CVE-2026-23001: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23001 Overview

CVE-2026-23001 is a use-after-free (UAF) vulnerability in the Linux kernel's macvlan driver. The flaw resides in the macvlan_forward_source() function, where the vlan pointer inside struct macvlan_source_entry lacked Read-Copy-Update (RCU) protection. When macvlan_hash_del_source() removed an entry, the entry->vlan pointer was not cleared before the RCU grace period began, allowing concurrent readers to dereference a freed object.

The vulnerability affects Linux kernel versions starting at 3.18 and including 6.19 release candidates (rc1 through rc8). Local attackers with low privileges can trigger memory corruption through the network packet forwarding path.

Critical Impact
Successful exploitation of the use-after-free condition can lead to kernel memory corruption, privilege escalation to root, or a kernel panic resulting in denial of service.

Affected Products

Linux Kernel 3.18 (and subsequent stable branches)
Linux Kernel 6.19-rc1 through 6.19-rc8
Distributions shipping vulnerable stable kernels using the macvlan driver

Discovery Timeline

2026-01-25 - CVE-2026-23001 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-23001

Vulnerability Analysis

The vulnerability is a classic use-after-free [CWE-416] in the Linux kernel networking stack. The macvlan driver allows multiple virtual network interfaces to share a single physical interface, each with its own MAC address. Source-mode macvlan devices maintain a hash table of macvlan_source_entry structures that map source MAC addresses to specific macvlan_dev instances.

The macvlan_forward_source() function walks this hash table during packet reception to deliver frames to the correct virtual device. Without RCU protection on the vlan field, a reader could obtain a pointer to a macvlan_dev that had already been queued for freeing, leading to dereference of stale memory.

Root Cause

The root cause is missing synchronization between the deletion path and concurrent readers. macvlan_hash_del_source() removed entries from the source hash without first clearing entry->vlan before the RCU grace period started. As a result, lookups running in parallel under RCU read-side critical sections could observe a non-NULL vlan pointer referencing memory scheduled for reclamation.

The upstream fix introduces __rcu annotation on the vlan pointer and clears it before the RCU grace period, allowing macvlan_forward_source() to skip entries already queued for freeing. Backing macvlan_dev objects are already RCU-protected through netdev_priv().

Attack Vector

Exploitation requires local access and low privileges on a host configured with macvlan interfaces in source mode. An attacker capable of triggering source-mode entry deletion concurrently with incoming traffic forwarding can race the RCU grace period. Successful races corrupt kernel memory, which sophisticated attackers can shape into local privilege escalation. Remote network-only exploitation is not feasible because configuration of macvlan source entries requires CAP_NET_ADMIN.

The vulnerability mechanism is described in prose only. See the upstream patch commit for the implementation of the RCU-protected pointer fix.

Detection Methods for CVE-2026-23001

Indicators of Compromise

Kernel oops or panic messages referencing macvlan_forward_source, macvlan_hash_lookup_source, or macvlan_hash_del_source in dmesg or /var/log/kern.log
KASAN (Kernel Address Sanitizer) reports flagging use-after-free in the macvlan driver on instrumented kernels
Unexpected process privilege transitions on hosts with active macvlan source-mode interfaces

Detection Strategies

Inventory Linux hosts running kernels older than the patched stable releases listed in the kernel.org advisories
Identify systems with macvlan interfaces in source mode using ip link show type macvlan and ip -d link show
Audit privilege escalation events and module load events correlated with macvlan configuration changes

Monitoring Recommendations

Forward kernel logs and audit events to a centralized SIEM to detect crashes and anomalous root-level activity
Monitor CAP_NET_ADMIN usage and namespace creation, since container runtimes commonly configure macvlan interfaces
Track kernel package versions across the fleet to confirm patch deployment status

How to Mitigate CVE-2026-23001

Immediate Actions Required

Apply the stable kernel updates from your Linux distribution vendor that incorporate the RCU protection fix
Reboot affected systems to load the patched kernel image
Restrict CAP_NET_ADMIN and unprivileged user namespace creation where feasible to reduce local attack surface

Patch Information

The fix has been merged into multiple stable kernel branches. Reference commits include 8518712a2ca9, 7470a7a63dc1, 6dbead9c7677, 484919832e2d, 232afc74a6dd, 15f6faf36e16, and 8133e85b8a3e. See the Linux kernel stable commit log for the canonical patch.

Workarounds

Avoid configuring macvlan interfaces in source mode on multi-tenant or untrusted-workload hosts until patches are applied
Disable the macvlan kernel module on systems that do not require it using modprobe -r macvlan and blacklisting via /etc/modprobe.d/
Limit access to network configuration capabilities by tightening container runtime security profiles (seccomp, AppArmor, SELinux)

bash

# Verify running kernel version and macvlan usage
uname -r
ip -d link show type macvlan

# Blacklist macvlan module if not required
echo "blacklist macvlan" | sudo tee /etc/modprobe.d/blacklist-macvlan.conf
sudo modprobe -r macvlan