uvnc UltraVNC up to 1.8.2.2 ClientConnection.cpp off-by-one

Summaryinfo

A vulnerability was found in uvnc UltraVNC up to 1.8.2.2. It has been rated as critical. This impacts an unknown function of the file vncviewer/ClientConnection.cpp. This manipulation causes off-by-one. The identification of this vulnerability is CVE-2026-7831. It is possible to initiate the attack remotely. There is no exploit available.

Detailsinfo

A vulnerability was found in uvnc UltraVNC up to 1.8.2.2. It has been declared as critical. Affected by this vulnerability is an unknown function of the file vncviewer/ClientConnection.cpp. The manipulation with an unknown input leads to a off-by-one vulnerability. The CWE definition for the vulnerability is CWE-193. A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

The weakness was shared by Arjun Basnet. This vulnerability is known as CVE-2026-7831 since 05/05/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. It demands that the victim is doing some kind of user interaction. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 07/01/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Type

• Connectivity Software

Vendor

• uvnc

Name

• UltraVNC

Version

• 1.8.2.0
• 1.8.2.1
• 1.8.2.2

License

• open-source

CPE 2.3info

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CPE 2.2info

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