CVE-2026-45929: Linux Kernel Use-After-Free Vulnerability

CVE-2026-45929 Overview

CVE-2026-45929 is a use-after-free vulnerability in the Linux kernel's OpenVPN data channel offload (ovpn) module. The flaw resides in the ovpn_net_xmit transmit path, where the code continues to dereference a stale skb pointer after skb_share_check may have freed the original socket buffer. Subsequent operations including peer lookup, skb_dst_drop, and ovpn_peer_stats_increment_tx then operate on freed memory. The issue has been resolved upstream through commits that reorder peer lookup and destination handling before segmentation.

Critical Impact

Triggering the freed-pointer path in ovpn_net_xmit can lead to kernel memory corruption, potentially enabling denial of service or local privilege escalation on systems running the affected ovpn driver.

Affected Products

• Linux kernel versions containing the ovpn (OpenVPN data channel offload) driver prior to the fix commits
• Distributions shipping vulnerable mainline and stable kernel branches
• Systems using the kernel-mode OpenVPN data channel offload feature

Discovery Timeline

• 2026-05-27 - CVE-2026-45929 published to NVD
• 2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-45929

Vulnerability Analysis

The vulnerability is a use-after-free [CWE-416] in the OpenVPN data channel offload transmit path. When ovpn_net_xmit builds its skb_list, it calls skb_share_check on the incoming socket buffer. If the buffer is shared, skb_share_check releases the original skb and returns a new one. The original implementation then continues to use the stale skb pointer for peer lookup, skb_dst_drop, and statistics accounting.

Dereferencing a freed skb allows an attacker who can control or influence the freed allocation to read or corrupt adjacent kernel memory. Because ovpn_net_xmit runs in the network transmit path, the issue is reachable any time traffic egresses through an ovpn interface and the kernel hands the driver a shared socket buffer.

Root Cause

The root cause is incorrect lifetime management of the skb pointer relative to skb_share_check. The function may free its input and return a replacement, but the driver assumed the original pointer remained valid throughout segmentation and post-processing. The fix relocates the peer lookup and skb_dst_drop before segmentation, ensuring the original skb is still live when used. The patch also switches ovpn_peer_stats_increment_tx to reference skb_list.next and returns early when every segment fails skb_share_check and the list ends up empty.

Attack Vector

Exploitation requires generating network traffic that traverses an ovpn interface under conditions where the kernel supplies a shared skb to the driver. The transmit-path nature of the bug means it is reachable from local processes producing traffic, and possibly from remote senders whose packets are bridged onto an ovpn interface. The defect manifests as kernel memory access after free, with consequences ranging from kernel panic to potential exploitation for privilege escalation depending on heap state. Refer to the upstream commits for the precise code paths: kernel commit 3e4fbcb4, kernel commit 442915c9, and kernel commit a5ec7baa.

Detection Methods for CVE-2026-45929

Indicators of Compromise

• Kernel oops or panic traces referencing ovpn_net_xmit, skb_share_check, or ovpn_peer_stats_increment_tx
• KASAN use-after-free reports tagged against sk_buff allocations during VPN transmit
• Unexpected crashes or interface resets on hosts running the ovpn data channel offload driver

Detection Strategies

• Compare running kernel versions against the fixed commits in the stable trees and flag hosts on unpatched builds
• Enable KASAN on test and staging kernels to surface use-after-free conditions in the ovpn transmit path
• Monitor dmesg and journald for kernel warnings, BUG reports, or RIP addresses inside net/ovpn/

Monitoring Recommendations

• Collect kernel crash dumps from production VPN gateways and correlate stack frames containing ovpn_net_xmit
• Alert on repeated ovpn interface restarts or NETDEV_UNREGISTER events that may indicate triggered crashes
• Track package and kernel inventory for Linux hosts terminating OpenVPN tunnels to identify drift from patched versions

How to Mitigate CVE-2026-45929

Immediate Actions Required

• Update affected Linux systems to a kernel release that includes the three upstream fix commits referenced in the NVD entry
• Identify all hosts loading the ovpn module and schedule patching on VPN concentrators and gateways first
• Validate distribution security advisories from your vendor and apply backported stable-kernel updates

Patch Information

The fix moves the peer lookup and skb_dst_drop operations ahead of segmentation so the original skb remains valid when dereferenced, switches ovpn_peer_stats_increment_tx to use skb_list.next, and returns early when all segments fail skb_share_check. The corrections are tracked in kernel commit 3e4fbcb4, kernel commit 442915c9, and kernel commit a5ec7baa.

Workarounds

• Unload the ovpn module (modprobe -r ovpn) on hosts that do not require kernel-mode OpenVPN data channel offload
• Fall back to userspace OpenVPN on affected hosts until a patched kernel is deployed
• Restrict which local users and containers can transmit traffic through ovpn interfaces to reduce the attack surface

# Verify whether the ovpn module is loaded and check kernel version
lsmod | grep -w ovpn
uname -r

# Temporarily disable the driver on hosts that do not need it
sudo modprobe -r ovpn
echo 'blacklist ovpn' | sudo tee /etc/modprobe.d/blacklist-ovpn.conf