CVE-2026-50538: libvnc libvncserver Heap-based Buffer Overflow

Vulnerability summary

CVE-2026-50538 is a high (CVSS 8.8) Heap-based Buffer Overflow vulnerability in libvnc libvncserver. EPSS estimates a 0.40% chance of exploitation in the next 30 days, higher than 33.1% of scored vulnerabilities.

Timeline

1. 21-08-2026
   PublishedVulnerability first disclosed.
2. 24-08-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-122Heap-based Buffer OverflowA heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc().
• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.