CVE-2026-31663: Linux Kernel Race Condition Vulnerability

CVE-2026-31663 Overview

CVE-2026-31663 is a use-after-free vulnerability in the Linux kernel's xfrm (IPsec) transformation framework. The flaw resides in the asynchronous crypto resumption path, where xfrm_input_resume() calls dev_put() on the socket buffer's device reference before the packet completes processing through transport_finish and its associated NF_HOOK. The premature reference release creates a race condition with device teardown, leaving skb->dev pointing to potentially freed memory while netfilter hooks and their okfn callbacks still dereference it.

The vulnerability affects multiple stable Linux kernel branches and was fixed across three upstream commits.

Critical Impact
A local attacker with the ability to trigger IPsec processing during network device teardown can exploit the use-after-free in skb->dev to corrupt kernel memory, escalate privileges, or crash the system.

Affected Products

Linux Kernel 4.15 (specific build)
Linux Kernel 7.0 release candidates rc1 through rc7
Multiple Linux kernel stable branches prior to the fix commits

Discovery Timeline

2026-04-24 - CVE-2026-31663 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31663

Vulnerability Analysis

The Linux kernel xfrm subsystem processes IPsec packets and supports asynchronous cryptographic operations. When async crypto completes, xfrm_input_resume() re-enters the input path to finish packet processing. The original code released the device reference held on the socket buffer (skb) immediately upon async resumption by calling dev_put() before the packet traversed transport_finish.

This ordering created a window where skb->dev could reference a device structure whose refcount had already been decremented. Subsequent code paths, including the netfilter NF_HOOK macro and its okfn continuation, dereference skb->dev to make routing and filtering decisions. If the device tore down during this window, those dereferences operate on freed memory.

Root Cause

The root cause is incorrect reference counting in the async resume path. The fix removes the early dev_put() from xfrm_input_resume() and instead drops the reference after the NF_HOOK call inside transport_finish. Because NF_HOOK can consume the skb through NF_DROP, NF_QUEUE, or NF_STOLEN verdicts, the patch saves the device pointer locally before invoking the hook and releases the reference using the saved pointer. For non-transport exits such as decapsulation, GRO, and drop paths, the reference is released inline when the async flag is set.

Attack Vector

Exploitation requires local access with the ability to trigger IPsec processing using asynchronous crypto offload. An attacker would need to race device teardown against in-flight async crypto completions. Successful exploitation leads to kernel memory corruption with potential for local privilege escalation. The vulnerability is not remotely exploitable and is classified under [CWE-noinfo] pending detailed analysis.

The vulnerability is described in prose only; no public proof-of-concept code is available. See the Kernel Git Commit 5002beda for the upstream fix.

Detection Methods for CVE-2026-31663

Indicators of Compromise

Unexpected kernel oops or panic messages referencing xfrm_input_resume, xfrm_trans_reinject, or dev_put in dmesg
KASAN reports flagging use-after-free in network device structures during IPsec workloads
Sudden network interface instability correlated with active IPsec tunnels using async crypto

Detection Strategies

Audit running kernel versions against the fixed commits 0f451b43, 1c428b03, and 5002beda to identify vulnerable hosts
Enable KASAN on test kernels to detect the use-after-free pattern during IPsec stress testing
Monitor kernel ring buffers for repeated faults in the xfrm input path on systems with crypto offload

Monitoring Recommendations

Centralize kernel logs from IPsec-enabled hosts and alert on xfrm-related stack traces
Track network namespace and device teardown events in environments running container or VPN gateways
Correlate kernel crash dumps with IPsec session activity to identify suspected exploitation attempts

How to Mitigate CVE-2026-31663

Immediate Actions Required

Identify Linux hosts running kernels that lack the three remediation commits and prioritize patching
Apply vendor-supplied kernel updates from your distribution as soon as they are available
Restrict local account access on IPsec gateways and systems exposing async crypto paths

Patch Information

The vulnerability is resolved in upstream Linux kernel commits 0f451b43, 1c428b03, and 5002beda. The patch relocates the dev_put() call to occur after NF_HOOK returns inside transport_finish and adds inline reference release in non-transport exit paths.

Workarounds

Disable asynchronous crypto offload for IPsec where operationally acceptable to avoid the resume path
Limit local user access on systems that process IPsec traffic until patching is complete
Avoid frequent dynamic teardown of network devices on hosts handling active xfrm sessions

bash

# Verify the running kernel version and check distribution security advisories
uname -r
# Example: apply vendor kernel update on Debian/Ubuntu
sudo apt update && sudo apt install --only-upgrade linux-image-$(uname -r)
sudo reboot