Linux kernel: Mount namespace copy reveals covered mounts in OPEN_TREE_NAMESPACE
Summary
The Linux kernel's open_tree() call with OPEN_TREE_NAMESPACE copies mounts into a new mount namespace without keeping mounts covered. An unprivileged local user who first creates a user namespace can read content that is hidden under other mounts, for example files masked for containers in /proc or /sys. The attack requires a local account that can create a user namespace.
Affected
- Linux kernel from commit 9b8a0ba68246a61d903ce62c35c303b1501df28b onward, before the fixes listed below
- Version 7.0 is listed as affected; versions before 7.0 and 7.2.11 and later are listed as unaffected
- Exact affected ranges per stable branch: Unknown.
Fixed in
- Upstream fix commits: 0febe278ec45b1ccdbe9bd5ff1ba365d8fd55e2a and b8a94a405ddcf45e9470789e0a821d8f2aa4ca10
- Linux 7.2.11 is listed as fixed in OSV.
- Fixed versions per stable branch and distribution backports: Unknown.
Workaround
- No vendor workaround known.
- Restricting unprivileged user namespaces can mitigate the issue but may break rootless containers and browsers; test before applying.
Exploitation
- No known exploitation in the wild; not listed in the CISA KEV catalog as of the research date.
- No public exploit found. A kernel regression selftest exists in the public tree.
- EPSS: 0.00164 (low).
Our recommendation
- Install the kernel update from your distribution as soon as it is available, following the normal update cycle.
- Prioritise hosts with local users, CI runners and container hosts.
- Check whether your kernel version is affected and whether your distribution has backported the fix.
- Reduce local shell access on shared hosts and monitor for unexpected user namespace creation.
- Single-tenant servers without local users are at low risk.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Linux kernel stable commit 0febe278ec45b1ccdbe9bd5ff1ba365d8fd55e2a git.kernel.org
- Linux kernel stable commit b8a94a405ddcf45e9470789e0a821d8f2aa4ca10 git.kernel.org
- Ubuntu security tracker CVE-2026-98988 ubuntu.com
- NVD record via Vulners vulners.com
- SUSE CVE page via Vulners vulners.com
- CISA KEV alert 26 Oct 2026 www.cisa.gov
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 31 Oct 2026, 10:44.
Original description (NVD)
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.
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.
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