---
id: "CVE-2026-98988"
kind: "vulnerability"
aliases: ["CVE-2026-98988"]
title: "Linux kernel: open_tree namespace copy reveals mounts hidden under other mounts"
severity: "unknown"
cvss: null
epss: 0.00198
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["containers", "kernel", "linux", "runtime", "self-hosted"]
not_affected_if: ["unprivileged-userns-disabled"]
published_at: "2026-10-23T13:17:20+00:00"
first_seen_at: "2026-10-23T21:57:33+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98988", "first_seen_at": "2026-10-23T21:57:33+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-98988", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel: open_tree namespace copy reveals mounts hidden under other mounts

An unprivileged user can call open_tree with OPEN_TREE_NAMESPACE to copy a mount tree that includes mounts that were meant to cover directories, so the content under those covering mounts becomes readable. The copy ignores the covering mounts and unbindable mounts that the source namespace would have kept hidden from that user.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` and compare against the fixed kernel builds listed in the NVD or distribution advisory for CVE-2026-98988
- Check whether unprivileged user namespaces are enabled with `sysctl kernel.unprivileged_userns_clone` or `sysctl user.max_user_namespaces`

## Fix

- Update the Linux kernel to a build that includes the upstream fix for CVE-2026-98988 (exact fixed version unknown from sources)
- Until patched, disable unprivileged user namespaces if your workload does not need them, for example `sysctl -w user.max_user_namespaces=0`

## Timeline

- 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98988
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-98988

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

namespace: keep covered mounts covered in OPEN_TREE_NAMESPACE

open_tree(OPEN_TREE_NAMESPACE) creates a new mount namespace from a
copy of the tree at the given path. The caller needs to be privileged
over its current user namespace because the new mount namespace will be
owned by it. It doesn't need to be privileged over the mount namespace
the tree is copied from. An unprivileged user just needs to create a
user namespace first.

The copy follows the rules of a detached bind mount. Without
AT_RECURSIVE only the mount itself is cloned and its children are left
out. With AT_RECURSIVE unbindable mounts are skipped. Both is fine when
the caller has privileges over the source mount namespace because it
could unmount those mounts anyway. Not so for an unprivileged user in
that mount namespace. For them a mount namespace copy is what unshare()
does. copy_mnt_ns() copies everything including unbindable mounts and
lock_mnt_tree() makes sure nothing can be unmounted in the copy. Nothing
that was covered gets revealed.

create_new_namespace() only does the locking and so the covered content
is right there in the new mount namespace:

  # mount -t tmpfs none /tmp/otn; mkdir /tmp/otn/covered
  # echo hidden > /tmp/otn/covered/under.txt
  # mount -t tmpfs none /tmp/otn/covered

  uid 1000, after unshare(CLONE_NEWUSER):
  fd = open_tree(AT_FDCWD, "/tmp/otn", OPEN_TREE_NAMESPACE);
  setns(fd, CLONE_NEWNS);
  open("/covered/under.txt", O_RDONLY)  -> "hidden"

Covering paths with mounts is how container runtimes mask parts of
/proc and /sys and how admins hide things.

When the caller's user namespace doesn't own the source mount namespace
copy the way unshare() does and refuse a non-recursive copy of a mount
that has anything mounted below the requested directory and copy
unbindable mounts in a recursive copy. A caller that is privileged over
the source mount namespace sees no change.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_