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Vulnerability record · CVE-2026-50538 · published 21 August 2026

CVE-2026-50538: Heap-based buffer overflow vulnerability

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.

8.8 CVSS 3.1 High EPSS 0.61% · top 53.2% CWE-122 · Heap-based buffer overflowCWE-787 · Out-of-bounds write Received
8.8CVSS 3.1 base score
0.61%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
3References
24 Aug 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

References

LinkTags
https://github.com/LibVNC/libvncserver/commit/540332be3e0acc566fa64da6f1b4680c72c724dd
https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8
https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8

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Source: NIST National Vulnerability Database (record CVE-2026-50538), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.