Linux Kernel copy.fail Vulnerability Enables Local Privilege Escalation

copy.fail is a Linux kernel local privilege escalation (LPE) that was disclosed by Theori on 29 April 2026. The exploit abuses the kernel crypto API (AF_ALG sockets) and splice() to write four bytes at a time directly into the page cache of a file the attacker does not own. It works unmodified across Ubuntu, RHEL, Debian, SUSE, Amazon Linux, Fedora and most other distributions, with no race condition or per‑distro offsets. The file on disk is never modified, so AIDE, Tripwire and checksum‑based monitoring see nothing. Kubernetes Pod Security Standards (Restricted) and the default RuntimeDefault seccomp profile do not block the syscall used; a custom seccomp profile is required.

The mainline kernel fix landed on 1 April 2026, and distros are rolling kernels out now. The vulnerability is critical for shared infrastructure: every container on a shared Kubernetes node, every tenant on a shared hosting box, every CI/CD job that runs untrusted code, every WSL2 instance on a Windows laptop, and every containerised AI agent with shell access share one Linux kernel with their neighbours. A local LPE collapses that boundary.

In summary, copy.fail allows LPE via AF_ALG sockets and splice, works on all major distributions, does not modify the target file, is not blocked by standard Kubernetes security profiles, requires a custom seccomp profile, was fixed on 1 April 2026, and distros are rolling out patched kernels now.