CVE-2026-98988 namespace: keep covered mounts covered in OPEN_TREE_NAMESPACE
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-98988?
CVE-2026-98988 is a high-severity vulnerability in Linux, published on 23 Oct 2026.
Official description
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.
Which versions are affected by CVE-2026-98988?
How do I fix CVE-2026-98988?
Upgrade to a fixed version.
Is CVE-2026-98988 being exploited?
There is no known exploitation of CVE-2026-98988 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-98988, GHSA-6v5x-qw46-2jph, UBUNTU-CVE-2026-98988.