Copy Fail (CVE-2026-31431): 9-year-old Linux kernel LPE found by AI-assisted fuzzing

What it is

Copy Fail (CVE-2026-31431), a Linux kernel local privilege escalation in the algif_aead crypto path (the page names the authencesn code), reachable by any unprivileged local user. The page adds operational detail the catalog summary omits: exact commands, a prioritization of which hosts to patch first, and detection queries.

Preconditions / Affected

Kernels built between 2017 and the patch — "essentially every mainstream distribution" out of the box, since the kernel crypto API (AF_ALG) ships enabled by default. Needs only a local unprivileged account: no network, no kernel debug features, no pre-installed primitives. It grants no remote access by itself, but converts any local code execution into root.

What to do

Patch first — update to a kernel including mainline commit a664bf3d603d, which reverts the 2017 in-place optimization so page-cache pages can no longer land in the writable destination scatterlist. Before you can patch, disable the module:

# echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif.conf
# rmmod algif_aead

For untrusted workloads (containers, sandboxes, CI), block AF_ALG socket creation via seccomp regardless of patch state. To check whether disabling breaks anything, look for active AF_ALG users:

lsof | grep AF_ALG
ss -xa

The published PoC is Python 3.10+, stdlib only (os, socket, zlib). It targets /usr/bin/su by default; pass another setuid binary as argv[1]. Issue tracker: https://github.com/theori-io/copy-fail-CVE-2026-31431

Patch-first priority

The page ranks exposure rather than treating all hosts equally:

flowchart TD
    A[Affected kernel] --> B{Who has shell?}
    B -->|Multiple users: dev boxes, jump hosts, build servers| C[Patch first]
    B -->|CI runners executing untrusted PR code| C
    B -->|Multi-tenant containers / notebook / serverless| C
    B -->|Single-tenant, only your team| D[Lower urgency]
    C --> E[Node compromise crosses tenant boundaries via shared page cache]

A container with the right primitives compromises the node and crosses tenant boundaries because the page cache is shared across the host.

Caveats

Directly verified distributions are a listed subset; Debian, Arch, Fedora, Rocky, Alma, Oracle and embedded systems are stated to "behave the same" but noted as not all individually tested. Disabling algif_aead breaks the AF_ALG.afalg engine, some embedded crypto offload paths, and applications binding aead/skcipher/hash sockets directly — otherwise "nothing measurable." The AI-fuzzing origin (Theori's Xint Code, ~1 hour of scan time) is a vendor claim tied to a marketing write-up.