CVE-2026-98988 (CVSS 8.8)

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.

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CVSS v3.1 Score

8.8

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS — Exploit Prediction

0.0016

Probability of exploitation

0.06%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

References

Other References

https://git.kernel.org/stable/c/0febe278ec45b1ccdbe9bd5ff1ba365d8fd55e2a https://git.kernel.org/stable/c/b8a94a405ddcf45e9470789e0a821d8f2aa4ca10

Frequently Asked Questions

What is CVE-2026-98988? +

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. It has a CVSS v3.1 base score of 8.8 (HIGH).

How severe is CVE-2026-98988? +

CVE-2026-98988 has a CVSS v3.1 score of 8.8 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0016, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-98988? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

CVE-2026-98988