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The Copy Fail vulnerability CVE-2026-31431 is a high-severity local privilege escalation flaw in the Linux kernel cryptographic subsystem that affects Linux kernel versions shipped from 2017 through 6.18.21 and 6.19.11. This Linux kernel vulnerability impacts all major distributions including Ubuntu (including 24.04 LTS), Amazon Linux 2023, Red Hat Enterprise Linux (including RHEL 10.1), SUSE 16, Debian, Fedora, Arch Linux, AlmaLinux, CloudLinux, Oracle Linux, Rocky Linux, and CentOS. The Copy Fail vulnerability also threatens Kubernetes, Docker, and LXC container hosts running affected Linux kernels, making it particularly dangerous for cloud-native infrastructure.
The Copy Fail Linux kernel flaw resides in the algif_aead module of the AF_ALG userspace crypto API and enables any unprivileged local user to gain root access through a logic defect that allows controlled memory writes into the Linux kernel page cache. Successful exploitation of the Copy Fail vulnerability results in full root privilege escalation with high impact on confidentiality, integrity, and availability. The Copy Fail vulnerability was first observed on March 23, 2026, and CISA has added CVE-2026-31431 to its Known Exploited Vulnerabilities catalog, mandating Federal Civilian Executive Branch agencies to patch by May 15, 2026.
Organizations whose isolation strategy depends on shared-kernel containers without microVM, gVisor, or dedicated-host boundaries should treat any container compromise on vulnerable Linux kernels as a potential host compromise. The Copy Fail vulnerability demonstrates exceptional exploit characteristics including broad applicability across Linux distributions, cross-architecture reliability, deterministic exploitation without race conditions, a sub-kilobyte exploit footprint, and publicly available proof-of-concept code. The combination of these factors makes the Copy Fail Linux kernel vulnerability one of the most dangerous privilege escalation flaws affecting enterprise Linux infrastructure, particularly in multi-tenant cloud environments and containerized workloads.
The Copy Fail vulnerability CVE-2026-31431 is a high-severity local privilege escalation flaw affecting the Linux kernel cryptographic subsystem. This Linux kernel vulnerability specifically targets a logic defect within the algif_aead module of the AF_ALG userspace crypto API. The Copy Fail vulnerability enables any unprivileged local user to gain root access on essentially every major Linux distribution shipped since 2017. Affected Linux distributions include Ubuntu, Amazon Linux, Red Hat Enterprise Linux, SUSE, Debian, Fedora, Arch Linux, and downstream rebuilds such as AlmaLinux, CloudLinux, Oracle Linux, Rocky Linux, and CentOS. The Copy Fail vulnerability represents a fundamental breakdown in Linux kernel memory isolation that bypasses traditional security boundaries.
The Copy Fail exploit leverages a compact 732-byte Python proof-of-concept that performs a controlled four-byte write into the Linux kernel in-memory page cache of any readable file. The Copy Fail attack typically targets setuid binaries such as /usr/bin/su to inject attacker-supplied shellcode. A critical characteristic of the Copy Fail vulnerability is that modifications never touch disk and affected pages are never marked dirty for writeback, causing file-integrity monitoring tools that rely on on-disk checksums to completely miss the tampering. When the corrupted binary is next invoked, it executes the attacker shellcode and yields a root shell. While no named threat actor has been publicly linked to in-the-wild exploitation of the Copy Fail vulnerability, Go and Rust ports of the original Python proof-of-concept have already appeared in open-source repositories, signaling rapid weaponization of the Linux kernel flaw.
The Copy Fail vulnerability resides in authencesn, an AEAD (Authenticated Encryption with Associated Data) wrapper used by IPsec for 64-bit Extended Sequence Numbers. The Linux kernel flaw is the cumulative product of three individually benign code changes spread over six years. The 2011 authencesn implementation repurposed the caller’s destination buffer as scratch space for ESN byte rearrangement. A 2015 migration to the new AEAD interface quietly introduced an out-of-bounds write at offset assoclen plus cryptlen. The 2017 in-place optimization to algif_aead.c (Linux kernel commit 72548b093ee3) chained page-cache pages from splice() into the writable destination scatterlist. None of these Linux kernel commits was unsafe in isolation, which explains why the Copy Fail vulnerability went undetected for almost a decade. This demonstrates the complexity of modern Linux kernel code and the difficulty of detecting subtle security flaws that only emerge through the interaction of multiple changes over time.
The published Copy Fail exploit has been validated against Ubuntu 24.04 LTS (6.17.0-1007-aws), Amazon Linux 2023 (6.18.8-9.213.amzn2023), RHEL 10.1 (6.12.0-124.45.1.el10_1), and SUSE 16 (6.12.0-160000.9-default). The Copy Fail exploit binary runs unmodified across Linux distributions and architectures with no per-distro offsets, no recompilation, and no version checks required. This universal reliability dramatically increases the Copy Fail vulnerability’s threat profile. Because Copy Fail exploitation requires no root privileges inside a container, no kernel modules, and no network access, the Linux kernel vulnerability slots cleanly into virtually any post-exploitation scenario including compromised CI runners, hijacked web containers, or any unprivileged foothold on multi-tenant Linux hosts.
Organizations must update to patched Linux kernel versions as the primary remediation for the Copy Fail vulnerability CVE-2026-31431. The upstream Linux kernel fix (mainline commit a664bf3d603d) reverts the algif_aead in-place optimization and is included in Linux kernel versions 6.18.22, 6.19.12, and 7.0. Distribution-specific fixed Linux kernels are available from CloudLinux (kernel-4.18.0-553.121.1 for CL7h/CL8, kernel-5.14.0-611.49.2.el9_7 for CL9, kernel-6.12.0-124.52.2.el10_1 for CL10), AlmaLinux, Ubuntu, Amazon Linux, Red Hat, and SUSE through normal kernel package update channels. System reboot is required after updating Linux kernel packages. Federal Civilian Executive Branch agencies have been mandated by CISA to apply Copy Fail vulnerability patches by May 15, 2026, underscoring the critical nature of this Linux kernel security flaw.
Organizations that cannot tolerate immediate Linux system reboots should deploy live kernel patching solutions such as KernelCare to address the Copy Fail vulnerability. KernelCare has released CVE-2026-31431 livepatches to its main feed for AlmaLinux 8/9, CloudLinux 7h/8, CentOS 8, RHEL 8/9, Rocky Linux 8/9, Oracle Linux 7/8/9 (including UEK6), Proxmox VE 7, Ubuntu Bionic/Focal/Jammy (including AWS and Azure variants), Debian 11/12, and equivalents. Subscribed Linux systems receive the Copy Fail vulnerability fix automatically on the next kcarectl update invocation, with verification available via kcarectl info grep CVE-2026-31431. Live kernel patching provides immediate Copy Fail vulnerability remediation without service interruption.
Where immediate Linux kernel patching is not possible, organizations should block AF_ALG AEAD interface registration at boot to mitigate the Copy Fail vulnerability. Use the kernel command-line parameter initcall_blacklist equals algif_aead_init applied via grubby and reboot. This closes the Copy Fail attack surface without replacing the Linux kernel and can be reversed quickly once patched kernels are installed. The modprobe-based workaround (install algif_aead /bin/false) circulating on security mailing lists does not work on RHEL-family distributions because algif_aead is compiled into the Linux kernel (CONFIG_CRYPTO_USER_API_AEAD equals y) rather than loaded as a module. Compatibility analysis confirms that dm-crypt/LUKS, kTLS, IPsec, SSH, and default OpenSSL/GnuTLS builds do not depend on AF_ALG and remain unaffected by this Copy Fail mitigation.
Organizations should treat any container remote code execution incident on vulnerable Linux kernels as a potential host compromise and enforce rapid node recycling after compromise indicators are detected. Because the Linux page cache is shared between containers and the host operating system, namespace-based isolation does not contain the Copy Fail vulnerability. For workloads executing untrusted code including multi-tenant Kubernetes clusters, CI/CD runners accepting pull requests from forks, and AI agent code-execution sandboxes, organizations should evaluate migration to hardware-or-VM isolation boundaries. Recommended isolation technologies include Firecracker microVMs, gVisor userspace kernels, or dedicated-host configurations that do not share a Linux kernel across tenants. This architectural change provides defense-in-depth protection against Linux kernel vulnerabilities like Copy Fail that bypass traditional container security boundaries.
T1082: System Information Discovery – Attackers perform Linux system information discovery to identify vulnerable kernel versions susceptible to the Copy Fail exploit and locate suitable setuid binaries for modification.
T1059: Command and Scripting Interpreter T1059.006: Python – The Copy Fail proof-of-concept exploit is delivered and executed as a Python script, enabling privilege escalation on vulnerable Linux systems.
T1068: Exploitation for Privilege Escalation – Threat actors exploit the Copy Fail Linux kernel vulnerability to escalate from unprivileged user accounts to full root access on affected systems.
T1611: Escape to Host – In containerized environments, the Copy Fail vulnerability enables attackers to escape from containers to compromise the underlying Linux host operating system by exploiting the shared kernel.
T1564: Hide Artifacts – The Copy Fail exploit modifies Linux kernel in-memory page cache without touching disk or marking pages dirty, evading file-integrity monitoring tools that rely on disk-based checksums.
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