# CVE-2026-128321: TightVNC and UltraVNC heap-based buffer overflow in Tight JPEG decoding

Source: https://junglewise.ai/threats/cve-2026-128321-tight-jpeg-decoding-in-tightvnc-and-ultravnc-viewers-contains-a  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-13. Updated: 2026-11-13  
License: CC BY 4.0 with a link back

CVE-2026-128321 · Severity: info · Published 2026-11-13

TightVNC and UltraVNC are remote desktop viewers that allow users to access computers over the network. A malicious VNC server can exploit a heap-based buffer overflow in the Tight JPEG decoder to crash the viewer or potentially corrupt memory when a user connects, compromising the integrity and availability of the remote session.

Vendors: [UltraVNC](https://junglewise.ai/threats/vendors/ultravnc).

## Quick answers

**Is CVE-2026-128321 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-128321 being exploited in the wild.

**How severe is CVE-2026-128321?** Informational severity, published 13 November 2026.

**Which products and versions are affected?** TightVNC TightVNC Viewer; UltraVNC UltraVNC Viewer.

**Is there a fix for CVE-2026-128321?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-128321 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-128321?** Check whether you run TightVNC Viewer or UltraVNC Viewer. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An attacker-controlled framebuffer rectangle width is not validated against a fixed-size decoding buffer in the Tight JPEG decoding component, leading to a heap-based buffer overflow. A malicious VNC server can trigger out-of-bounds heap writes when a user connects to it, potentially causing application crash or other memory corruption impacts. The vulnerability affects both TightVNC and UltraVNC viewers.

## Affected products

- TightVNC TightVNC Viewer
- UltraVNC UltraVNC Viewer

## Timeline

- 2026-11-13: disclosed

## References

- [https://github.com/ultravnc/ultravnc](https://github.com/ultravnc/ultravnc)
- [https://www.tightvnc.com/](https://www.tightvnc.com/)

## Related threats

- [UltraVNC Viewer off-by-one stack buffer overflow in RFB handler](https://junglewise.ai/threats/cve-2026-7831-ultravnc-viewer-off-by-one-stack-buffer-overflow-in-rfb-handler)