What’s Vulnerable › Vulnerabilities

CVE-2026-50538: libvncclient Tight decoder has an attacker-controlled heap out-of-bounds write

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-21 · updated 2026-10-12
• Weakness: CWE-122

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.

Affected products

• LibVNCServer: from 0.9.12, <= 0.9.15

References

• 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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