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.
Problem types
Product status
Any version
Timeline
| 2026-06-02: | Vulnerability discovered during security audit |
| 2026-06-17: | Reported to vendor (coordinated disclosure) |
| 2026-09-15: | Planned public disclosure (90-day window) |
Credits
Arjun Basnet, Securin (arjun.basnet@securin.io)
References
uvnc.com/ (UltraVNC project page)
github.com/ultravnc/UltraVNC (UltraVNC source repository)
www.securin.io/...-stack-overflow-viewer-namelength-ultravnc