CVE-2026-53227: Linux Kernel Use-After-Free Vulnerability

CVE-2026-53227 Overview

CVE-2026-53227 affects the Linux kernel's openvswitch networking subsystem. The flaw involves an error-handling path that may call kfree_skb() on an ERR_PTR value rather than a valid socket buffer pointer. After an earlier fix changed the allocation ordering of the reply skb, the cleanup logic remained in its original reversed order. When allocation occurs after locking ovs_mutex and fails, the reply pointer is left holding an ERR_PTR, which is then passed to the freeing routine during cleanup.

The issue was resolved upstream by setting the reply pointer to NULL after saving its error value, preventing the invalid pointer from reaching the cleanup path.

Critical Impact
A failed skb allocation under contention can trigger kernel memory corruption or a kernel oops by freeing an ERR_PTR value, potentially leading to denial of service on systems running Open vSwitch datapath.

Affected Products

Linux kernel (mainline) with the net/openvswitch datapath module
Stable kernel branches receiving the openvswitch reply skb allocation reordering patch
Distributions shipping affected stable kernels with Open vSwitch enabled

Discovery Timeline

2026-06-25 - CVE-2026-53227 published to NVD
2026-06-25 - Last updated in NVD database

Technical Details for CVE-2026-53227

Vulnerability Analysis

The vulnerability resides in the Linux kernel's Open vSwitch (openvswitch) Netlink handling code. A prior patch (referenced in the upstream "Fixes" tag) changed when the reply socket buffer (skb) is allocated relative to acquiring ovs_mutex. The allocation can now occur either before or after the mutex is taken.

However, the error-handling unwind logic was not updated to match. The cleanup path assumes the classical reversed order: that reply was allocated before locking, and therefore must be freed only after unlocking. When allocation happens after the lock and fails, the kernel stores the error code returned by the allocation routine into reply using ERR_PTR. Execution then jumps to a goto cleanup label that eventually calls kfree_skb(reply) on an encoded error pointer rather than a valid sk_buff *.

Freeing an ERR_PTR is undefined behavior in the kernel and can produce a NULL or invalid pointer dereference, a kernel BUG, or in some configurations memory corruption. This class of bug is tracked as a Use-After-Free / invalid free [CWE-763] pattern.

Root Cause

The root cause is an inconsistency between the allocation order and the cleanup order. The fix introduced two possible allocation sites for the reply skb without updating the error-handling goto targets to distinguish between the two cases. The classical reversed-order cleanup assumption no longer holds.

Attack Vector

Triggering the bug requires reaching the affected openvswitch Netlink code path under conditions where the reply skb allocation fails. This generally requires the CAP_NET_ADMIN capability and memory pressure or allocation failure injection. The result is a kernel crash or undefined behavior in the host networking stack. No verified remote, unprivileged exploitation vector has been published.

No public proof-of-concept has been released. Refer to the upstream commits for the precise affected code path and patch diff: Kernel commit 0bb5b2dc1b90, Kernel commit 25fdf5369853, and Kernel commit ee30dd2909d8.

Detection Methods for CVE-2026-53227

Indicators of Compromise

Kernel oops or BUG messages in dmesg referencing kfree_skb invoked from openvswitch Netlink handlers
Stack traces showing ovs_mutex involvement followed by an invalid pointer dereference in skb_release_* functions
Unexpected host networking outages on systems running Open vSwitch datapaths under memory pressure

Detection Strategies

Inventory kernel versions across Linux hosts and compare against the fixed commits listed in the upstream stable tree
Audit hosts that load the openvswitch module (lsmod | grep openvswitch) and prioritize them for patching
Correlate kernel crash telemetry with workloads that exercise Open vSwitch Netlink commands, such as OVN, Kubernetes CNI plugins, or OpenStack Neutron

Monitoring Recommendations

Forward kmsg/journald kernel logs to a centralized log platform and alert on BUG:, Oops:, and kfree_skb patterns
Monitor host availability and network namespace health on virtualization and container hosts that use Open vSwitch
Track memory pressure metrics (MemAvailable, OOM events) on OVS hosts, since allocation failure is the precondition for the bug

How to Mitigate CVE-2026-53227

Immediate Actions Required

Upgrade to a Linux kernel release that includes the openvswitch reply skb cleanup fix from the upstream stable branches
Restrict CAP_NET_ADMIN and Open vSwitch Netlink access to trusted administrative accounts and workloads only
Schedule rolling reboots for OVS-enabled hosts after applying the patched kernel

Patch Information

The upstream fix sets the reply pointer to NULL after saving its error value, ensuring the cleanup path does not invoke kfree_skb() on an ERR_PTR. The patch is distributed across multiple stable branches via the following commits: 0bb5b2dc1b90, 25fdf5369853, 895d1dd9057c, 971b1b37774f, e248fb2e680d, e3d509a1b713, ecc55aad3390, and ee30dd2909d8. Apply the kernel update provided by your Linux distribution vendor.

Workarounds

If patching cannot be performed immediately, unload the openvswitch module on hosts that do not require it (modprobe -r openvswitch)
Reduce the likelihood of skb allocation failure by ensuring adequate free memory and tuning vm.min_free_kbytes on OVS hosts
Limit the set of users and containers permitted to issue Open vSwitch Netlink commands by enforcing strict capability and namespace policies

bash

# Verify whether the openvswitch module is loaded and identify running kernel
uname -r
lsmod | grep -i openvswitch

# Unload the module on hosts that do not require Open vSwitch
sudo systemctl stop openvswitch-switch 2>/dev/null || true
sudo modprobe -r openvswitch

# Confirm the patched kernel after upgrade and reboot
dmesg | grep -iE 'openvswitch|kfree_skb'