CVE-2026-31431: Copy Fail vs. rootless containers

It's a shame, this seems like an interesting topic but I can't get past the blatant AI-isms littered throughout.

>This is not raw shellcode — it is a fully formed ELF executable

Running sstrip on an ELF binary is called ELF "golfing"? TIL…

tl;dr - within the container, the exploit works, and elevates to root (uid 0) within the container - BUT because that namespace actually maps to uid 1000 (the user) outside the container, the escalation does not flow up to the host.

But… does this escape the container? If not (the author seems to indicate it does not) then does it matter if you are in Docker or rootless Podman, right, since the end result is always: you have elevated to root within the container. If the rest of the container filesystem isolation does its job, the end result is the same? Though I guess another chained exploit to escape the container would be worse in Docker? Do I have that right?

Sigh.

1. I would hope the default seccomp policy blocks AF_ALG in these containers. I bet it doesn’t. Oh well.

2. The write-to-RO-page-cache primitive STILL WORKED! It’s just that the particular exploit used had no meaningful effect in the already-root-in-a-container context. If you think you are safe, you’re probably wrong. All you need to make a new exploit is an fd representing something that you aren’t supposed to be able to write. This likely includes CoW things where you are supposed to be able to write after CoW but you aren’t supposed to be able to write to the source.

So:

- Are you using these containers with a common image or even a common layer in an image to isolate dangerous workloads from each other. Oops, they can modify the image layers and corrupt each other. There goes any sort of cross-tenant isolation.

- What if you get an fd backed by the zero page and write to it? This can’t result in anything that the administrator would approve of.

- What if you ro-bind-mount something in? It’s not ro any more.

This isn't a new CVE. It's just documenting what happened when this person ran the exploit inside a certain type of container.