CVE-2026-76642 in util-linux
Summary
by MITRE • 09/03/2026
util-linux versions through 2.41.5 and 2.42.2 fail to check mount helper exit status before running post-mount hooks, allowing unprivileged users to execute privileged operations on pre-existing filesystems. Attackers can exploit X-mount.idmap or X-mount.owner hooks to clone filesystems with inherited suid bits or modify target inode permissions after a helper fails, achieving privilege escalation.
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Analysis
by VulDB Data Team • 09/27/2026
The vulnerability identified in util-linux versions through 2.41.5 and 2.42.2 represents a critical logic flaw within the mount utility's handling of filesystem operations, specifically concerning the execution sequence of post-mount hooks relative to external helper processes. This issue stems from an insufficient validation mechanism where the system fails to verify the exit status or success state of a designated mount helper before proceeding to execute subsequent hook scripts defined in the mount configuration. In standard Linux mounting procedures, particularly when dealing with complex filesystem types such as NFS, CIFS, or FUSE-based solutions, external helpers are often invoked to perform authentication, mapping, or other preparatory tasks required for successful attachment. The flaw arises because the utility assumes that if a helper is configured and launched, its execution concludes successfully without explicitly checking return codes, thereby allowing control flow to continue immediately into post-mount hook execution regardless of whether the actual mount operation succeeded or failed.
This architectural oversight creates a significant security gap that allows unprivileged users to escalate privileges by manipulating filesystem attributes on pre-existing mounts. When an attacker triggers a mount operation using specific options such as X-mount.idmap or X-mount.owner, they can induce a failure in the underlying helper process while still triggering the associated post-mount hooks. Because these hooks are executed with elevated privileges typically reserved for root or system administrators to ensure proper filesystem configuration and permission setting, their execution during a failed state allows an attacker to modify inode permissions on already mounted directories. Specifically, attackers can exploit this race condition or logic error to clone existing filesystems that contain setuid binaries or sensitive files, inheriting the suid bits which grant elevated execution privileges. Furthermore, by modifying target inode permissions after the helper fails but before proper cleanup occurs, an attacker can alter access controls on critical system directories or user home directories, effectively bypassing standard permission restrictions and gaining unauthorized root-level access to resources that should remain protected.
From a classification perspective, this vulnerability aligns with CWE-94 Improper Control of Generation of Code Command Injection under the broader category of improper validation of inputs and states, as well as CWE-268 Incorrectness at Checkpoint where state verification is skipped. In terms of offensive security frameworks, this exploit path corresponds to ATT&CK technique T1055 Process Injection or more accurately T1068 Exploitation for Privilege Escalation via misconfigured system utilities and hooks. The attack vector relies on the attacker's ability to trigger a mount operation with specific options that invoke external helpers while simultaneously ensuring those helpers fail in a controlled manner, thereby forcing the privileged hook execution path without the intended safeguards of a successful mount transaction. This is particularly dangerous because it leverages legitimate administrative tools for malicious purposes, making detection through standard application whitelisting difficult as the executed binaries are trusted system utilities.
The operational impact of this vulnerability extends beyond simple privilege escalation to include potential data integrity compromise and lateral movement within a compromised environment. By modifying inode permissions on pre-existing filesystems, an attacker can expose sensitive configuration files, steal user credentials stored in home directories, or plant persistent backdoors that survive reboots if the modifications persist across mount cycles. In multi-tenant environments or systems running containerized workloads where users may have limited but non-zero privileges to trigger certain mounts for development or testing purposes, this flaw provides a direct path from low-level user access to full system control. The ability to inherit suid bits on cloned filesystems further amplifies the risk by allowing attackers to execute arbitrary code with root privileges without needing to exploit additional kernel vulnerabilities, effectively turning standard file management operations into powerful privilege escalation vectors.
Mitigation strategies for this vulnerability require both immediate patching and long-term architectural improvements within util-linux and dependent systems. The primary remediation is to upgrade util-linux to a version that includes the fix for checking mount helper exit statuses before executing post-mount hooks, ensuring that privileged hook scripts are only invoked when the underlying filesystem operation has completed successfully. System administrators should also audit their /etc/fstab configurations and any custom mount helpers to remove unnecessary X-mount.idmap or X-mount.owner options unless strictly required by specific application logic, thereby reducing the attack surface. Additionally implementing strict file integrity monitoring on critical system directories can help detect unauthorized permission changes resulting from such exploits. For environments where upgrading is not immediately feasible, restricting user privileges related to mounting operations via PAM modules or systemd service restrictions can provide a temporary layer of defense against exploitation attempts until the software update is applied across all affected endpoints and servers.