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

High 7.5 Published 1 Jul 2026 Updated 9 Jul 2026

A malicious VNC server can trigger a stack buffer overflow in UltraVNC viewer, potentially leading to code execution or denial of service.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: A user has to click or open something
Fix: None listed

What is CVE-2026-7831?

CVE-2026-7831 is a high-severity memory corruption vulnerability in uvnc UltraVNC, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official 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.

Weakness: CWE-787, CWE-193

Which versions are affected by CVE-2026-7831?

It affects 1 product.

Products include Uvnc Ultravnc.

How do I fix CVE-2026-7831?

No fix has been published for CVE-2026-7831 yet. Limit exposure of the affected software until one is available.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-7831 being exploited?

There is no known exploitation of CVE-2026-7831 in the wild and no public exploit code.

Sources

Also known as GHSA-38w7-mq68-3h8q.