CVE-2018-16872 in QEMU
Summary
by MITRE
A flaw was found in qemu Media Transfer Protocol (MTP). The code opening files in usb_mtp_get_object and usb_mtp_get_partial_object and directories in usb_mtp_object_readdir doesn't consider that the underlying filesystem may have changed since the time lstat(2) was called in usb_mtp_object_alloc, a classical TOCTTOU problem. An attacker with write access to the host filesystem shared with a guest can use this property to navigate the host filesystem in the context of the QEMU process and read any file the QEMU process has access to. Access to the filesystem may be local or via a network share protocol such as CIFS.
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Analysis
by VulDB Data Team • 10/26/2024
The vulnerability identified as CVE-2018-16872 represents a critical time-of-check to time-of-use flaw within QEMU's Media Transfer Protocol implementation, specifically affecting the usb_mtp subsystem. This issue stems from a fundamental race condition where the system performs a file system check at one point in time but accesses the same resources at a later point, creating an exploitable window. The flaw manifests in multiple functions including usb_mtp_get_object, usb_mtp_get_partial_object, and usb_mtp_object_readdir, all of which fail to validate that the file system state remains consistent between the initial check and subsequent access operations. The vulnerability is particularly dangerous because it allows attackers to manipulate the file system state between the time when lstat(2) is called in usb_mtp_object_alloc and when the actual file operations occur, effectively bypassing access controls.
This time-of-check to time-of-use vulnerability directly maps to CWE-367, which specifically addresses the issue of time-of-check to time-of-use flaws where the state of a resource changes between the time it is checked and when it is used. The attack vector is particularly insidious because it leverages the shared filesystem environment between host and guest systems, where an attacker with write privileges on the host filesystem can manipulate the target files or directories. The exploitation requires the attacker to have write access to the shared filesystem that is mounted in the guest environment, but once achieved, the attacker can navigate the host filesystem through the QEMU process context, effectively escalating their privileges to the same level as the QEMU process itself. This creates a significant security boundary violation where guest operating systems can potentially access host resources that should remain isolated.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential data exfiltration and system compromise. When QEMU processes are running with elevated privileges, particularly when using the MTP functionality for USB device passthrough, attackers can leverage this flaw to read files that the QEMU process has access to, potentially including sensitive system files, configuration data, or user information. The vulnerability is not limited to local filesystem access but also extends to network shares such as CIFS, making it applicable in enterprise environments where shared storage is common. This affects virtualized environments where QEMU is used for desktop virtualization, server virtualization, or containerized applications, particularly in scenarios where guest operating systems are granted access to host filesystems through shared directories or mounted volumes.
Mitigation strategies for CVE-2018-16872 should focus on implementing proper synchronization mechanisms to prevent the race condition, such as using atomic file operations or maintaining consistent state checks throughout the execution flow. The most effective immediate solution involves updating QEMU to versions that have patched this vulnerability by ensuring that file system operations are performed with consistent state validation, eliminating the TOCTTOU window. Organizations should also implement strict access controls on shared filesystems, particularly those used in virtualized environments, limiting write privileges to only trusted users and processes. Network-based mitigations include implementing proper firewall rules and access control lists to limit network share access, while system-level protections should involve running QEMU processes with minimal required privileges and implementing additional monitoring for suspicious file access patterns. From an ATT&CK perspective, this vulnerability aligns with techniques such as privilege escalation and credential access, specifically targeting the T1068 privilege escalation technique and T1552 credential access, making it a critical concern for organizations implementing security controls against these attack vectors.