Cybersecurity professionals consider CVE-2026-7838 an immediate threat requiring urgent mitigation.
Status: Received on 01 Jul 2026, 05:16 UTC
Published on: 01 Jul 2026, 05:16 UTC
CVSS Release: version 3
33c584b5-0579-4c06-b2a0-8d8329fcab9c
Secondary
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
CVE-2026-7838: 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 exploitability of CVE-2026-7838 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).
With low attack complexity and no required privileges, CVE-2026-7838 is an easy target for cybercriminals. Organizations should prioritize immediate mitigation measures to prevent unauthorized access and data breaches.
A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.
Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.
Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.
Above is the CVSS Sub-score Breakdown for CVE-2026-7838, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.
Below is the Impact Analysis for CVE-2026-7838, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.
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