---
id: "CVE-2026-128321"
page: "https://infrastack.ai/live/CVE-2026-128321"
kind: "vulnerability"
aliases: ["CVE-2026-128321"]
title: "TightVNC and UltraVNC viewers: malicious server can overflow heap memory"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["other", "tightvnc", "ultravnc", "vnc", "windows"]
not_affected_if: ["trusted-servers-only"]
published_at: "2026-11-13T02:18:56+00:00"
first_seen_at: "2026-11-13T02:41:37+00:00"
updated_at: "2026-11-13T02:43:14+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-128321", "first_seen_at": "2026-11-13T02:41:37+00:00"}]
exploit_sources: []
---

# TightVNC and UltraVNC viewers: malicious server can overflow heap memory

Link for people: https://infrastack.ai/live/CVE-2026-128321 (this .md is for agents)

TightVNC and UltraVNC viewers mishandle Tight JPEG frames because a rectangle width from the server is not checked against a fixed-size buffer. A malicious VNC server can trigger an out-of-bounds heap write when a user connects, crashing the viewer and possibly corrupting memory further.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed TightVNC or UltraVNC viewer version against the vendor's release notes, since the sources do not name affected version ranges

## Timeline

- 2026-11-13T02:41:37+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-128321

## From the sources

Tight JPEG decoding in TightVNC and UltraVNC viewers contains a heap-based buffer overflow because an attacker-controlled framebuffer rectangle width is not validated against a fixed-size decoding buffer. A malicious VNC server can cause an out-of-bounds heap write when a user connects, resulting in application crash and potentially other memory-corruption impacts.

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_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_