CVE-2026-58090: Unix Socket Use-After-Free Vulnerability

CVE-2026-58090 Overview

CVE-2026-58090 is a use-after-free vulnerability [CWE-416] in the FreeBSD unix socket implementation. The SOCK_STREAM receive path fails to fully detach control messages from the socket buffer before processing them. Certain error paths free those messages, leaving freed data mbufs referenced in the receive socket buffer. An unprivileged local user can trigger the dangling references to corrupt kernel memory and escalate privileges to root.

The issue affects the FreeBSD kernel and is documented in FreeBSD Security Advisory SA-26:57. The vulnerability requires local access and low privileges, but no user interaction.

Critical Impact
Local unprivileged users can exploit the use-after-free in the unix socket receive path to gain kernel-level code execution and full root privileges.

Affected Products

FreeBSD operating system (unix socket / SOCK_STREAM implementation)
Systems running FreeBSD kernels prior to the SA-26:57 patch
Any FreeBSD-derived distribution that ships the unaffected kernel unix socket code

Discovery Timeline

2026-08-26 - CVE-2026-58090 published to NVD
2026-08-27 - Last updated in NVD database

Technical Details for CVE-2026-58090

Vulnerability Analysis

The vulnerability resides in the receive path for SOCK_STREAM unix domain sockets in the FreeBSD kernel. When a receiver processes ancillary data (control messages) attached to stream data, the kernel begins consuming the control messages from the socket buffer without fully detaching the associated mbuf chain. If a subsequent step along an error path frees those control message mbufs, the receive socket buffer retains pointers to memory that has been returned to the kernel allocator.

The socket buffer now holds freed mbufs that later code paths continue to treat as valid. Reads, teardown routines, or subsequent recvmsg calls dereference these pointers, producing classic use-after-free behavior in kernel context. Because unix sockets are used broadly for inter-process communication, the code path is reachable by any local user with the ability to create a socket pair.

Root Cause

The root cause is incomplete state synchronization between control-message processing and socket buffer bookkeeping. The receive routine advances buffer state as if the control messages were detached, while the underlying mbuf chain remains linked. Error paths that free the messages do not repair the socket buffer, so the buffer references memory it no longer owns [CWE-416].

Attack Vector

An unprivileged local process opens a SOCK_STREAM unix socket pair using socketpair(2). The attacker sends control messages such as SCM_RIGHTS file descriptor passing alongside stream data using sendmsg(2). The receiver invokes recvmsg(2) under conditions that trigger the vulnerable error path, freeing the control-message mbufs while leaving references in the socket buffer. The attacker then races to reallocate the freed memory with controlled content, transforming the dangling reference into kernel memory corruption and, ultimately, privilege escalation.

See the FreeBSD Security Advisory SA-26:57 for authoritative technical details.

Detection Methods for CVE-2026-58090

Indicators of Compromise

Unexplained kernel panics or page faults referencing mbuf, sbappend, or unix socket receive functions in crash dumps.
Sudden appearance of processes running with UID 0 that were spawned by unprivileged users.
Anomalous sendmsg/recvmsg activity from non-privileged processes passing SCM_RIGHTS control messages in tight loops.
New setuid binaries, kernel module loads, or sysctl changes following local shell activity.

Detection Strategies

Monitor kernel crash reports and dmesg output for repeated faults in unix socket receive paths.
Audit process lineage for unprivileged users whose child processes suddenly transition to root without a legitimate setuid invocation.
Correlate execve and setuid telemetry from endpoint agents to flag privilege transitions inconsistent with normal system usage.

Monitoring Recommendations

Enable FreeBSD audit(4) with policies covering AUE_SENDMSG, AUE_RECVMSG, and process credential changes.
Forward kernel and audit logs to a centralized data lake for retention and correlation across hosts.
Alert on kernel core dumps generated on production systems, especially those referencing socket buffer code.

How to Mitigate CVE-2026-58090

Immediate Actions Required

Apply the FreeBSD kernel update referenced in SA-26:57 and reboot affected systems.
Inventory all FreeBSD hosts, including appliances and jails, to identify kernels shipped prior to the fix.
Restrict local shell and interactive access on high-value FreeBSD servers until patches are deployed.
Prioritize multi-tenant hosts, developer workstations, and jail hosts where untrusted local users are present.

Patch Information

FreeBSD has published fixes in FreeBSD Security Advisory SA-26:57. Administrators should update to the patched kernel version identified in the advisory using freebsd-update fetch install on supported branches, or rebuild from the corrected source tree. Reboot is required for the kernel change to take effect.

Workarounds

No official workaround eliminates the vulnerability; patching is the supported remediation.
Reduce exposure by minimizing local user accounts and disabling interactive login on production hosts.
Constrain untrusted workloads inside jails with strict resource limits, recognizing that jails do not fully isolate kernel bugs.
Monitor for kernel crashes and anomalous privilege transitions as a compensating control until the patch is applied.

bash

# Apply FreeBSD binary updates and reboot
sudo freebsd-update fetch
sudo freebsd-update install
sudo shutdown -r now

# Verify running kernel matches the patched version
uname -a
freebsd-version -k