CVE-2020-14403 in LibVNCServer
Summary
by MITRE
An issue was discovered in LibVNCServer before 0.9.13. libvncserver/hextile.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-2020-14403 represents a critical out-of-bounds memory access flaw within LibVNCServer version 0.9.12 and earlier. This issue specifically affects the hextile.c component of the library, which is responsible for handling hextile encoding in remote desktop protocol implementations. The flaw arises from insufficient input validation and boundary checking during the processing of encoded data streams, creating a potential avenue for arbitrary code execution or denial of service attacks. The vulnerability impacts any system utilizing LibVNCServer for remote desktop functionality, including virtual machine management systems, remote access solutions, and network administration tools that depend on this library for VNC protocol implementation.
The technical root cause of this vulnerability stems from improper handling of encoding data structures within the hextile encoding module. When processing incoming VNC client data, the code fails to properly validate the bounds of memory allocations or verify the integrity of encoding parameters before accessing memory regions. This creates a scenario where maliciously crafted VNC client data could trigger memory access violations, potentially leading to buffer overflows or memory corruption. The vulnerability manifests when the server processes hextile-encoded pixel data, where the encoding parameters specify dimensions and data lengths that exceed allocated memory boundaries. This flaw aligns with CWE-129, which addresses insufficient bounds checking, and represents a classic example of how improper input validation can lead to memory safety issues in network protocol implementations.
The operational impact of CVE-2020-14403 extends beyond simple denial of service conditions to encompass potential remote code execution capabilities. An attacker positioned to send crafted VNC client data could exploit this vulnerability to execute arbitrary code on systems running vulnerable versions of LibVNCServer. This risk is particularly concerning in virtualized environments where VNC servers are commonly used for remote machine management, as it could enable attackers to compromise entire virtual infrastructure. The vulnerability also affects systems using VNC for remote desktop access, potentially allowing unauthorized access to sensitive systems. Organizations relying on VNC-based solutions for remote administration, system monitoring, or cloud computing environments face significant exposure risks, as the flaw can be exploited without requiring authentication or special privileges.
Mitigation strategies for this vulnerability primarily focus on immediate software updates and deployment of patched versions of LibVNCServer. System administrators should prioritize upgrading to version 0.9.13 or later, which includes the necessary fixes for the hextile encoding boundary checking. Additionally, network segmentation and access control measures should be implemented to limit exposure of VNC servers to untrusted networks. The use of network firewalls to restrict VNC traffic to trusted IP ranges, combined with mandatory authentication and encryption protocols, can significantly reduce the attack surface. Organizations should also consider implementing intrusion detection systems capable of monitoring for suspicious VNC protocol traffic patterns that might indicate exploitation attempts. From an operational security perspective, regular vulnerability assessments and penetration testing should be conducted to identify and remediate similar issues within the broader network infrastructure, as this vulnerability demonstrates the importance of proper input validation in network protocol implementations. The remediation process should include comprehensive testing of patched systems to ensure that the fix does not introduce compatibility issues with existing VNC client implementations while maintaining the security improvements.