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How to Mitigate Copy Fail (CVE-2026-31431) with Seccomp

Mitigate the Copy Fail (CVE-2026-31431) vulnerability in Linux kernels. Learn how to implement seccomp profiles for Kubernetes to prevent local privilege escalation.

How to Mitigate Copy Fail (CVE-2026-31431) with Seccomp
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Fixing Copy Fail (CVE-2026-31431): Seccomp Hardening Guide

The Copy Fail vulnerability, identified as CVE-2026-31431, allows local privilege escalation in Linux kernels (versions 4.14 to 6.19.12). This flaw is found within the algif_aead module, which is part of the kernel’s AF_ALG (Address Family for Algorithms) subsystem. A malicious local user or a compromised container can exploit this to gain root privileges on the host system, severely compromising isolation in shared environments like Kubernetes clusters or multi-tenant CI/CD pipelines. Immediate mitigation is essential to prevent container breakouts and secure system operations. This guide details how to implement a specific seccomp profile to block the vulnerable syscalls and deploy it effectively in Kubernetes.

Understanding CVE-2026-31431 (Copy Fail)

CVE-2026-31431 is a local privilege escalation vulnerability affecting Linux kernel versions from 4.14 up to 6.19.12. The flaw exists within the algif_aead module, a component of the AF_ALG subsystem. This subsystem provides a user-space interface to kernel-side cryptographic algorithms. The vulnerability, known as “Copy Fail,” enables an attacker to corrupt kernel memory, resulting in local privilege escalation.

The issue arises when handling specific AF_ALG socket operations. A specially crafted sequence of operations can trigger a kernel heap overflow, allowing an attacker to write arbitrary data into sensitive kernel memory. This capability is then used for privilege escalation, as it can alter kernel data structures or inject malicious code that executes with elevated privileges. The impact is significant because it enables a low-privileged user or process to gain root access on the host system.

Impact and Risk Analysis

The implications of CVE-2026-31431 are far-reaching, especially in environments where multiple tenants or untrusted workloads share a single Linux kernel.

Key risks include:

  • Host Compromise: An attacker can elevate privileges from a standard user to root on the affected Linux host. This provides full control over the operating system, including access to all data, installation of rootkits and manipulation of system services.
  • Container Breakouts: In containerized environments like Docker or Kubernetes, a compromised container can exploit this vulnerability to escape its isolation boundary and gain root access to the underlying host. This entirely undermines the security benefits of containerization. This is a critical risk you must address quickly. You can read more about preventing other types of container escapes in our article, How to Fix CVE-2026-43284: Preventing Dirty Frag Pod Escapes.
  • Kubernetes Cluster Takeover: If an attacker gains root access to a Kubernetes node, they could potentially access sensitive credentials, manipulate Kubelets and pivot to other nodes or even the control plane, leading to a complete cluster compromise.
  • CI/CD Pipeline Exploitation: Multi-tenant CI/CD runners often execute untrusted code in containers on shared hosts. An attacker could embed malicious code in a build job to escape the container and compromise the build agent, potentially leading to supply chain attacks.

A critical point is that even with hardened container runtimes, fundamental kernel vulnerabilities like Copy Fail expose a shared surface. While a container runtime like containerd or CRI-O provides isolation, they still rely on the underlying Linux kernel. If the kernel itself has a privilege escalation flaw, the container isolation can be bypassed. This makes kernel patching and proactive mitigations like seccomp essential layers of defense.

Introduction to Seccomp for Linux Hardening

Seccomp, or Secure Computing mode, is a Linux kernel security feature that allows a process to restrict the system calls it can make. It acts as a filter, defining a whitelist or blacklist of syscalls a process is permitted to execute. If a process attempts a disallowed syscall, the kernel can terminate it, send a signal or return an error.

When seccomp is enabled for a process, you provide a seccomp profile. This JSON document specifies the defaultAction for syscalls not explicitly listed and then lists specific syscalls with their permitted action and any required args filters. This fine-grained control lets you harden processes by removing access to kernel functionalities they do not legitimately need. For example, a web server typically does not need to interact with raw network sockets or kernel modules, so these syscalls can be blocked.

Seccomp is effective for mitigating kernel vulnerabilities because it can prevent a compromised application from triggering the vulnerable kernel code path. By denying access to the specific syscalls or arguments used by an exploit, even if the application has a bug, it cannot reach the vulnerable kernel function. This reduces the attack surface of applications and containers, making it a key component in a defense-in-depth strategy. More details on seccomp can be found in the official Kubernetes documentation on seccomp profiles.

Seccomp Mitigation Strategy for Copy Fail

The Copy Fail (CVE-2026-31431) vulnerability is rooted in the AF_ALG subsystem, specifically accessed via the socket and socketpair syscalls with the AF_ALG address family. To mitigate this vulnerability using seccomp, the strategy is simple: block any attempts to create AF_ALG sockets.

This is achieved by applying a seccomp profile that restricts the use of socket and socketpair syscalls when called with AF_ALG (which corresponds to the numerical value 38 for the domain argument on x86-64 Linux). By specifically denying this combination, you prevent malicious processes, including those within compromised containers, from interacting with the vulnerable kernel module. This mitigation works without fully disabling the AF_ALG module or immediately updating the kernel, providing a critical temporary fix until patches become available.

This approach offers surgical precision. Instead of broadly disabling syscalls that other applications might need, you are targeting only the specific interaction vector for CVE-2026-31431. This minimizes the risk of breaking legitimate application functionality while effectively neutralizing the exploit path.

Crafting the copy-fail-deny.json Seccomp Profile

The following seccomp profile blocks AF_ALG operations associated with CVE-2026-31431. Save this content as copy-fail-deny.json.

{
  "defaultAction": "SCMP_ALLOW",
  "architectures": [
    "SCMP_ARCH_X86_64",
    "SCMP_ARCH_AARCH64"
  ],
  "syscalls": [
    {
      "names": [
        "socket",
        "socketpair"
      ],
      "action": "SCMP_SYSRET_EPERM",
      "args": [
        {
          "index": 0,
          "value": 38,
          "valueTwo": 0,
          "op": "SCMP_CMP_EQ"
        }
      ]
    }
  ]
}

Here is a breakdown of the profile’s components:

  • defaultAction: "SCMP_ALLOW": This setting permits any syscall not explicitly listed in the syscalls array. This ensures legitimate applications continue to function normally. For specific vulnerability mitigation, layering this rule on top of a base profile with SCMP_ALLOW is often safer than a complete SCMP_DENY and whitelisting approach.
  • architectures: Specifies the CPU architectures this profile applies to. X86_64 and AARCH64 cover most modern Linux systems.
  • syscalls: This array contains the specific rules for syscall filtering.
    • names: ["socket", "socketpair"]: These are the syscalls being targeted, used to create new sockets.
    • action: "SCMP_SYSRET_EPERM": If a syscall matches the defined criteria, this action will occur. SCMP_SYSRET_EPERM means the syscall returns an EPERM (Operation not permitted) error code to the calling process, denying the operation without crashing the process.
    • args: This section allows filtering based on syscall arguments.
      • index: 0: Refers to the first argument of the socket or socketpair syscall.
      • value: 38: On Linux x86-64, the AF_ALG address family constant is 38. This condition checks if the first argument (the domain) of the socket or socketpair call is AF_ALG.
      • op: "SCMP_CMP_EQ": Specifies an “equals” comparison operation.

This profile effectively blocks the creation of AF_ALG sockets by returning EPERM when a process attempts to call socket or socketpair with AF_ALG as the domain.

Merging Profiles

Many environments, especially Kubernetes, already use a default seccomp profile (often runtime/default from containerd or Docker). You should not replace these existing profiles with copy-fail-deny.json. Instead, you must integrate this specific rule into your existing base profile.

If your environment uses the default runtime/default seccomp profile, you would typically download it, add the socket/socketpair rule for AF_ALG and then use the modified profile. Tools like seccomp-tools or manual JSON manipulation are required for this. For a rapid mitigation, however, deploying this specific profile with a defaultAction of SCMP_ALLOW can work directly if no other socket or socketpair rules conflict.

Deploying Seccomp Profiles in Kubernetes

Deploying a custom seccomp profile across a Kubernetes cluster involves two main steps: making the profile available on each node and then instructing your pods to use it. A DaemonSet is well-suited for the first part, ensuring the profile is present on every node.

Step 1: Create a ConfigMap for the Seccomp Profile

First, create a Kubernetes ConfigMap from your copy-fail-deny.json file. This stores the seccomp profile content in the cluster.

kubectl create configmap copy-fail-seccomp-profile --from-file=copy-fail-deny.json -n kube-system

Verify the ConfigMap was created:

kubectl get configmap copy-fail-seccomp-profile -n kube-system -o yaml

You should see the content of copy-fail-deny.json under the data key.

Step 2: Deploy a DaemonSet to Distribute the Profile

Next, deploy a DaemonSet that mounts this ConfigMap onto each node’s filesystem at a well-known path, typically /var/lib/kubelet/seccomp/profiles/. Kubelet expects to find custom seccomp profiles in this location.

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: seccomp-profile-distributor
  namespace: kube-system
  labels:
    app: seccomp-profile-distributor
spec:
  selector:
    matchLabels:
      app: seccomp-profile-distributor
  template:
    metadata:
      labels:
        app: seccomp-profile-distributor
    spec:
      tolerations:
      - operator: Exists
      hostPID: true # Required to write to host filesystem
      containers:
      - name: profile-copier
        image: busybox:1.36.1 # Use a lightweight image for copying
        command: ["sh", "-c"]
        args:
        - |
          mkdir -p /host/var/lib/kubelet/seccomp/profiles/copy-fail-deny;
          cp /profiles/copy-fail-deny.json /host/var/lib/kubelet/seccomp/profiles/copy-fail-deny/copy-fail-deny.json;
          echo "Seccomp profile copied successfully.";
          tail -f /dev/null # Keep the container running
        securityContext:
          privileged: true # Required for host filesystem access
        volumeMounts:
        - name: host-kubelet-path
          mountPath: /host/var/lib/kubelet
        - name: seccomp-profile-config
          mountPath: /profiles
      volumes:
      - name: host-kubelet-path
        hostPath:
          path: /var/lib/kubelet
          type: Directory
      - name: seccomp-profile-config
        configMap:
          name: copy-fail-seccomp-profile

Apply this DaemonSet:

kubectl apply -f seccomp-daemonset.yaml

Once deployed, verify the DaemonSet pods are running and the profile exists on your nodes. SSH into a node and check:

# On a Kubernetes node (e.g., via 'gcloud compute ssh node-name')
sudo ls /var/lib/kubelet/seccomp/profiles/copy-fail-deny/

You should see copy-fail-deny.json listed.

Step 3: Apply the Seccomp Profile to Your Pods

Now, apply this custom seccomp profile to your Kubernetes pods. For Kubernetes v1.19 and later, use the securityContext.seccompProfile field in your Pod or container definition.

The type must be Localhost and localhostProfile should be the path relative to Kubelet’s seccomp profiles directory.

Here is an example of how to apply it to a test pod:

apiVersion: v1
kind: Pod
metadata:
  name: seccomp-test-pod
  labels:
    app: seccomp-test
spec:
  containers:
  - name: my-container
    image: busybox:1.36.1
    command: ["sh", "-c", "echo 'Container running with seccomp profile.' && sleep 3600"]
    securityContext:
      seccompProfile:
        type: Localhost
        localhostProfile: 'profiles/copy-fail-deny/copy-fail-deny.json'
  restartPolicy: Never

Apply the pod:

kubectl apply -f seccomp-test-pod.yaml

For production applications, you would typically integrate this securityContext into your deployment manifests. Consider using Pod Security Admission (PSA) or validating admission webhooks to enforce this profile across your cluster for all new workloads, ensuring consistent application of the mitigation. This helps prevent pods from being deployed without the necessary security hardening. For new teams starting with Kubernetes, understanding how to apply these security contexts is a fundamental step after learning to deploy your first application.

Verifying Seccomp Enforcement

Confirming that your seccomp profile is correctly applied and actively enforcing syscall restrictions is critical.

1. Check Pod Description

After deploying a pod with the seccomp profile, inspect its describe output:

kubectl describe pod seccomp-test-pod

Look for the seccompProfile section in the Security Context output:

...
Containers:
  my-container:
    Container ID:   containerd://...
    Image:          busybox:1.36.1
    Image ID:       docker.io/library/busybox@sha256:...
    Port:           <none>
    Host Port:      <none>
    Command:
      sh
      -c
      echo 'Container running with seccomp profile.' && sleep 3600
    State:          Running
      Started:      Mon, 15 Jul 2024 10:30:00 -0700
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:         <none>
  Conditions:
    Type              Status
    Initialized       True
    Ready             True
    ContainersReady   True
    PodScheduled      True
  Volumes:            <none>
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                 node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:          <none>
SeccompProfile:  Localhost -> profiles/copy-fail-deny/copy-fail-deny.json

The SeccompProfile: Localhost -> profiles/copy-fail-deny/copy-fail-deny.json line confirms Kubernetes is attempting to apply the profile.

2. Test Syscall Restriction using a test binary

The ultimate test is to try to perform the blocked operation from inside a pod that should be restricted. Since busybox does not include strace by default, a minimal C program can be used to attempt AF_ALG socket creation.

First, create a simple C program named test_af_alg.c:

#include <stdio.h>
#include <sys/socket.h>
#include <errno.h>
#include <string.h>
#include <linux/if_alg.h> // For AF_ALG
#include <unistd.h>     // For close

int main() {
    // AF_ALG is 38 on x86-64 Linux.
    // SOCK_SEQPACKET is often used with AF_ALG for authenticated encryption.
    int sock = socket(AF_ALG, SOCK_SEQPACKET, 0); 
    if (sock == -1) {
        perror("Failed to create AF_ALG socket");
        printf("Error: %s\n", strerror(errno));
        if (errno == EPERM) {
            printf("AF_ALG socket creation blocked by seccomp (expected).\n");
            return 0; // Success for the test, as it was blocked
        }
        return 1; // Unexpected error
    }
    printf("Successfully created AF_ALG socket (UNEXPECTED, seccomp failed).\n");
    close(sock); // Clean up the socket if created
    return 1; // Failure, seccomp did not block
}

Now, build a Docker image for this test. Replace your-registry/your-repo with your actual registry path.

# Dockerfile
FROM gcc:12.3.0-bullseye

WORKDIR /app

COPY test_af_alg.c .

RUN gcc -o test_af_alg test_af_alg.c

CMD ["./test_af_alg"]

Build and push the image to your container registry:

docker build -t your-registry/your-repo/af-alg-tester:1.0.0 .
docker push your-registry/your-repo/af-alg-tester:1.0.0

Now, deploy a pod using this image, applying the seccomp profile:

apiVersion: v1
kind: Pod
metadata:
  name: af-alg-blocked-test-pod
spec:
  containers:
  - name: af-alg-tester
    image: your-registry/your-repo/af-alg-tester:1.0.0
    securityContext:
      seccompProfile:
        type: Localhost
        localhostProfile: 'profiles/copy-fail-deny/copy-fail-deny.json'
  restartPolicy: Never

After the pod runs, check its logs:

kubectl logs af-alg-blocked-test-pod

Expected output (seccomp correctly enforced):

Failed to create AF_ALG socket: Operation not permitted
Error: Operation not permitted
AF_ALG socket creation blocked by seccomp (expected).

Unexpected output (seccomp failed):

Successfully created AF_ALG socket (UNEXPECTED, seccomp failed).

If you receive the “Operation not permitted” error, your seccomp profile is working as intended. If it succeeds, double-check your DaemonSet deployment, ConfigMap path and the seccompProfile annotation on the pod.

Long-Term Remediation: Kernel Patching

While seccomp offers an effective and immediate mitigation against CVE-2026-31431, it is fundamentally a workaround. The primary and most comprehensive solution remains updating and patching the Linux kernel to a fixed version. Seccomp prevents the exploit from leveraging the vulnerability, but it does not remove the vulnerability itself.

As soon as patches become available from your distribution maintainers (for example, Ubuntu, Red Hat, Debian), prioritize updating all your Linux hosts, including Kubernetes nodes, CI/CD runners and any other systems running affected kernel versions. Always follow your distribution’s security update procedures, which typically involve:

  1. Backup: Back up critical data and configurations before any kernel update.
  2. Staging: Test the patched kernel in a staging environment to ensure no regressions occur with your applications.
  3. Update: Apply the kernel updates using your distribution’s package manager (for example, apt upgrade linux-image-generic, yum update kernel).
  4. Reboot: A reboot is almost always required for kernel updates to take effect. Schedule this downtime carefully.

Once your kernels are patched to a version that fixes CVE-2026-31431 (versions 6.19.13 and newer, or backported patches to older stable series), you can consider removing the specific AF_ALG seccomp rule if it is no longer deemed necessary for other reasons. However, a defense-in-depth strategy often includes keeping such targeted mitigations as an additional layer of protection, even after patching.

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