CVE-2026-49425: FreeBSD Kernel Information Disclosure

CVE-2026-49425 Overview

CVE-2026-49425 is an information disclosure vulnerability in the FreeBSD kernel's 32-bit compatibility layer. The compat32kevent() handler translates a 64-bit kevent struct into a 32-bit struct declared on the kernel stack. The handler fails to zero the destination struct before use, leaving residual kernel stack memory in fields that are subsequently copied to userspace. An unprivileged local user can invoke the affected syscall and observe uninitialized kernel stack bytes. The disclosed bytes may contain sensitive kernel data such as pointers, credentials, or fragments of prior syscall state. The issue is tracked as [CWE-908: Use of Uninitialized Resource] and documented in the FreeBSD Security Advisory SA-26:48.

Critical Impact
Unprivileged local users on FreeBSD systems running 32-bit compatibility binaries can leak small amounts of uninitialized kernel stack memory, potentially exposing pointers useful for defeating kernel address space layout randomization (KASLR) or other sensitive data.

Affected Products

FreeBSD operating system (versions supporting compat32kevent())
Systems executing 32-bit binaries on 64-bit FreeBSD kernels
Environments relying on the FreeBSD kqueue/kevent event notification subsystem

Discovery Timeline

2026-08-19 - CVE-2026-49425 published to NVD
2026-08-19 - Last updated in NVD database

Technical Details for CVE-2026-49425

Vulnerability Analysis

The FreeBSD kernel exposes kevent() through the kqueue event notification framework. On 64-bit kernels running 32-bit userland binaries, the compat32 shim translates between the native 64-bit struct kevent and the 32-bit ABI layout expected by the calling process. The translation routine allocates a 32-bit kevent struct on the kernel stack and populates it field-by-field from the 64-bit source. Because the stack allocation is not zeroed before field assignment, any bytes not explicitly written retain data from prior stack frames. When the compat32 handler copies the struct back to userspace, those unwritten bytes leak kernel memory contents. The vulnerability requires only local, unprivileged access and does not depend on any timing race or memory corruption primitive.

Root Cause

The root cause is missing initialization of a stack-declared structure prior to partial population. Structure padding, reserved fields, and any conditionally assigned members are not overwritten before the struct is emitted to userspace, matching the pattern described by [CWE-908: Use of Uninitialized Resource].

Attack Vector

An attacker executes a 32-bit binary on a vulnerable FreeBSD host and issues kevent() calls through the compat32 path. The returned event structures contain residual kernel stack bytes in uninitialized fields. Repeated invocations can be used to harvest additional stack contents. See the FreeBSD Security Advisory SA-26:48 for the authoritative technical description; no verified public proof-of-concept code is available.

Detection Methods for CVE-2026-49425

Indicators of Compromise

Unexpected execution of 32-bit binaries invoking kevent() on 64-bit FreeBSD hosts where such workloads are not part of normal operation.
High-frequency kevent() syscalls originating from unprivileged users or short-lived processes.
Presence of unknown 32-bit ELF binaries in user-writable directories such as /tmp or /var/tmp.

Detection Strategies

Audit kernel version and confirm whether the running FreeBSD build predates the fix referenced in FreeBSD-SA-26:48.
Enable FreeBSD audit(4) with rules targeting AUE_KEVENT events and review invocations from unprivileged principals.
Correlate process execution telemetry with syscall traces to identify processes issuing atypical kevent() patterns via the compat32 path.

Monitoring Recommendations

Ingest FreeBSD audit logs and process telemetry into a centralized analytics platform for baseline deviation analysis.
Alert on execution of 32-bit ELF binaries by users who do not normally require 32-bit compatibility.
Track kernel patch state across the fleet so hosts missing the SA-26:48 update are flagged for remediation.

How to Mitigate CVE-2026-49425

Immediate Actions Required

Apply the kernel update from FreeBSD Security Advisory SA-26:48 and reboot affected hosts.
Inventory hosts with COMPAT_FREEBSD32 enabled in the running kernel configuration and prioritize them for patching.
Restrict shell and execution access on multi-tenant FreeBSD systems until the patch is deployed.

Patch Information

FreeBSD has published a fix in Security Advisory SA-26:48. Administrators should follow the standard FreeBSD update procedure using freebsd-update fetch install on supported releases, or rebuild the kernel from patched sources on custom builds, then reboot to activate the corrected compat32kevent() handler.

Workarounds

Disable 32-bit compatibility by building and running a kernel without the COMPAT_FREEBSD32 option where 32-bit userland support is not required.
Prevent execution of untrusted 32-bit binaries by restricting write and execute permissions on user-writable filesystems using mount options such as noexec.
Limit local shell access to trusted administrators until patched kernels are deployed.

bash

# Apply the FreeBSD security update and reboot
freebsd-update fetch
freebsd-update install
shutdown -r now

# Verify kernel version after reboot
uname -a

# Optional: mount user-writable filesystems noexec to limit binary execution
# /etc/fstab example
# /dev/ada0p4  /tmp  ufs  rw,noexec,nosuid  2  2