Copy Fail (CVE-2026-31431): Critical Linux Kernel Vulnerabil

Copy Fail (CVE-2026-31431): Critical Linux Kernel Vulnerability Lets Any User Become Root

What Happened

On April 29, 2026, a critical Linux kernel vulnerability named “Copy Fail” (CVE-2026-31431) was publicly disclosed. This is a local privilege escalation vulnerability affecting essentially every mainstream Linux distribution shipped since 2017. What makes this vulnerability particularly concerning is that it can be exploited with a tiny 732-byte Python script that requires no special conditions, race conditions, or kernel debugging features.

The vulnerability affects major enterprise Linux distributions including Ubuntu 24.04 LTS, Amazon Linux 2023, Red Hat Enterprise Linux 10.1, and SUSE Linux Enterprise. Public proof-of-concept exploits are already available, making this an immediate threat to unpatched systems.

Technical Mechanism: How the Vulnerability Works

Copy Fail exists in the Linux kernel’s algif_aead module, which is the AEAD socket interface of the kernel’s userspace crypto API (AF_ALG). The flaw was introduced in 2017 through commit 72548b093ee3, which implemented a performance optimization for AEAD operations.

Here’s how the attack works step by step:

1. AF_ALG socket binding: An unprivileged user creates an AF_ALG socket bound to the authencesn algorithm (which combines HMAC-SHA256 with AES-CBC)
2. File splicing: The attacker uses splice() to copy page cache pages from a setuid binary (like /usr/bin/su) into the crypto pipeline
3. Controlled memory corruption: When the crypto operation processes the data, it performs a 4-byte write to memory that ends up modifying the page cache of the target setuid binary
4. Root shell execution: The attacker can stage shellcode in the corrupted page cache, and when the legitimate setuid binary is executed, it runs the attacker’s code instead of the original program

Unlike previous kernel vulnerabilities that required precise timing conditions, Copy Fail is a straight-line logic flaw that works reliably across different Linux distributions without requiring any race conditions.

Affected Systems and Distribution Impact

Any system running a vulnerable Linux kernel is at risk. The vulnerability affects kernels built between 2017 and the availability of the patch, which covers nearly all mainstream Linux distributions released in the past 9 years.

Directly Verified Affected Distributions:

• Ubuntu 24.04 LTS (kernel 6.17.0-1007-aws)
• Amazon Linux 2023 (kernel 6.18.8-9.213.amzn2023)
• Red Hat Enterprise Linux 10.1 (kernel 6.12.0-124.45.1.el10_1)
• SUSE Linux Enterprise 16 (kernel 6.12.0-160000.9-default)

Other Potentially Affected Distributions:

• Debian
• Arch Linux
• Fedora
• Rocky Linux
• AlmaLinux
• Oracle Linux
• Various embedded Linux distributions

Risk Assessment by Environment

The severity of Copy Fail varies significantly depending on the environment and threat model. Multi-tenant systems and cloud infrastructure face the highest risks, while single-user workstations have lower immediate impact but still require attention.

High-Risk Environments

Multi-tenant Linux Hosts

Shared development boxes, shell-as-a-service platforms, and jump servers represent the highest risk scenario. In these environments, any unprivileged user can exploit Copy Fail to gain root access, potentially compromising other users’ data and applications.

Kubernetes and Container Clusters

The page cache sharing mechanism makes container environments particularly vulnerable. A single compromised pod can potentially escape its container and gain access to the underlying host, leading to cluster-wide compromise. The vulnerability affects both container runtimes and orchestration platforms.

CI/CD Runners and Build Farms

Continuous integration and deployment systems are prime targets. Untrusted pull request code or build artifacts can exploit Copy Fail to gain root access on shared build infrastructure, potentially leading to supply chain attacks or code injection.

Medium-Risk Environments

Cloud SaaS Platforms

Infrastructure-as-a-service and platform-as-a-service providers face medium risk. Tenant isolation is typically stronger than in multi-tenant shared hosts, but compromised tenant applications can still exploit the vulnerability to gain elevated privileges.

Standard Linux Servers

Traditional single-tenant production servers face medium risk primarily as a post-exploitation vector. Once an attacker gains initial access through other means (web application compromise, stolen credentials), Copy Fail provides a reliable method to escalate privileges to root.

Lower-Risk Environments

Single-User Workstations

Desktop and laptop systems face the lowest immediate risk since the user already has full access to the system. However, the vulnerability still represents a risk in scenarios where malware or unauthorized access could be introduced.

Current Status and Patch Availability

As of May 1, 2026, patch status varies by distribution. The upstream fix (commit a664bf3d603d) was committed to the mainline Linux kernel on April 1, 2026, and reverts the 2017 optimization that caused the vulnerability.

Fixed Kernel Versions

• Linux kernel 7.0
• Linux kernel 6.19.12
• Linux kernel 6.18.22

Patch Status by Distribution (as of May 1, 2026)

Immediate Mitigation Steps

While waiting for official patches from your distribution, you should apply these interim mitigations immediately. The following steps disable the vulnerable algif_aead module and implement compensating controls.

Kernel Module Disabling

For systems where you have root access, create a persistent configuration to disable the algif_aead module:

echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif.conf
rmmod algif_aead 2>/dev/null || true

This configuration persists across reboots and prevents the vulnerable module from loading.

Impact Assessment: What This Breaks

Services and Applications That Will NOT Be Affected:

• dm-crypt/LUKS full disk encryption
• Kernel TLS (kTLS) for encrypted network traffic
• IPsec and XFRM kernel encryption
• OpenSSL, GnuTLS, and NSS default builds
• SSH protocol implementations
• Kernel keyring cryptographic operations
• Most standard cryptographic libraries and applications

Potentially Affected Applications:

• Applications specifically configured to use the AF_ALG interface
• OpenSSL with the afalg engine explicitly enabled
• Some embedded crypto offload implementations
• Applications that bind AEAD/skcipher/hash sockets directly
• Specialized cryptographic acceleration tools

To determine if your system uses AF_ALG interfaces:

# Check active AF_ALG sockets
lsof | grep AF_ALG

# Or check network socket details
ss -xa | grep ALG

Container Runtime Protections

For containerized environments, implement seccomp profiles to block AF_ALG socket creation. This provides an additional layer of defense even if the kernel remains unpatched.

Docker seccomp Configuration:

Create a custom seccomp profile that blocks AF_ALG socket creation:

{
  "defaultAction": "SCMP_ACT_ALLOW",
  "syscalls": [
    {
      "action": "SCMP_ACT_ERRNO",
      "args": [],
      "comment": "Block AF_ALG socket creation for vulnerability protection",
      "includes": {},
      "excludes": {},
      "syscall": "socket",
      "args": [
        {
          "index": 0,
          "op": "SCMP_CMP_EQ",
          "value": 38,
          "valueTwo": 0
        }
      ]
    }
  ]
}

Kubernetes seccomp Profiles:

For Kubernetes clusters, create namespace-specific seccomp profiles or use the PodSecurity admission controller to restrict AF_ALG usage in untrusted workloads.

Detection and Monitoring Strategies

System-Level Monitoring

Implement comprehensive monitoring to detect attempts to exploit Copy Fail. Key indicators include:

• Creation of AF_ALG sockets by non-cryptographic applications
• Privilege escalation events following crypto operations
• Unexpected modifications to setuid binaries
• Suspicious process execution patterns

Runtime Detection Rules

For organizations using advanced security tools like Sysdig Secure or Falco, implement these detection rules:

Sysdig Secure Integration:
The “AF_ALG Page Cache Poisoning Leading to Privilege Escalation” rule is automatically included in the Sysdig Runtime Behavioral Analytics managed policy. This provides comprehensive detection and response capabilities.

Falco Rules for Open Source Environments:

# Define known legitimate AF_ALG users
list: known_af_alg_binaries
  items: [cryptsetup, "systemd-cryptse", "systemd-cryptsetup", veritysetup, 
          integritysetup, "cryptsetup-resh", kcapi-enc, kcapi-dgst, 
          kcapi-rng, kcapi-sym]

# Macro for successful AF_ALG socket creation
macro: successful_af_alg_socket
  condition: >
    evt.type = socket and
    evt.rawres >= 0 and
    (evt.arg.domain contains AF_ALG or evt.rawarg.domain = 38)

# Macro for AF_ALG SEQPACKET socket (required for AEAD operations)
macro: successful_af_alg_seqpacket_socket
  condition: >
    successful_af_alg_socket and
    (evt.arg.type = 5 or
    evt.arg.type = 2053 or
    evt.arg.type = 524293 or
    evt.arg.type = 526341)

# Main detection rule
rule: Unexpected Process Using Kernel AEAD Crypto Socket
  desc: >
    Detects creation of an AF_ALG SEQPACKET socket - the kernel AEAD crypto API interface 
    exclusively required by AEAD operations (authencesn, ccm, gcm) - from a process outside 
    the known disk-encryption toolchain. This is the mandatory first step of CVE-2026-31431, 
    a Linux kernel LPE that uses AF_ALG + splice() to corrupt a SUID binary in the page cache.
  condition: >
    successful_af_alg_seqpacket_socket and
    not proc.name in (known_af_alg_binaries)
  output: >
    CRITICAL: Unexpected process %proc.name (PID %proc.pid) opened AF_ALG AEAD 
    kernel crypto socket with parent %proc.pname under user %user.loginname
    (socket.domain=%evt.arg.domain socket.type=%evt.arg.type 
     proc.exe=%proc.exe proc.name=%proc.name 
     container.name=%container.name image=%container.image.repository:%container.image.tag
     proc.cmdline=%proc.cmdline user=%user)
  priority: CRITICAL
  tags: [host, container, kernel, cve, CVE-2026-31431, MITRE, 
          MITRE_TA0004_privilege_escalation, 
          MITRE_T1068_exploitation_for_privilege_escalation]

Comparison to Previous Kernel Vulnerabilities

Technical Comparison:

Exploitation Comparison:

Dirty Pipe (CVE-2022-0847):

• Required precise pipe buffer manipulation
• Version-specific targeting needed
• Dependent on race conditions
• Exploit size: ~150 lines of C code

Copy Fail (CVE-2026-31431):

• Straight-line logic flaw
• Works across all distributions since 2017
• No race conditions required
• Exploit size: 732 bytes of Python

Recommended Action Plan

Immediate Actions (Next 24-48 Hours):

1. System Inventory: Create comprehensive inventory of all Linux systems and their kernel versions
2. Risk Assessment: Identify which systems are running vulnerable kernels
3. Apply Interim Mitigation: Disable algif_aead module on all vulnerable systems
4. Implement Detection: Deploy runtime monitoring rules to detect exploitation attempts
5. Container Protection: Block AF_ALG socket creation in container environments
6. Alert Configuration: Set up alerts for suspicious privilege escalation events

Short-term Actions (Next 1-2 Weeks):

1. Patch Testing: Test official patches from your distribution vendor in non-production environments
2. Patch Deployment: Develop and execute patch rollout plan for all production systems
3. Container Updates: Update container images and seccomp profiles with new protections
4. Incident Response: Review and update incident response procedures for this vulnerability
5. Security Architecture Review: Assess overall security architecture for similar vulnerabilities

Long-term Actions (Next 1-3 Months):

1. Update Procedures: Implement accelerated kernel patch procedures for critical vulnerabilities
2. Security Controls: Deploy additional security controls for multi-tenant environments
3. Regular Testing: Implement regular vulnerability testing and validation procedures
4. Architecture Hardening: Review and implement architectural hardening measures
5. Training and Awareness: Conduct security training for development and operations teams

FAQ and Frequently Asked Questions

Q: How can I determine if my specific system is vulnerable to Copy Fail?

A: You can check your kernel version with uname -r. If your kernel version falls in the range 4.14 to 7.0-rc (excluding the fixed versions 6.18.22, 6.19.12, and 7.0), your system is vulnerable. You can also check if the algif_aead module is loaded with lsmod | grep algif_aead. The most definitive test is to run the exploit code on a non-production system you own, but this should only be done in isolated test environments.

Q: Does disabling algif_aead affect encryption performance or functionality?

A: No, disabling algif_aead has minimal impact on most systems. It does not affect standard encryption implementations like dm-crypt/LUKS, kernel TLS (kTLS), IPsec, or most cryptographic libraries. The only potential impact is on applications that specifically use the AF_ALG interface, which is uncommon in most deployments. You can verify if any applications on your system use AF_ALG with lsof | grep AF_ALG or ss -xa | grep ALG.

Q: What additional protections should I implement for Kubernetes environments?

A: For Kubernetes, implement seccomp profiles that block AF_ALG socket creation, use Pod Security Admission to restrict privileged containers, and consider security contexts that limit capabilities. Monitor for suspicious AF_ALG usage and implement network policies to restrict pod-to-host communication. Additionally, consider runtime security tools that can detect anomalous behavior in container environments.

Q: How quickly do organizations need to patch systems against Copy Fail?

A: This should be treated as a critical security priority with immediate action required. Given that there’s a working public exploit and the vulnerability allows any unprivileged user to gain root access, patching should be completed within hours to days depending on risk level. Multi-tenant systems, Kubernetes nodes, CI/CD runners, and cloud infrastructure should be patched first.

Q: Are there any alternatives to kernel patches for production systems that cannot be patched immediately?

A: Yes, several compensating controls can be implemented: disable the algif_aead module as described earlier, implement runtime detection monitoring, use container security controls to block AF_ALG usage, and consider additional access controls like SELinux or AppArmor profiles. Network segmentation and host-based firewalls can also help limit the blast radius if exploitation occurs.

References and Further Reading

Official Sources:

• Copy Fail Research Website – Official vulnerability details and PoC
• CERT-EU Security Advisory 2026-005 – Official EU cybersecurity assessment
• NIST CVE Database – Official vulnerability entry

Technical Analysis:

• Sysdig Threat Research – Comprehensive technical analysis
• Bugcrowd Analysis – Security perspective and exploitation details
• Official Exploit Repository – Reference implementation and source code

Vendor Advisories:

• AlmaLinux Security Notice
• CloudLinux Security Advisory
• Ubuntu Security Notice
• SUSE Security Advisory
• Red Hat Security Advisory

Additional Resources:

• Xint Code Research Blog – Original discovery and technical details
• Help Net Security – Industry analysis and impact assessment
• OVHcloud Blog – Cloud-specific mitigation strategies