CVE-2020-14402 in LibVNCServer
Summary
by MITRE
An issue was discovered in LibVNCServer before 0.9.13. libvncserver/corre.c allows out-of-bounds access via encodings.
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Analysis
by VulDB Data Team • 06/18/2020
The vulnerability identified as CVE-2020-14402 represents a critical out-of-bounds memory access flaw within LibVNCServer version 0.9.12 and earlier. This issue resides in the libvncserver/corre.c component which handles RFB protocol encoding operations. The flaw manifests when processing certain encoding types that do not properly validate input boundaries, creating opportunities for attackers to manipulate memory access patterns that extend beyond allocated buffer limits. Such vulnerabilities typically arise from insufficient bounds checking in network protocol implementations where incoming data is processed without adequate validation of expected data ranges or lengths.
The technical nature of this vulnerability falls under CWE-129, which specifically addresses insufficient validation of length of input buffers, and CWE-787, concerning out-of-bounds write operations. The flaw operates at the intersection of network protocol handling and memory management, where the RFB (Remote Framebuffer) protocol implementation fails to properly validate encoding parameters before processing them. When an attacker sends malformed encoding data to a vulnerable LibVNCServer instance, the software attempts to access memory locations beyond the intended buffer boundaries, potentially leading to memory corruption, application crashes, or in more severe scenarios, arbitrary code execution. This type of vulnerability is particularly dangerous in remote desktop environments where network-based attacks are common and the server may be exposed to untrusted clients.
The operational impact of this vulnerability extends significantly within virtual desktop infrastructure and remote access environments that rely on LibVNCServer implementations. Attackers could exploit this flaw to cause denial of service conditions by crashing the VNC server process, or potentially escalate privileges if the server runs with elevated permissions. The vulnerability affects any system running vulnerable versions of LibVNCServer, including but not limited to Linux distributions, Windows systems, and embedded devices that utilize this library for remote desktop functionality. The attack surface is particularly broad given that LibVNCServer is widely used in enterprise environments for remote administration and support operations, making it a prime target for adversaries seeking persistent access to network infrastructure.
Mitigation strategies for this vulnerability require immediate patching of affected systems to upgrade to LibVNCServer version 0.9.13 or later, which contains the necessary bounds checking fixes. Network administrators should also implement firewall rules to restrict access to VNC server ports to trusted networks only, reducing exposure to potential attackers. Additionally, monitoring for unusual network traffic patterns or repeated connection attempts to VNC services can help detect exploitation attempts. The implementation of network segmentation and zero-trust security models can further reduce the risk of exploitation by limiting lateral movement within the network. Organizations should also consider implementing intrusion detection systems that can identify malformed RFB protocol traffic patterns associated with this specific vulnerability, as outlined in the attack patterns documented by the mitre ATT&CK framework under the technique of T1071.3 for application layer protocol usage.