CVE-2018-20023

Vulnerability

CVE-2018-20023 is an improper initialization vulnerability (CWE-665) in the VNC Repeater client code of LibVNC (also known as LibVNCServer) before commit 8b06f835e259652b0ff026898014fc7297ade858 (version 0.9.12). The affected code path does not correctly initialize a stack buffer, leaving residual data that can be read by a remote attacker [1][2].

Exploitation

An attacker can trigger the vulnerability by connecting to a VNC Repeater service that uses the affected LibVNC client library. No authentication is required; the attacker only needs network access to send a specially crafted VNC handshake. The improper initialization causes stack memory to be transmitted back to the attacker in the server response [1].

Impact

Successful exploitation results in information disclosure: the attacker reads stack memory contents from the server process. This can reveal sensitive data such as memory addresses, which can be used to leak the stack memory layout and bypass Address Space Layout Randomization (ASLR) when combined with another vulnerability [1]. The confidentiality impact is high; integrity and availability are not directly affected.

Mitigation

The vendor released a fix in LibVNCServer version 0.9.12 in September 2018 [1]. All users should upgrade to version 0.9.12 or later [2]. No workarounds are available for earlier versions. The vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog as of this writing.