CVE-2026-98987 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
namespace: look at the topmost mount for a mount namespace file
attach_recursive_mnt() preallocates a mountpoint for the root of the topmost mount of the source so that a mount that already sits at the destination can be put on top of the source or on top of one of its propagated copies.
The copies are made without CL_COPY_MNT_NS_FILE so their chain of overmounts is shorter than the source's. It ends below the first bind mount of a mount namespace file. So while the loop walks up the chain of the source it remembers the mountpoint of that mount in "shorter" and the existing mount is put there for the copies.
But the loop ends at the topmost mount without ever looking at it. If the topmost mount is the only mount namespace file in the chain "shorter" stays NULL and mnt_change_mountpoint() attaches the existing mount of the copy below the root of the mount namespace file. That's a dentry of nsfs. No path walk in the copy's mount namespace ever gets there. The mount is gone until its parent goes and the copy that took its place can't be unmounted synchronously because it has a child:
S bind mount of a file N bind mount of a mount namespace file on top of S A shared mount, B a slave of A, Q mounted on B/file
move_mount(S, A/file) cat B/file -> the content of S instead of Q umount B/file -> EBUSY
Look at every mount of the chain including the topmost one.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel contains a logic flaw within the attach_recursive_mnt function that affects how bind mounts involving mount namespace files are handled during recursive mounting operations. This vulnerability arises from an incomplete traversal of the source mount hierarchy, specifically failing to inspect the topmost mount in the chain when determining where to place copied mounts. The core issue lies in the loop structure used to walk up the mount tree; while it correctly identifies and remembers the mountpoint for intermediate bind mounts of namespace files, it terminates before evaluating the highest-level mount. This oversight leads to incorrect placement of mounted filesystems, resulting in data visibility errors and resource management failures that can disrupt system stability and security posture.
The technical mechanism involves the preallocation of a mount point for the root of the topmost source mount, intended to allow existing mounts at the destination to be placed atop the source or its propagated copies. However, when creating these copies without the CL_COPY_MNT_NS_FILE flag, the resulting chain of overmounts is shorter than the original because it stops below the first bind mount of a mount namespace file. The algorithm tracks this intermediate point in a variable referred to as shorter but fails to account for scenarios where the topmost mount itself is the sole instance of a mount namespace file within that specific branch. Consequently, if no lower-level namespace file exists, the shorter pointer remains null. This causes mnt_change_mountpoint to attach the existing copy below the root of the mount namespace file rather than correctly positioning it relative to the source hierarchy.
The operational impact of this flaw manifests in two primary ways: incorrect data exposure and unmounting failures. In a typical scenario involving a shared mount A, its slave B, and a Q mounted on B/file, moving S onto A/file results in subsequent access to B/file revealing content from S instead of the expected Q due to the misplacement of mounts. More critically for system integrity, attempting to unmount B/file triggers an EBUSY error because the copy that replaced the original mount has become a child of another mount and cannot be removed synchronously. This leaves orphaned or improperly linked filesystem structures in memory, potentially leading to resource leaks and preventing administrators from cleanly managing mounted volumes, which is essential for container isolation and secure multi-tenant environments.
From a vulnerability classification perspective, this defect aligns with CWE-841 Improper Enforcement of Behavioral Workflow as the logic fails to follow the correct sequence of mount hierarchy evaluation. It also relates to CWE-200 Exposure of Sensitive Information to an Unauthorized Actor because users may inadvertently access data from unintended mounts due to the incorrect overlaying behavior. In terms of MITRE ATT&CK, this could facilitate lateral movement or privilege escalation if an attacker exploits the unmount failure to maintain persistence through lingering mount points that are difficult to detect and remove using standard tools. The flaw represents a subtle but significant deviation in kernel filesystem logic that undermines the reliability of namespace isolation mechanisms.
Mitigation strategies should focus on patching the attach_recursive_mnt function to ensure the traversal loop includes the topmost mount in its evaluation criteria. Developers must verify that all mounts in the source chain, including those at the highest level, are considered when determining the attachment point for copied mounts. System administrators relying on affected kernel versions should apply vendor-provided security updates as soon as they become available. Until patches are deployed, careful management of bind mount operations and avoiding complex recursive mounting scenarios involving namespace files can reduce the risk of triggering this specific code path. Regular auditing of mounted filesystem states using tools like findmnt or /proc/mounts may help identify any anomalous mounts resulting from prior exploitation attempts.