CVE-2026-23003: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23003 Overview

A vulnerability has been discovered in the Linux kernel's IPv6 tunnel implementation (ip6_tunnel) involving uninitialized memory use in the __ip6_tnl_rcv() function. The issue arises because the previous commit did not properly handle VLAN encapsulations when processing tunnel traffic, leading to use of uninitialized memory during ECN (Explicit Congestion Notification) decapsulation operations.

This vulnerability was identified by syzbot, Google's continuous fuzzing infrastructure for the Linux kernel. The flaw specifically affects the handling of VLAN-tagged packets within IPv6 tunnels, where pskb_inet_may_pull() was being used instead of the proper skb_vlan_inet_prepare() function.

Critical Impact

Local attackers with access to network interfaces (such as TUN devices) may be able to trigger uninitialized memory reads in the kernel, potentially leading to information disclosure or denial of service conditions.

Affected Products

• Linux kernel with IPv6 tunnel support enabled
• Systems using ip6_tunnel, ip6_gre, or similar IPv6 tunneling mechanisms
• Network configurations with VLAN encapsulation over IPv6 tunnels

Discovery Timeline

• January 25, 2026 - CVE-2026-23003 published to NVD
• January 26, 2026 - Last updated in NVD database

Technical Details for CVE-2026-23003

Vulnerability Analysis

The vulnerability is classified as Uninitialized Memory Use within the Linux kernel's networking subsystem. When IPv6 tunnel traffic with VLAN encapsulation is received, the __ip6_tnl_rcv() function calls ECN decapsulation routines that expect properly initialized packet data. However, the previous implementation using pskb_inet_may_pull() does not adequately prepare VLAN-tagged socket buffers, resulting in access to uninitialized memory regions.

The KMSAN (Kernel Memory Sanitizer) reports show the uninitialized value being accessed in IP6_ECN_decapsulate() through the call chain: ip6_tnl_rcv() → __ip6_tnl_rcv() → ip6ip6_dscp_ecn_decapsulate() → IP6_ECN_decapsulate() → INET_ECN_decapsulate() → __INET_ECN_decapsulate().

The uninitialized memory originates from socket buffer allocation in tun_alloc_skb() via the slab allocator, where the allocated memory is not properly initialized before use in the ECN decapsulation path.

Root Cause

The root cause lies in the improper function selection for handling VLAN-encapsulated packets. The pskb_inet_may_pull() function does not account for VLAN header handling, leaving portions of the socket buffer in an uninitialized state. The fix replaces this with skb_vlan_inet_prepare(), which properly handles VLAN encapsulations and ensures the necessary packet data is initialized before processing.

This is a classic case where network protocol layering (IPv6 tunnels with VLAN tags) creates edge cases that can be missed during implementation, particularly when different encapsulation types are combined.

Attack Vector

The vulnerability can be triggered through the TUN/TAP interface by crafting malicious packets with specific VLAN encapsulation over IPv6 tunnel protocols. The attack path demonstrated by syzbot shows:

1. An attacker with access to a TUN device writes crafted packet data
2. The packet traverses the IPv6 receive path with GRE or tunnel encapsulation
3. VLAN-tagged inner packets trigger the uninitialized memory access in ECN decapsulation
4. The kernel reads uninitialized slab memory, potentially leaking information or causing undefined behavior

The attack requires local access to create or write to TUN/TAP devices, which typically requires elevated privileges or specific capabilities (CAP_NET_ADMIN).

Detection Methods for CVE-2026-23003

Indicators of Compromise

• Kernel log messages containing BUG: KMSAN: uninit-value related to inet_ecn.h or ip6_tunnel.c
• Unexpected kernel warnings or crashes in the IPv6 tunnel receive path
• Anomalous TUN/TAP device activity with malformed VLAN-tagged IPv6 tunnel traffic
• System instability when processing IPv6 GRE or tunnel traffic with VLAN encapsulation

Detection Strategies

• Enable KMSAN or KASAN in development/testing kernels to detect uninitialized memory access patterns
• Monitor kernel logs for warnings related to __ip6_tnl_rcv, IP6_ECN_decapsulate, or related functions
• Deploy SentinelOne Singularity for real-time kernel vulnerability detection and anomaly monitoring
• Implement network traffic analysis to identify unusual VLAN-over-IPv6-tunnel packet patterns

Monitoring Recommendations

• Configure alerting for kernel KMSAN/KASAN sanitizer messages in production environments where enabled
• Monitor for unusual crash patterns or kernel panics related to IPv6 networking subsystems
• Track TUN/TAP device creation and usage patterns for suspicious activity
• Implement SentinelOne endpoint monitoring to detect exploitation attempts targeting kernel network code

How to Mitigate CVE-2026-23003

Immediate Actions Required

• Apply the kernel patches from the stable kernel tree to affected systems
• If patching is not immediately possible, consider disabling IPv6 tunnel functionality if not required
• Restrict access to TUN/TAP device creation to trusted users and processes
• Monitor systems for signs of exploitation until patches can be applied

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix replaces the use of pskb_inet_may_pull() with skb_vlan_inet_prepare() in the __ip6_tnl_rcv() function to properly handle VLAN encapsulations.

The following kernel commits contain the fix:

• Kernel Git Commit 81c734dae203
• Kernel Git Commit b9f915340f25
• Kernel Git Commit df5ffde96693

System administrators should update to kernel versions containing these commits or apply backported patches from their distribution vendor.

Workarounds

• Disable IPv6 tunneling (ip6_tunnel, ip6_gre modules) if not required for operations
• Restrict CAP_NET_ADMIN capability to minimize the attack surface for TUN/TAP device exploitation
• Use network namespaces to isolate untrusted processes from tunnel interfaces
• Apply kernel security hardening configurations to limit exploitation impact

# Configuration example - Disable IPv6 tunnel modules if not needed
# Add to /etc/modprobe.d/disable-ip6-tunnel.conf
blacklist ip6_tunnel
blacklist ip6_gre
blacklist ip6_vti

# Reload module configuration
sudo update-initramfs -u