CVE-2018-1049 in systemd
Summary
by MITRE
In systemd prior to 234 a race condition exists between .mount and .automount units such that automount requests from kernel may not be serviced by systemd resulting in kernel holding the mountpoint and any processes that try to use said mount will hang. A race condition like this may lead to denial of service, until mount points are unmounted.
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Analysis
by VulDB Data Team • 02/04/2021
The vulnerability described in CVE-2018-1049 represents a critical race condition within the systemd service manager that affects versions prior to 234. This flaw manifests in the interaction between .mount and .automount units, creating a scenario where the kernel's automount requests fail to be properly serviced by systemd's daemon. The race condition occurs during the synchronization process between kernel-level mount requests and systemd's service management framework, fundamentally undermining the reliability of automated mount operations.
The technical nature of this vulnerability stems from the timing discrepancy in how systemd processes automount requests versus how the kernel handles mountpoint operations. When a kernel-initiated automount request arrives, there exists a window where systemd has not yet completed its processing of the mount unit, leaving the mountpoint in an inconsistent state. This creates a deadlock condition where the kernel holds the mountpoint lock while waiting for systemd to service the request, and systemd itself cannot process the request because the mountpoint is already occupied. This dual dependency creates a circular wait condition that results in process hanging and system resource exhaustion.
The operational impact of this vulnerability extends beyond simple service disruption to encompass complete denial of service conditions within affected systems. Any application or process attempting to access the problematic mountpoint will hang indefinitely until the mount is manually unmounted, effectively blocking all I/O operations to that location. This behavior creates cascading failures throughout the system as dependent services and applications become unresponsive, potentially leading to complete system lockup depending on the criticality of the affected mountpoints. The vulnerability particularly affects systems relying heavily on automated mount operations and network file systems where mountpoint availability is crucial for continued operation.
From a cybersecurity perspective, this vulnerability aligns with CWE-362, which describes race conditions that can lead to security flaws through improper synchronization. The issue also maps to ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion. The vulnerability demonstrates how improper state management in system-level services can create exploitable conditions that adversaries might leverage to disrupt service availability. Organizations should prioritize patching this vulnerability to prevent potential exploitation that could result in system-wide denial of service conditions, particularly in environments where automated mount operations are critical for system functionality and where attackers might attempt to trigger the race condition to disable services.
Mitigation strategies should include immediate deployment of systemd version 234 or later, which contains the necessary synchronization fixes to resolve the race condition. System administrators should also implement monitoring solutions to detect hanging processes and mountpoint lockups, enabling rapid identification of affected systems. Additionally, organizations should review their mount point configurations to minimize the impact of such conditions and consider implementing alternative mount strategies that reduce reliance on the vulnerable automount mechanisms. The patch addresses the underlying synchronization issues by ensuring proper ordering and locking mechanisms between kernel mount requests and systemd's processing of mount units, thereby eliminating the race condition that led to the denial of service scenario.