Linux Kernel: Mount lock bypass in namespace handling allows covered path access
Summary
A flaw in the Linux kernel mount namespace code lets the owner of a user namespace move a mount beneath a locked mount with MOVE_MOUNT_BENEATH and then unlock the cover. The owner can then unmount the cover and read paths that should stay hidden, such as a secret under a shared host mount. The attack needs a local account that can create user namespaces and the attacker must be able to reach the mount setup, so it is not remotely exploitable.
Affected
- Linux kernel from commit c62a4766937edec2962d52e583276b459b739f2d (move_mount: transfer MNT_LOCKED) up to the fixes d74d66aa708ccf2bc0b1e265a83fed0f2308d5c4 and f1f0c24ca701d7d3061c7f69f7450f1734c11745
- CNA version data lists Linux 7.1 as affected; unaffected 0 to before 7.1
- OSV: introduced in 7.1.0
Fixed in
- Fixed in the upstream stable tree by commits d74d66aa708ccf2bc0b1e265a83fed0f2308d5c4 and f1f0c24ca701d7d3061c7f69f7450f1734c11745
- OSV: fixed in 7.2.11
- Distribution-specific fixed versions: Unknown.
Workaround
- No vendor workaround known.
- If patching is delayed, restricting unprivileged user namespaces may reduce exposure, but this can break rootless containers and sandboxes and must be tested first.
Exploitation
- Exploited in the wild: Unknown. No reports found.
- Public proof of concept: none found. The upstream selftest exercises the same code path but is a regression test.
Our recommendation
- Identify hosts with multiple local users, shared CI runners or container hosts where untrusted users can create user namespaces.
- Install a patched kernel from your distribution or a stable kernel that contains the fix (7.2.11 or later per OSV) in the normal patch cycle, with priority on exposed multi-user systems.
- Verify the running kernel version after the reboot.
- Review the related entries CVE-2026-98978 and CVE-2026-99170 in the same mount-lock area.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Linux stable commit d74d66aa git.kernel.org
- Linux stable commit f1f0c24c git.kernel.org
- Ubuntu security tracker CVE-2026-98976 ubuntu.com
- OSV entry CVE-2026-98976 vulners.com
- Strix CVE-2026-98976 www.strix.ai
- Strix CVE-2026-98978 (related) www.strix.ai
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 22:36.
Original description (NVD)
In the Linux kernel, the following vulnerability has been resolved: namespace: keep the lock on a mount that a propagated copy is moved beneath attach_recursive_mnt() transfers MNT_LOCKED from the top mount to the mount that is moved beneath it with MOVE_MOUNT_BENEATH. 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.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
Affected? We can help.
We check whether your systems are affected, install updates and close the gap – continuously with the System Health Checker if you wish.