uvnc UltraVNC up to 1.8.2.2 RFB Protocol ClientConnection.cpp CheckBufferSize reasonLen integer overflow

Summaryinfo

A vulnerability identified as critical has been detected in uvnc UltraVNC up to 1.8.2.2. This affects the function CheckBufferSize of the file vncviewer/ClientConnection.cpp of the component RFB Protocol. This manipulation of the argument reasonLen causes integer overflow. This vulnerability is tracked as CVE-2026-7838. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability has been found in uvnc UltraVNC up to 1.8.2.2 and classified as critical. Affected by this vulnerability is the function CheckBufferSize of the file vncviewer/ClientConnection.cpp of the component RFB Protocol. The manipulation of the argument reasonLen with an unknown input leads to a integer overflow vulnerability. The CWE definition for the vulnerability is CWE-190. The product performs a calculation that can produce an integer overflow or wraparound, when the logic assumes that the resulting value will always be larger than the original value. This can introduce other weaknesses when the calculation is used for resource management or execution control. 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 integer overflow leading to a heap buffer overflow in the RFB protocol failure-response parsing path. In vncviewer/ClientConnection.cpp, the 4-byte network-supplied reasonLen field (type CARD32) is passed as reasonLen+1 to CheckBufferSize(). Because both operands are unsigned 32-bit, a reasonLen of 0xFFFFFFFF overflows to 0, causing CheckBufferSize to allocate only 256 bytes. The subsequent ReadString(m_netbuf, reasonLen) call then performs ReadExact for the original 4 GiB length into that 256-byte heap buffer. This overflow is reachable via rfbConnFailed (auth-scheme negotiation) and rfbVncAuthFailed (post-handshake) message types without successful authentication. A malicious VNC server, or any man-in-the-middle on the RFB stream, can trigger this condition when the victim viewer connects, potentially resulting in remote code execution as the user running the viewer. The crash was confirmed with AddressSanitizer on a portable reproduction harness (heap-buffer-overflow WRITE at offset 256).

The weakness was disclosed by Arjun Basnet. This vulnerability is known as CVE-2026-7838 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.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Type

• Connectivity Software

Vendor

• uvnc

Name

• UltraVNC

Version

• 1.8.2.0
• 1.8.2.1
• 1.8.2.2

License

• open-source

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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