CVE-2026-49427 Overview
CVE-2026-49427 is a use-after-free vulnerability in the FreeBSD kernel affecting POSIX shared memory objects backed by large pages. Pages belonging to largepage shared memory objects were not explicitly wired. When sendfile(2) transmitted such an object with the SF_NOCACHE flag, the kernel freed the underlying pages after transmission even though existing mappings still referenced them. An unprivileged local user can abuse this behavior to access freed kernel memory and escalate privileges. The vulnerability is tracked under CWE-826: Premature Release of Resource During Expected Lifetime.
Critical Impact
Local privilege escalation through kernel memory corruption, granting attackers full control over affected FreeBSD systems.
Affected Products
- FreeBSD operating system (kernel component)
- Systems using POSIX shared memory (shm_open) with largepage support
- Configurations exercising sendfile(2) with the SF_NOCACHE flag on shared memory objects
Discovery Timeline
- 2026-08-19 - CVE-2026-49427 published to NVD
- 2026-08-20 - Last updated in NVD database
Technical Details for CVE-2026-49427
Vulnerability Analysis
The flaw resides in the FreeBSD kernel's handling of largepage-backed POSIX shared memory objects. Normally, pages referenced by active mappings must remain resident and reference-counted until all consumers release them. In this code path, largepage shared memory pages were not explicitly wired into the kernel's page management system.
When a process invoked sendfile(2) against such an object with the SF_NOCACHE flag, the kernel released the underlying pages once transmission completed. Existing memory mappings, however, still pointed to these freed pages. Subsequent kernel or userland access to those mappings operated on memory that had already been returned to the free pool.
This condition creates a use-after-free window that a local attacker can leverage to read or manipulate kernel data structures. Because kernel memory is shared across privilege boundaries, the primitive can be extended into privilege escalation.
Root Cause
The root cause is missing wiring semantics on largepage shared memory pages. The kernel treated these pages as safe to reclaim after SF_NOCACHE transmission without verifying that no other mappings retained references. This mismatch between the object's lifetime and the mappings' lifetime produces the dangling reference described by [CWE-826].
Attack Vector
An unprivileged local user creates a largepage-backed POSIX shared memory object using shm_open with the SHM_LARGEPAGE flag, maps it into the process address space, and issues sendfile(2) with SF_NOCACHE targeting a socket. The transmission triggers premature page release. The attacker then interacts with the still-valid mapping to access freed physical memory, positioning kernel objects into the reclaimed slots to achieve arbitrary read or write primitives.
The vulnerability manifests entirely at the kernel boundary and does not require special privileges beyond the ability to open shared memory and sockets. Full exploitation details are documented in the FreeBSD Security Advisory SA-26:44.
Detection Methods for CVE-2026-49427
Indicators of Compromise
- Unexpected kernel panics or page fault traces referencing shm_largepage or sendfile code paths
- Local processes creating largepage POSIX shared memory objects and immediately invoking sendfile(2) with SF_NOCACHE
- Unprivileged processes performing subsequent privileged actions without an authenticated escalation event
Detection Strategies
- Audit system call telemetry for the combination of shm_open with SHM_LARGEPAGE, mmap, and sendfile with SF_NOCACHE originating from unprivileged users
- Correlate kernel crash dumps and dmesg output for use-after-free signatures in the virtual memory subsystem
- Monitor for anomalous transitions of process credentials (uid changes) following heavy shared memory activity
Monitoring Recommendations
- Enable FreeBSD audit framework (auditd) with policies capturing shm_open, mmap, and sendfile events
- Forward kernel logs to a centralized analytics platform for correlation across hosts
- Alert on repeated allocations of largepage shared memory objects by non-root accounts
How to Mitigate CVE-2026-49427
Immediate Actions Required
- Apply the FreeBSD kernel patch published in FreeBSD-SA-26:44 and reboot affected systems
- Inventory hosts running FreeBSD and confirm kernel version alignment with the fixed release
- Restrict local shell access on multi-tenant FreeBSD systems until patches are deployed
Patch Information
The FreeBSD Project has released a security advisory and updated kernel binaries addressing the missing wiring behavior for largepage shared memory pages. Administrators should follow the update procedure documented in the advisory, which includes freebsd-update steps and source rebuild instructions. A reboot is required for the patched kernel to take effect.
Workarounds
- Disable or restrict use of largepage POSIX shared memory where feasible by tuning security.bsd.unprivileged_proc_debug and related sysctls that limit unprivileged capabilities
- Prevent untrusted local users from executing arbitrary binaries on affected hosts through mandatory access control policies
- Constrain sendfile(2) usage in application designs that pair it with SF_NOCACHE against shared memory objects until the patch is applied
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
