Eight bytes on 64-bit, from its own pool.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel's Advanced Linux Sound Architecture (ALSA) subsystem contains an information disclosure vulnerability within the sequencer component, specifically in the handling of bounced error events for variable-length sequences. This flaw arises from a failure to properly sanitize internal kernel pointers before they are exposed to user-space applications. The core issue is located in the bounce_error_event function, which constructs a payload containing details about a failed event delivery attempt. When an application queues a variable-length event to a non-existent port, the system generates this error event and attempts to return it to the sender via a bounce mechanism. During this process, the code incorrectly embeds a pointer to internal kernel data structures directly into the extension cell of the payload without stripping or replacing sensitive memory addresses.
The technical root cause involves the interaction between snd_seq_event_dup and snd_seq_expand_var_event functions. When an event is duplicated for processing, its data.ext.ptr field is populated with the address of its first extension cell within the kernel's memory space. The vulnerability occurs because this raw pointer value is passed verbatim through the expansion routine into the final payload sent to userspace. This behavior mirrors a previously identified issue addressed in commit 705dd6dcbc0e, which cleared variable event pointers from the main event header during read operations but failed to apply similar sanitization logic to the bounce error path. Consequently, an unprivileged user can trigger this code path by setting the SNDRV_SEQ_FILTER_BOUNCE flag and queuing a malformed or invalid event, thereby causing the kernel to leak eight bytes of memory address information on 64-bit systems back into their application space.
From a security impact perspective, while the leaked data originates from the attacker's own process pool rather than arbitrary sensitive kernel memory, it still constitutes an unauthorized disclosure of internal implementation details and can aid in further exploitation attempts such as heap layout fingerprinting or bypassing address space layout randomization protections if combined with other vulnerabilities. This vulnerability aligns with CWE-200: Information Exposure, where the system inadvertently reveals security-relevant information to potential adversaries. In terms of offensive security frameworks like MITRE ATT&CK, this behavior corresponds to T1608: Link Exploitation or more broadly to techniques involving memory disclosure that facilitate subsequent privilege escalation or code execution attacks by providing attackers with critical knowledge about the target environment's memory layout and kernel structure offsets.
The resolution involves modifying the bounce_error_event function to embed a sanitized copy of the event data rather than passing raw pointers directly into the payload. This approach mirrors how snd_seq_read handles header sanitization, ensuring that any extension cell references are neutralized before transmission. By treating the bounced error event with the same level of scrutiny as standard read operations, the kernel prevents the leakage of internal memory addresses to unprivileged userspace processes. To mitigate this risk in environments where immediate patching is not feasible, administrators should restrict access to ALSA sequencer devices using appropriate file system permissions and ensure that only trusted applications can interact with these interfaces. Additionally, deploying security modules such as SELinux or AppArmor can help limit the potential impact of any successful exploitation by enforcing strict confinement policies on processes attempting to utilize advanced sequencing features.