Vulnerability Description
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).
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Uvnc | Ultravnc | <= 1.8.2.2 |
Related Weaknesses (CWE)
References
- https://github.com/ultravnc/UltraVNCProduct
- https://uvnc.com/ProductRelease Notes
- https://www.securin.io/zero-days/cve-2026-7838-heap-overflow-viewer-reasonlen-in
FAQ
What is CVE-2026-7838?
CVE-2026-7838 is a vulnerability with a CVSS score of 8.8 (HIGH). 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-...
How severe is CVE-2026-7838?
CVE-2026-7838 has been rated HIGH with a CVSS base score of 8.8/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-7838?
Check the references section above for vendor advisories and patch information. Affected products include: Uvnc Ultravnc.