CVE-2017-15124 in QEMU
Summary
by MITRE
VNC server implementation in Quick Emulator (QEMU) before 2.14.3 was found to be vulnerable to an unbounded memory allocation issue, as it did not throttle the framebuffer updates sent to its client. If the client did not consume these updates, VNC server allocates growing memory to hold onto this data. A malicious remote VNC client could use this flaw to cause DoS to the server host.
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Analysis
by VulDB Data Team • 01/29/2021
The vulnerability identified as CVE-2017-15124 represents a critical memory exhaustion flaw within the VNC server implementation of Quick Emulator QEMU versions prior to 2143. This issue stems from inadequate memory management practices in the VNC protocol handling mechanism that governs framebuffer update transmission. The vulnerability manifests when the VNC server fails to implement proper throttling mechanisms for managing the flow of graphical updates to connected clients, creating a scenario where memory allocation becomes unbounded and continuously grows without proper constraints.
The technical exploitation of this vulnerability occurs through a specific memory management flaw that operates at the protocol level of VNC communication. When a VNC client connects to the QEMU server, the server begins transmitting framebuffer updates to maintain screen synchronization. However, in vulnerable versions, the server does not implement proper flow control or memory limiting mechanisms to prevent excessive allocation of memory buffers for pending updates. This design flaw allows for a memory leak scenario where each update that cannot be immediately consumed by the client remains allocated in memory, leading to progressive memory consumption that can eventually exhaust available system resources.
The operational impact of this vulnerability extends beyond simple resource exhaustion to constitute a full denial of service condition that can compromise entire host systems. An attacker positioned remotely with access to the VNC server can exploit this vulnerability by maintaining an inactive or slow client connection while continuously sending graphical updates that accumulate in memory. This creates a memory starvation condition that can cause the QEMU process to crash, become unresponsive, or force the host system to terminate the service to prevent system instability. The vulnerability is particularly dangerous in virtualized environments where QEMU serves as a core component of hypervisor infrastructure, potentially affecting multiple virtual machines hosted on the same physical system.
This vulnerability maps directly to CWE-772, which describes "Missing Release of Resource after Effective Lifetime," and represents a classic example of resource exhaustion through improper memory management. The flaw aligns with ATT&CK technique T1499.004, "Endpoint Denial of Service," where adversaries exploit weaknesses in system resources to render services unavailable. The vulnerability also demonstrates characteristics of T1071.004, "Application Layer Protocol: DNS," in terms of how the attacker can leverage legitimate VNC protocol mechanisms to conduct malicious activity, though the specific protocol used here is VNC rather than DNS. Additionally, this issue reflects ATT&CK tactic TA0040, "Execution," as the successful exploitation can result in complete system compromise through denial of service conditions that may be leveraged as part of broader attack chains.
The recommended mitigation strategy involves immediate upgrade to QEMU version 2.14.3 or later, which implements proper memory throttling mechanisms and buffer management for VNC framebuffer updates. System administrators should also implement network-level controls to restrict VNC access to trusted networks and establish monitoring for unusual memory consumption patterns. Additionally, implementing resource limits and process monitoring can help detect and prevent exploitation attempts before they cause system-wide denial of service conditions. Organizations should also consider implementing VNC authentication mechanisms and encryption to reduce the attack surface while the primary fix is implemented.