CVE-2026-7831 | UltraVNC viewer off-by-one stack overflow in ServerInit desktop name parsing

UltraVNC viewer through 1.8.2.2 contains an off-by-one stack buffer overflow in the RFB ServerInit message handler. In vncviewer/ClientConnection.cpp, when the server-supplied nameLength equals exactly 2024 the code declares a 2024-byte stack buffer _dn[2024] and calls ReadString(_dn, 2024). ReadString writes the NUL terminator at buf[length], i.e., _dn[2024], one byte past the end of the stack buffer. A malicious VNC server can trigger this condition by advertising a desktop name of length 2024 in its ServerInit message. On release builds without stack canaries the single-byte NUL overwrite adjacent stack data. On builds with /GS stack protection the canary is corrupted and the process terminates, resulting in denial of service. User interaction (connecting the viewer to the malicious server) is required.

Published: 2026-07-01 Last update: 2026-07-09 Assigner: 33c584b5-0579-4c06-b2a0-8d8329fcab9c Source: 33c584b5-0579-4c06-b2a0-8d8329fcab9c
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-7831 is rated Moderate Risk (44.8/100): CVSS High severity, with low exploitation likelihood (EPSS 0.53%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-7831

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-07-09 0.42% 0.53% +0.11%
2 2026-07-01 — 0.42% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-7831

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.6 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 2.8 4.7 33c584b5-0579-4c06-b2a0-8d8329fcab9c

Weakness enumeration for CVE-2026-7831

GitHub Security Advisory for CVE-2026-7831

GHSA-38w7-mq68-3h8q · Severity: high — UltraVNC viewer through 1.8.2.2 contains an off-by-one stack buffer overflow in the RFB...

Affected software / configurations for CVE-2026-7831

Vendor Product Version Raw CPE
uvnc ultravnc <= 1.8.2.2 cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:*

References for CVE-2026-7831

cvelogic Threat Intelligence