CVE-2017-7394 in TigerVNC
Summary
by MITRE
In TigerVNC 1.7.1 (SSecurityPlain.cxx SSecurityPlain::processMsg), unauthenticated users can crash the server by sending long usernames.
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Analysis
by VulDB Data Team • 11/24/2022
The vulnerability identified as CVE-2017-7394 affects TigerVNC 1.7.1 server implementations where the SSecurityPlain::processMsg function in SSecurityPlain.cxx fails to properly validate username length before processing incoming authentication requests. This flaw represents a classic buffer overflow condition that occurs when unauthenticated users send excessively long usernames to the VNC server, causing the application to crash and potentially leading to a denial of service condition. The vulnerability exists within the plain authentication security module which is designed to handle simple username/password authentication scenarios. The specific technical implementation issue stems from insufficient input validation where the server does not enforce reasonable limits on username length, allowing malicious actors to exploit this weakness through crafted network packets.
The operational impact of this vulnerability extends beyond simple service disruption as it creates a potential vector for attackers to systematically target VNC server availability. When an attacker sends a username exceeding the buffer capacity allocated by the SSecurityPlain::processMsg function, the server experiences a memory corruption event that results in an abrupt termination of the VNC service. This behavior aligns with CWE-121, which describes stack-based buffer overflow conditions, and represents a direct violation of secure coding practices that mandate proper input validation and bounds checking. The vulnerability affects any system running TigerVNC 1.7.1 server components that utilize plain authentication, making it particularly concerning for environments where VNC services are exposed to untrusted networks or where authentication bypasses are possible.
From an adversarial perspective, this vulnerability maps directly to ATT&CK technique T1499.004 which covers network disruption through resource exhaustion or service interruption. The attack requires minimal sophistication as it only requires sending malformed network traffic to the target VNC server, making it suitable for automated exploitation. Security professionals should note that the vulnerability does not provide direct access to system resources or data, but rather creates a persistent availability issue that can be leveraged as part of broader attack campaigns. The crash condition affects the server's ability to maintain legitimate connections and can be used in conjunction with other denial of service vectors to maximize operational impact. Organizations should consider implementing network segmentation and access controls to limit exposure to this vulnerability while awaiting official patches from TigerVNC maintainers.
The recommended mitigations for CVE-2017-7394 include immediate application of the patched version of TigerVNC that addresses the buffer overflow condition in the plain authentication module. System administrators should also implement network-level controls such as firewall rules that limit VNC service exposure and restrict access to trusted IP ranges only. Additionally, monitoring solutions should be configured to detect unusual authentication patterns or repeated connection attempts that may indicate exploitation attempts. The vulnerability highlights the importance of adhering to secure coding practices and proper input validation, particularly in authentication modules where user-supplied data directly influences application behavior. Organizations should conduct regular vulnerability assessments of their remote access infrastructure and maintain updated inventory of all VNC server installations to ensure comprehensive remediation efforts.