CVE-2026-98976: namespace: keep the lock on a mount that a propagated copy is moved beneath

Published Oct 23, 2026

Updated

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

namespace: keep the lock on a mount that a propagated copy is moved beneath

attachrecursivemnt() transfers MNTLOCKED from the top mount to the mount that is moved beneath it with MOVEMOUNTBENEATH. This allows the owner of a user namespace to replace its locked /proc or its root. The mount beneath takes on the job of covering the underlying mountpoint allowing the top mount to be unmounted.

Consider two mount namespaces:

(H) The host H has a shared mount P with a secret in P/d (Z) Z is a user namespace made from M. Its copy of P receives propagation from the host's P and its copy of M's cover on P/d is locked Z's owner must not get to see P/d.

Now (H) mounts X on P/d. This propagates into (Z). The copy of X lands beneath (Z)'s locked cover. The locked property is transfered from (Z)'s cover to the copy of X propagated beneath it. The cover is now unlocked.

Now (H) unmounts X again. The copy of X in (Z) gets unmounted and the covering mount is left unlocked on top of P/d. (Z) can now unmount it:

Z: umount2(P/d) = EINVAL / the cover is locked / H: mount X on P/d, umount X / both propagate into Z / Z: umount2(P/d) = 0 / the cover is now unlocked / Z: read P/d/secret = "covered-by-root" / secret revealed /

So only transfer the locked property to the mount beneath for mounts the caller has placed. A propagated copy that lands beneath a locked mount is locked as well so that the mount at the bottom of the stack carries a lock the way every check expects. The mount on top of it remains locked to ensure that it keeps covering even if the propagated mount is unmounted again.

Affected Software

1 affected component

Linux Linux kernel

Event History

Oct 23, 2026

CVE Published

via MITRE·12:19 PM

Data Sourced

via MITRE·12:19 PM

Description

Frequently Asked Questions

Who is exposed to this issue?

Systems where a user namespace has a propagated copy of a shared host mount and a locked covering mount are exposed. The described impact is that the user-namespace owner can replace its locked /proc or root and potentially access a host-mounted path that should remain covered.

What conditions are needed for exploitation?

Exploitation requires mount propagation from a host shared mount into the user namespace, plus a host mount and unmount operation on the covered path. The propagated mount must land beneath the user namespace's locked cover, causing the lock to be transferred and leaving the cover unlockable after the propagated mount is removed.

How can an administrator tell whether the vulnerable condition is present?

The relevant condition is a user namespace containing a locked cover over a path in a mount that receives propagation from a shared host mount. If host mounts and unmounts at that covered path propagate into the namespace, the cover may become unlocked and subsequently removable by the namespace owner.

What is the immediate mitigation if the fix cannot be deployed?

Avoid configurations in which untrusted user namespaces receive propagated copies of shared host mounts beneath locked covers. In particular, prevent host mount and unmount operations on paths whose propagated copies land beneath such covers.

CVE-2026-98976: namespace: keep the lock on a mount that a propagated copy is moved beneath

Affected Software

Event History

Frequently Asked Questions

Who is exposed to this issue?

What conditions are needed for exploitation?

How can an administrator tell whether the vulnerable condition is present?

What is the immediate mitigation if the fix cannot be deployed?

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Frequently Asked Questions

Who is exposed to this issue?

What conditions are needed for exploitation?

How can an administrator tell whether the vulnerable condition is present?

What is the immediate mitigation if the fix cannot be deployed?