LibVNCClient is a library for easy implementation of a VNC client. In versions 0.9.12 through 0.9.15, a malicious (or man-in-the-middle) VNC server can force a connecting `libvncclient` to write attacker-controlled data past the end of its framebuffer. This is an out-of-bounds heap write with attacker-controlled length, contents, and offset. It needs no authentication (the attacker is the server), works in a default build with default settings, and fires from a single `FramebufferUpdate` the moment the victim connects. It crashes any client unconditionally (denial of service); we also demonstrated it overwriting an application callback pointer and redirecting execution to attacker-chosen code (code execution) under the default configuration. Commit 540332be3e0acc566fa64da6f1b4680c72c724dd patches the issue.

CVE-2026-50538 affects LibVNCClient versions 0.9.12 through 0.9.15, a widely used VNC client library. A malicious or man-in-the-middle VNC server can trigger an out-of-bounds heap write with attacker-controlled length, contents, and offset by sending a single FramebufferUpdate packet upon connection. No authentication is required to exploit this vulnerability. The flaw unconditionally crashes any connecting client, constituting a denial-of-service condition. Researchers also demonstrated successful code execution by overwriting an application callback pointer, redirecting execution to attacker-chosen code. The vulnerability works in default builds with default settings, making it trivially exploitable. A patch has been issued via commit 540332be3e0acc566fa64da6f1b4680c72c724dd in the LibVNC/libvncserver repository.