CVE-2026-68415: Linux Kernel Privilege Escalation Flaw

CVE-2026-68415 Overview

CVE-2026-68415 is a Linux kernel vulnerability in the xfrm (IPsec transform) subsystem. The flaw affects the IPTFS (IP Traffic Flow Security) mode setup path. When __xfrm_init_state() caches x->mode_cbs and IPTFS setup subsequently fails with -ENOMEM before publishing mode_data, the dead state retains a stale callback table pointer. Once the xfrm_iptfs module unloads and deferred garbage collection runs, xfrm_state_gc_task dereferences the stale x->mode_cbs, producing a kernel page fault and panic. The issue can be triggered locally by an authenticated user with the ability to configure IPsec state and manipulate module loading conditions.

Critical Impact
A local attacker can trigger a kernel panic or potentially exploit the stale pointer dereference to achieve elevated code execution in kernel context.

Affected Products

Linux kernel versions containing the xfrm_iptfs module prior to the fix commits
Distributions shipping IPTFS support in xfrm
Systems with IPsec configuration privileges available to local users

Discovery Timeline

2026-08-10 - CVE-2026-68415 published to NVD
2026-08-13 - Last updated in NVD database

Technical Details for CVE-2026-68415

Vulnerability Analysis

The vulnerability resides in the Linux kernel xfrm state initialization logic. During __xfrm_init_state(), the kernel calls xfrm_get_mode_cbs() to look up the mode callback table for the requested transform mode. This lookup takes a temporary module reference and caches the callback pointer in x->mode_cbs. When IPTFS mode initialization or cloning subsequently fails before publishing mode_data, the temporary module reference is dropped immediately. However, the x->mode_cbs pointer remains cached on the dead state.

The stale state is later queued to the deferred garbage collector through xfrm_state_put(). If the xfrm_iptfs module unloads before xfrm_state_gc_task executes, the callback table backing x->mode_cbs no longer exists. When the GC worker dereferences the pointer, it accesses freed module memory, producing a fatal exception. Reproduction under fault injection triggered a Kernel panic - not syncing: Fatal exception inside xfrm_state_gc_task+0x142/0x650.

Root Cause

The root cause is a lifetime mismatch between the callback table pointer cached on the xfrm state and the module reference protecting that pointer. The failed setup path never installs mode-specific state and never takes the long-term IPTFS module pin, yet it leaves x->mode_cbs populated. The dead state outlives the module, causing a use-after-free style dereference [CWE-416] during deferred garbage collection.

Attack Vector

Local exploitation requires the ability to create xfrm states with IPTFS mode and to induce an allocation failure during setup. An attacker who can load and unload the xfrm_iptfs module, or coerce the module lifecycle indirectly, can then race deferred GC against module unload. The result is a kernel-mode dereference of a dangling function table pointer, which can produce denial of service or, under favorable memory reuse conditions, control-flow hijack in kernel context.

See the upstream fix commits for the exact code changes: kernel.org commit 2538bd3c, kernel.org commit 9845a359, and kernel.org commit c37a0792. The fix clears x->mode_cbs when mode init or clone fails before publishing mode_data.

Detection Methods for CVE-2026-68415

Indicators of Compromise

Kernel panic messages referencing xfrm_state_gc_task in the faulting instruction pointer
Modules linked in: lines showing [last unloaded: xfrm_iptfs] at the time of a crash
Unexpected xfrm_iptfs module load and unload sequences from non-administrative workflows
Repeated -ENOMEM failures observed during IPTFS state creation in kernel logs

Detection Strategies

Monitor dmesg and /var/log/kern.log for page fault stack traces originating in xfrm_state_gc_task or nearby xfrm symbols
Audit finit_module and delete_module syscalls correlated with IPsec configuration activity via auditd
Track invocations of ip xfrm state add with IPTFS mode from non-network-admin processes

Monitoring Recommendations

Enable kernel crash dump collection (kdump) to capture full context if xfrm_state_gc_task faults
Alert on any unexpected loading of the xfrm_iptfs kernel module on production hosts that do not use IPTFS
Correlate xfrm state creation failures with subsequent module unload events across the fleet

How to Mitigate CVE-2026-68415

Immediate Actions Required

Apply the upstream stable kernel patches referenced in the commits above as soon as vendor builds are available
Blocklist the xfrm_iptfs module on hosts that do not require IPTFS using /etc/modprobe.d/ blacklist entries
Restrict CAP_NET_ADMIN and CAP_SYS_MODULE to trusted administrative accounts only
Audit which local users and containers can configure IPsec state or influence kernel module loading

Patch Information

The fix clears x->mode_cbs when mode.init or clone_state() fails before publishing mode_data, ensuring the deferred garbage collector never dereferences a callback table belonging to an unloaded module. Merge the fix from the stable trees referenced in the kernel.org commit log and rebuild or update to a distribution kernel that includes these commits.

Workarounds

Prevent xfrm_iptfs from loading on systems that do not require IPTFS by adding install xfrm_iptfs /bin/false to a modprobe.d configuration file
Disable module auto-unload by setting kernel.modules_disabled=1 after boot on hardened systems that do not need to unload modules at runtime
Constrain unprivileged user namespaces if IPsec configuration is exposed through namespaced network administration

bash

# Blocklist the xfrm_iptfs module until patched kernels are deployed
echo 'install xfrm_iptfs /bin/false' | sudo tee /etc/modprobe.d/blacklist-xfrm-iptfs.conf
sudo depmod -a

# Verify the module is not currently loaded
lsmod | grep xfrm_iptfs