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Fencer Vulns
high 8.8

CVE-2026-50538 json

libvncclient Tight decoder has an attacker-controlled heap out-of-bounds write

published 2026-08-21 2 mo ago modified 2026-10-12 28 d ago also EUVD-2026-64167

cvss
8.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 34%
public exploits
0
none known

Summary

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.

Patch / fix

  • libvncserver Debian 12 0.9.14+dfsg-1+deb12u2
  • libvncserver Debian 13 0.9.15+dfsg-1+deb13u2
  • libvncserver Debian 14 0.9.15+dfsg-6

debian advisories DEBIAN-CVE-2026-50538

  • libvncserver Ubuntu 22.04 LTS 0.9.13+dfsg-3ubuntu0.2
  • libvncserver Ubuntu 24.04 LTS 0.9.14+dfsg-1ubuntu0.2
  • libvncserver Ubuntu 25.10 0.9.15+dfsg-1ubuntu0.2
  • libvncserver Ubuntu 26.04 LTS 0.9.15+dfsg-3ubuntu0.2

ubuntu advisories UBUNTU-CVE-2026-50538

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • LibVNCServer SLE Workstation Extension 15 SP7, SLE Module for Package Hub 15 SP7 0.9.14-150600.3.12.1
  • LibVNCServer SLES for SAP applications 16.0, SLES 16.0 0.9.14-160000.6.1
  • LibVNCServer SLE Module for Package Hub 15 SP7 0.9.10-150000.4.41.1
  • LibVNCServer SLES LTSS Extended Security 12 SP5, SLES 12 SP5-LTSS 0.9.9-17.50.1

suse advisories SUSE-SU-2026:4024-1SUSE-SU-2026:4022-1SUSE-SU-2026:23601-1SUSE-SU-2026:3803-1

  • LibVNCServer openSUSE Leap 16.0 0.9.14-160000.6.1
  • LibVNCServer openSUSE Tumbleweed 0.9.15-4.1

opensuse advisories openSUSE-SU-2026:21735-1openSUSE-SU-2026:11601-1

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
total
Assessed
2 mo ago

Exploit prediction

0.40% chance of exploitation in the next 30 days, higher than 34% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 8.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Affected products

  • LibVNC libvncserver

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.