Quick answers
- Is CVE-2026-7838 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-7838 being exploited in the wild.
- How severe is CVE-2026-7838?
- High severity, with a CVSS score of 8.8, published 1 July 2026.
- Which products and versions are affected?
- UltraVNC through 1.8.2.2.
- Is there a fix for CVE-2026-7838?
- No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
- How is CVE-2026-7838 attacked?
- It is attacked over the network, in a network appliance.
- Am I affected by CVE-2026-7838?
- Check whether you run UltraVNC at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
An integer overflow exists in vncviewer/ClientConnection.cpp where a 4-byte network-supplied 'reasonLen' field is incremented by 1 before being passed to CheckBufferSize(). If an attacker provides a value of 0xFFFFFFFF, the addition overflows to 0, causing the application to allocate a small default buffer (256 bytes). A subsequent ReadString call then attempts to read the full 4 GiB of data into this undersized heap buffer. This vulnerability is reachable via rfbConnFailed and rfbVncAuthFailed message types during the handshake phase, requiring no authentication. Successful exploitation could allow for remote code execution (RCE) with the privileges of the user running the VNC viewer.
Affected products
- uvnc UltraVNC through 1.8.2.2
Timeline
- 2026-07-01: advisory: NVD publication date