UltraVNC viewer off-by-one stack overflow in ServerInit desktop name parsing

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Summary

CVECVE-2026-7831
StatePUBLISHED
Assignersecurin
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-01 05:16:24 UTC
Updated2026-07-02 15:22:26 UTC
DescriptionUltraVNC 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.

Risk And Classification

Primary CVSS: v3.1 7.6 HIGH from 33c584b5-0579-4c06-b2a0-8d8329fcab9c

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H

EPSS: 0.004160000 probability, percentile 0.334490000 (date 2026-07-02)

Problem Types: CWE-193 | CWE-787 | CWE-193 Off-by-one Error | CWE-787 Out-of-bounds Write

VersionSourceTypeScoreSeverityVector
3.133c584b5-0579-4c06-b2a0-8d8329fcab9cSecondary7.6HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H
3.1CNACVSS7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Uvnc Ultravnc All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Uvnc UltraVNC affected 1.8.2.2 custom Not specified

References

ReferenceSourceLinkTags
uvnc.com 33c584b5-0579-4c06-b2a0-8d8329fcab9c uvnc.com Product, Release Notes
github.com/ultravnc/UltraVNC 33c584b5-0579-4c06-b2a0-8d8329fcab9c github.com Product
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Arjun Basnet, Securin ([email protected]) (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-06-02T00:00:00.000ZVulnerability discovered during security audit
CNA2026-06-17T00:00:00.000ZReported to vendor (coordinated disclosure)
CNA2026-09-15T00:00:00.000ZPlanned public disclosure (90-day window)
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