CVE-2018-20021

Vulnerability

CVE-2018-20021 is an infinite loop vulnerability (CWE-835) in the VNC client code of LibVNCServer. The bug exists in versions before commit c3115350eb8bb635d0fdb4dbbb0d0541f38ed19c, which corresponds to the 0.9.12 release [1]. The vulnerability is triggered when a VNC client processes specially crafted data from a server, causing the client to enter an infinite loop without proper exit conditions.

Exploitation

An attacker must operate a malicious VNC server and convince a user or automated system to connect to it using a vulnerable LibVNC client. No authentication is required; the infinite loop is triggered during the initial handshake or data exchange when the client parses server-supplied input. The attacker sends crafted messages that cause the client to loop indefinitely, consuming CPU and memory resources [1].

Impact

Successful exploitation leads to excessive consumption of CPU and RAM on the client machine, resulting in a denial of service (DoS). The client application becomes unresponsive and may crash or hang. No code execution or data compromise has been reported for this vulnerability [1].

Mitigation

The vulnerability is fixed in LibVNCServer version 0.9.12 and later [1][3]. Users should upgrade to at least this version. For Gentoo systems, the recommended action is to emerge >=net-libs/libvncserver-0.9.12 [3]. The SSVNC package (which bundles LibVNC) has been discontinued by Gentoo; users are advised to unmerge net-misc/ssvnc and use an alternative such as a manual SSH tunnel [2]. No workaround is available for unpatched versions.