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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-50538
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-21 21:16:59 UTC
Updated2026-08-21 21:16:59 UTC
DescriptionLibVNCClient 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.

Risk And Classification

Primary CVSS: v3.1 8.8 HIGH from [email protected]

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

EPSS: 0.003200000 probability, percentile 0.249450000 (date 2026-08-23)

Problem Types: CWE-122 | CWE-787 | CWE-122 CWE-122: Heap-based Buffer Overflow | CWE-787 CWE-787: Out-of-bounds Write

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary8.8HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
3.1CNADECLARED8.8HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA LibVNC Libvncserver affected >= 0.9.12, <= 0.9.15 Not specified

References

ReferenceSourceLinkTags
github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8 [email protected] github.com
github.com/LibVNC/libvncserver/commit/540332be3e0acc566fa64da6f1b4680c72... [email protected] github.com
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