LibVNC libvncserver up to 0.9.15 FramebufferUpdate out-of-bounds write

Summaryinfo

A vulnerability marked as critical has been reported in LibVNC libvncserver up to 0.9.15. This affects an unknown function of the component FramebufferUpdate Handler. Performing a manipulation results in out-of-bounds write. This vulnerability was named CVE-2026-44988. The attack may be initiated remotely. There is no available exploit. To fix this issue, it is recommended to deploy a patch.

Detailsinfo

A vulnerability classified as critical has been found in LibVNC libvncserver up to 0.9.15. This affects an unknown code of the component FramebufferUpdate Handler. The manipulation with an unknown input leads to a out-of-bounds write vulnerability. CWE is classifying the issue as CWE-787. The product writes data past the end, or before the beginning, of the intended buffer. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-44988 since 05/08/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 321679 (SUSE SLED15 / SLES15 Security Update : LibVNCServer (SUSE-SU-2026:2427-1)), which helps to determine the existence of the flaw in a target environment.

Applying the patch 5b270544b85233668b98161323297d418a8f5fd1 is able to eliminate this problem. The bugfix is ready for download at github.com.

The vulnerability is also documented in the vulnerability database at Tenable (321679). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Vendor

• LibVNC

Name

• libvncserver

Version

• 0.9.0
• 0.9.1
• 0.9.2
• 0.9.3
• 0.9.4
• 0.9.5
• 0.9.6
• 0.9.7
• 0.9.8
• 0.9.9
• 0.9.10
• 0.9.11
• 0.9.12
• 0.9.13
• 0.9.14
• 0.9.15

License

• open-source

Website

• Product: https://github.com/LibVNC/libvncserver/

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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