CVE-2026-58091 Overview
CVE-2026-58091 is a use-after-free vulnerability [CWE-416] in the FreeBSD sound subsystem. The flaw resides in the ioctl implementation that acquires locks on all channels within a sync group. When locking a channel would block, the implementation releases the sync group list lock and sleeps. Upon reawakening, the sync group structure may already be freed, and the code does not handle this condition. An unprivileged local user on a system with multiple audio devices can exploit this race to escalate privileges. FreeBSD published advisory SA-26:58 addressing the issue.
Critical Impact
Local unprivileged users can achieve kernel-level privilege escalation on FreeBSD systems with multiple audio devices through a use-after-free race in the sound sync group ioctl handler.
Affected Products
- FreeBSD operating system (sound subsystem)
- Systems with multiple audio devices exposing sync group ioctls
- Refer to FreeBSD Security Advisory SA-26:58 for version-specific details
Discovery Timeline
- 2026-08-26 - CVE-2026-58091 published to NVD
- 2026-08-27 - Last updated in NVD database
Technical Details for CVE-2026-58091
Vulnerability Analysis
The vulnerability exists in the FreeBSD kernel sound driver, specifically in the ioctl path that manages audio channel sync groups. Sync groups coordinate multiple audio channels so they start and stop together. The ioctl handler must lock every channel in the group before performing operations.
When the handler encounters a channel whose lock cannot be acquired without blocking, it drops the sync group list lock and sleeps to wait. During this sleep window, another thread can free the sync group structure. When the original thread wakes, it resumes operating on the now-freed memory. This dangling reference produces classic use-after-free behavior in kernel context.
The issue is classified as CWE-416 (Use After Free). Exploitation grants an attacker control over freed kernel memory contents, which can be shaped through heap grooming and reallocation primitives.
Root Cause
The root cause is missing validation of the sync group structure lifetime after re-acquiring the list lock. The code assumes the structure remains valid across the sleep boundary. No reference counting or re-lookup step confirms the object still exists before dereferencing it.
Attack Vector
Exploitation requires local access with an unprivileged user account and a system exposing multiple audio devices. The attacker triggers concurrent ioctl operations against sync groups from multiple threads or processes. One thread forces the blocking path, while another thread destroys the sync group. When the sleeping thread resumes, it operates on freed memory. By reallocating that memory with attacker-controlled content, the attacker can corrupt kernel state and escalate privileges to root.
No verified public exploit code is available. The vulnerability mechanism is described in FreeBSD Security Advisory SA-26:58.
Detection Methods for CVE-2026-58091
Indicators of Compromise
- Unexpected kernel panics or page faults referencing the sound driver (snd_* symbols) in /var/log/messages or crash dumps
- Unprivileged processes issuing high-frequency ioctl calls against /dev/dsp* or /dev/mixer* devices
- New root-owned processes spawned from user contexts that previously accessed audio devices
Detection Strategies
- Audit kernel crash dumps for use-after-free signatures in the sound subsystem stack traces
- Monitor ktrace or dtrace output for anomalous ioctl call patterns targeting sound device sync group operations
- Correlate audio device ioctl activity with subsequent privilege changes or process creation events
Monitoring Recommendations
- Enable kernel crash dump collection and forward dumps to a centralized analysis pipeline
- Track access to /dev/dsp*, /dev/mixer*, and related sound pseudo-devices by non-audio applications
- Alert on repeated failed ioctl operations from a single unprivileged user against audio devices
How to Mitigate CVE-2026-58091
Immediate Actions Required
- Apply the patches referenced in FreeBSD Security Advisory SA-26:58 and reboot affected systems
- Inventory FreeBSD hosts that expose multiple audio devices to unprivileged users and prioritize them for patching
- Restrict shell access on multi-user FreeBSD systems until patches are deployed
Patch Information
FreeBSD has released a security patch as documented in advisory SA-26:58. Administrators should follow the standard freebsd-update procedure or rebuild the kernel from source using the corrected sources. See the FreeBSD Security Advisory SA-26:58 for exact patch commits and version applicability.
Workarounds
- Restrict permissions on /dev/dsp* and /dev/mixer* devices so only trusted users can open them
- Remove or disable the sound kernel module on servers that do not require audio functionality
- Limit interactive local access on affected systems until the patch is applied
# Apply FreeBSD security patches
freebsd-update fetch
freebsd-update install
shutdown -r now
# Optional: restrict sound device access
chmod 0600 /dev/dsp* /dev/mixer*
# Optional: unload the sound module if audio is not required
kldunload snd_driver
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
