CVE-2026-49418: Kernel msync Use-After-Free Vulnerability

CVE-2026-49418 Overview

CVE-2026-49418 is a use-after-free vulnerability [CWE-416] in the FreeBSD kernel virtual memory subsystem. The flaw resides in the handling of msync(MS_INVALIDATE) calls against mappings backed by unmanaged device objects. When invoked, physical pages in the mapping range are marked invalid but remain in the pager's page list. A subsequent page fault causes the fault handler to re-insert the same page, corrupting the list. On object destruction, the kernel frees the page twice.

An unprivileged local user with access to a device exposing memory-mapped I/O can trigger the condition. Exploitation is constrained to a pool of fictitious pages that are never recycled, but privilege escalation to kernel context may be feasible.

Critical Impact
A local unprivileged attacker with access to a memory-mapped I/O device can corrupt kernel page lists and potentially escalate privileges through kernel use-after-free.

Affected Products

FreeBSD operating system (kernel virtual memory subsystem)
Systems exposing memory-mapped I/O device objects to unprivileged users
Refer to the FreeBSD Security Advisory for supported branch and release coverage

Discovery Timeline

2026-08-19 - CVE-2026-49418 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-49418

Vulnerability Analysis

The vulnerability sits in the FreeBSD VM pager path for unmanaged device objects. msync(MS_INVALIDATE) is intended to discard cached pages in a mapping range. For unmanaged device-backed mappings, the invalidation path marks pages invalid without removing them from the object's internal page list.

This leaves stale references to fictitious pages that the kernel treats as still owned by the object. A subsequent page fault against the same range invokes the fault handler, which re-inserts the same page structure into the object's list. The list now contains a duplicate entry pointing at a single fictitious page.

When the VM object is later destroyed, the destructor walks the page list and frees each entry. The duplicated page is freed twice, producing a use-after-free condition on fictitious page structures managed by the kernel.

Root Cause

The root cause is missing state synchronization between the invalidation path and the pager's page list for unmanaged device objects. Marking a page invalid does not remove it from the container, allowing the fault handler to violate the list invariant that each page appears at most once.

Attack Vector

Exploitation requires local access and permission to open a device node that exposes memory-mapped I/O through an unmanaged device pager. An attacker maps the device with mmap, triggers msync(MS_INVALIDATE) on the mapping, forces a page fault by touching the same range, and then releases the mapping to trigger object destruction and the double free. Because the affected structures are fictitious pages that are never recycled for other purposes, exploitation is constrained but may still allow kernel memory corruption sufficient for privilege escalation.

No verified public proof-of-concept code is available. See the FreeBSD Security Advisory for vendor technical detail.

Detection Methods for CVE-2026-49418

Indicators of Compromise

Unexpected kernel panics or page fault traces referencing device pager or fictitious page routines
Repeated msync system calls with the MS_INVALIDATE flag issued by non-root processes against device-backed mappings
Local processes opening device nodes that expose memory-mapped I/O without a legitimate operational reason

Detection Strategies

Audit mmap and msync system call sequences from unprivileged UIDs targeting /dev entries using DTrace or auditd equivalents on FreeBSD
Correlate kernel crash dumps that reference double-free assertions in the VM subsystem with recent user activity on device pagers
Monitor for anomalous local privilege changes following device access events

Monitoring Recommendations

Forward FreeBSD auditd logs and kernel crash telemetry to a centralized analytics platform for longitudinal analysis
Alert on any unprivileged process accessing device nodes historically used only by system services
Track kernel version and patch level across the FreeBSD fleet to identify unpatched hosts exposed to local users

How to Mitigate CVE-2026-49418

Immediate Actions Required

Apply the FreeBSD security patch referenced in FreeBSD-SA-26:37.vm on all affected hosts
Inventory systems where unprivileged users have local shell access, particularly multi-user and shared-tenant FreeBSD hosts
Restrict access to device nodes exposing memory-mapped I/O to trusted administrative accounts

Patch Information

FreeBSD has issued a security advisory for this issue. Consult the FreeBSD Security Advisory FreeBSD-SA-26:37.vm for the affected release branches, patch instructions, and reboot requirements. A kernel rebuild or binary update followed by reboot is required to load the fixed VM subsystem.

Workarounds

Tighten permissions on /dev nodes exposing memory-mapped I/O so unprivileged users cannot open them
Remove local shell access for untrusted users on hosts that cannot be immediately patched
Where feasible, disable loading of drivers that expose unmanaged device pagers to userspace

bash

# Example: restrict device node access to root and a trusted group
# Adjust the device path to match the driver exposing mmap on your system
chown root:wheel /dev/<device>
chmod 0600 /dev/<device>

# Verify FreeBSD version and apply vendor patch
freebsd-version -kru
freebsd-update fetch
freebsd-update install
shutdown -r now