Kubernetes Mitigation for Copy Fail Vulnerability CVE-2026-31431

Purpose

This document describes a Kubernetes DaemonSet used to apply a temporary mitigation for the Linux kernel Copy Fail vulnerability, tracked as CVE-2026-31431.

The DaemonSet is designed to run once on every Kubernetes node and disable the vulnerable algif_aead kernel module by writing a modprobe rule onto the host filesystem and attempting to unload the module if it is already loaded.

This is intended as a short-term mitigation only. The preferred long-term remediation is to apply the relevant vendor kernel and package updates, then reboot or roll nodes as required.

Background

Copy Fail, CVE-2026-31431, is a Linux kernel local privilege escalation vulnerability associated with the kernel cryptographic subsystem, specifically the algif_aead interface.

A successful exploit may allow a local unprivileged user or compromised workload with sufficient local execution capability to escalate privileges on an affected Linux host.

In a Kubernetes environment, the primary concern is that a compromised container, overly permissive workload, or attacker with local execution on a node could attempt to exploit the host kernel.

Because Kubernetes worker nodes share the host kernel across workloads, node-level mitigation is required.

Mitigation Summary

The mitigation disables loading of the vulnerable algif_aead kernel module on every Kubernetes node.

The DaemonSet performs the following actions:

1. Runs one privileged pod on each Kubernetes node.
2. Enters the host namespaces using nsenter.
3. Writes a modprobe configuration file to the host /etc/modprobe.d/disable-algif_aead.conf
4. Adds the following rule: install algif_aead /bin/false
5. Attempts to unload the module if currently loaded: rmmod algif_aead || true
6. Verifies whether the module is still loaded using lsmod.
7. Keeps the pod running with sleep infinity so that the mitigation pod remains visible on each node.

Kubernetes DaemonSet Manifest

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: copy-fail-fix
  namespace: kube-system
spec:
  selector:
    matchLabels:
      name: copy-fail-fix
  template:
    metadata:
      labels:
        name: copy-fail-fix
    spec:
      hostPID: true # Required to see host processes
      hostNetwork: true
      containers:
      - name: fix
        image: alpine
        securityContext:
          privileged: true
        command: ["/bin/sh", "-c"]
        args:
          - |
            echo "Applying CVE-2026-31431 mitigation..."
            # Write to the host filesystem via /proc/1/root
            nsenter -t 1 -m -u -n -i -- \
              sh -c 'echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif_aead.conf && rmmod algif_aead || true'
            echo "Fix applied. Verification:"
            nsenter -t 1 -m -u -n -i -- lsmod | grep algif_aead || echo "Module successfully disabled."
            sleep infinity

Why a DaemonSet Is Used

A DaemonSet is appropriate because the mitigation must be applied to every Kubernetes node, not just to a single application pod.

Kubernetes workloads share the node kernel. Applying this configuration inside a normal container would only affect the container filesystem, not the host. This DaemonSet uses privileged access and namespace entry to make the change directly on the host operating system.

Security Characteristics

This DaemonSet intentionally uses elevated privileges.

The following settings are required for this mitigation:

Because this pod is privileged, it should be treated as sensitive operational tooling. It should only be deployed by cluster administrators and should be removed once permanent patching is complete.

Deployment Procedure

1. Save the manifest

Save the manifest as:

copy-fail-fix-daemonset.yaml

2. Apply the DaemonSet

kubectl apply -f copy-fail-fix-daemonset.yaml

3. Confirm the DaemonSet is running

kubectl get daemonset copy-fail-fix -n kube-system

Expected result:

NAME            DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE
copy-fail-fix   <nodes>   <nodes>   <nodes> <nodes>      <nodes>

4. Confirm pods are running on all expected nodes

kubectl get pods -n kube-system -l name=copy-fail-fix -o wide

Each eligible node should have one copy-fail-fix pod.

Verification

Check pod logs

kubectl logs -n kube-system -l name=copy-fail-fix

Expected log output should include:

Applying CVE-2026-31431 mitigation...
Fix applied. Verification:
Module successfully disabled.

If algif_aead still appears in the output, the module may still be loaded on that node.

Verify directly from a node

If node access is available, run:

lsmod | grep algif_aead

Expected result:

<no output>

Confirm the modprobe rule exists:

cat /etc/modprobe.d/disable-algif_aead.conf

Expected result:

install algif_aead /bin/false

Verify through the DaemonSet pod

Select a pod:

kubectl get pods -n kube-system -l name=copy-fail-fix

Then run:

kubectl exec -n kube-system <pod-name> -- \
  nsenter -t 1 -m -u -n -i -- lsmod | grep algif_aead

Expected result:

<no output>

Operational Notes

• This mitigation prevents future loading of algif_aead using modprobe.
• The DaemonSet also attempts to unload the module immediately with rmmod.
• If the module is in use, rmmod may fail. The command uses || true so the script continues even if unloading fails.
• Nodes should still receive the official kernel or vendor security update.
• A reboot or node replacement may still be required depending on the vendor patching process.
• This DaemonSet does not patch the kernel; it only disables a vulnerable kernel module path.

Risks and Considerations

Privileged access

The DaemonSet runs as a privileged workload in kube-system. This is powerful and should only be applied by trusted administrators.

Compatibility

Disabling algif_aead may affect software that relies on the Linux AF_ALG AEAD crypto interface. This is uncommon for typical Kubernetes application workloads, but it should be checked in environments with specialist cryptographic, storage, VPN, or security software.

Persistence

The modprobe configuration file is written to the node filesystem and should persist across reboot, provided the node filesystem is not ephemeral.

For managed Kubernetes node groups that replace nodes from a base image, the DaemonSet should remain deployed until the underlying image or launch template includes the official fix.

Temporary mitigation

This should not be considered a substitute for kernel patching. It is a compensating control to reduce exposure until nodes are fully updated.

Rollback Procedure

Rollback should only be performed after the affected nodes have been patched or the risk has otherwise been accepted.

1. Remove the DaemonSet

kubectl delete daemonset copy-fail-fix -n kube-system

2. Remove the modprobe configuration from each node

On each node:

sudo rm -f /etc/modprobe.d/disable-algif_aead.conf

3. Reload module configuration if required

sudo depmod -a

4. Confirm the node has the permanent fix

Verify the installed kernel/package version against vendor guidance before allowing the module to be loaded again.

Troubleshooting

DaemonSet pods are not created

Check whether the DaemonSet exists:

kubectl get daemonset copy-fail-fix -n kube-system

Check events:

kubectl get events -n kube-system --sort-by=.lastTimestamp

Pods are pending

Check scheduling problems:

kubectl describe pod -n kube-system <pod-name>

Common causes include node taints, insufficient resources, or admission policies blocking privileged pods.

Pods fail due to Pod Security admission

The DaemonSet requires privileged access. If the cluster enforces restricted Pod Security standards, the workload may be blocked.

This mitigation must be deployed using an approved administrative exception or an appropriate privileged system namespace policy.

nsenter is not found

The manifest uses the alpine image. If the image does not include nsenter in the deployed environment, use an image that includes util-linux, or build a minimal internal image containing nsenter.

Alternative images may include:

• A trusted internal operations image
• A hardened image containing util-linux
• A distro image with nsenter installed

Module remains loaded

If logs still show algif_aead, the module may be in use and cannot be unloaded immediately.

Actions:

1. Confirm the modprobe rule was written successfully.
2. Drain and reboot the node when operationally safe.
3. Confirm the module is not loaded after reboot.
4. Apply the official vendor patch.

Recommended Follow-Up Actions

After applying this DaemonSet:

1. Identify all affected Kubernetes node pools.
2. Apply vendor kernel and package updates.
3. Roll or reboot nodes according to the cluster maintenance process.
4. Confirm the vulnerable kernel/module path is no longer exploitable.
5. Remove the DaemonSet after permanent remediation is complete.
6. Remove the modprobe configuration only when vendor guidance confirms it is safe.
7. Record the mitigation and patch status in the incident or change record.

Change Record Template

Change: Apply Kubernetes DaemonSet mitigation for Copy Fail CVE-2026-31431

Reason:
Temporary mitigation for Linux kernel local privilege escalation vulnerability affecting Kubernetes worker nodes.

Action taken:
Deployed privileged DaemonSet copy-fail-fix in kube-system to disable algif_aead module loading and attempt module unload on each node.

Validation:
- DaemonSet desired/current/ready counts confirmed
- Pod logs reviewed
- algif_aead not present in lsmod output on target nodes
- /etc/modprobe.d/disable-algif_aead.conf present on target nodes

Rollback:
Delete DaemonSet and remove /etc/modprobe.d/disable-algif_aead.conf from nodes after permanent vendor patching is confirmed.

Owner:
<name/team>

Date:
<date>

Status

This DaemonSet provides a node-level mitigation for CVE-2026-31431 by disabling the vulnerable algif_aead module path. It should remain in place only until all Kubernetes nodes have received the official kernel or vendor security fix and have been rebooted or replaced as required.

Updated on: 08/05/2026