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CVE-2026-44988: LibVNCClient Tight Gradient decoding allows malicious server-triggered heap/stack OOB writes

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-05-27 · updated 2026-10-26
• Weakness: CWE-787

Description

LibVNCClient is a library for easy implementation of a VNC client. In 0.9.15 and earlier, LibVNCClient's Tight encoding decoder uses fixed-size 2048-pixel scratch buffers for the Gradient filter, but it does not reject Tight rectangles whose width is larger than 2048 pixels. A malicious VNC server can send a crafted FramebufferUpdate rectangle using Tight encoding with NoZlib | ExplicitFilter and the Gradient filter. When a LibVNCClient-based client connects, the client processes the server-controlled rectangle width and writes beyond fixed-size Gradient buffers. This vulnerability is fixed with commit 5b270544b85233668b98161323297d418a8f5fd1.

Affected products

• LibVNCServer: through 0.9.15

References

• https://github.com/LibVNC/libvncserver/commit/5b270544b85233668b98161323297d418a8f5fd1
• https://github.com/LibVNC/libvncserver/security/advisories/GHSA-jcc5-8wj4-7c58

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