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).
References
| Link | Resource |
|---|---|
| https://github.com/ultravnc/UltraVNC | Product |
| https://uvnc.com/ | Product Release Notes |
| https://www.securin.io/zero-days/cve-2026-7838-heap-overflow-viewer-reasonlen-integer-overflow-ultravnc |
Configurations
History
09 Jul 2026, 06:16
| Type | Values Removed | Values Added |
|---|---|---|
| References |
|
02 Jul 2026, 15:25
| Type | Values Removed | Values Added |
|---|---|---|
| First Time |
Uvnc
Uvnc ultravnc |
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| References | () https://github.com/ultravnc/UltraVNC - Product | |
| References | () https://uvnc.com/ - Product, Release Notes | |
| CPE | cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:* |
01 Jul 2026, 05:16
| Type | Values Removed | Values Added |
|---|---|---|
| New CVE |
Information
Published : 2026-07-01 05:16
Updated : 2026-07-09 06:16
NVD link : CVE-2026-7838
Mitre link : CVE-2026-7838
CVE.ORG link : CVE-2026-7838
JSON object : View
Products Affected
uvnc
- ultravnc