CVE-2026-7840

UltraVNC repeater through 1.8.2.2 contains a global buffer overflow in its embedded HTTP administration server. The functions wi_senderr() and wi_replyhdr() in repeater/webgui/webutils.c write the caller-supplied HTTP request URI into a fixed 1000-byte global buffer (hdrbuf) via unchecked sprintf calls. The HTTP receive buffer accepts URIs up to approximately 150 KB (WI_RXBUFSIZE = 153600), so an unauthenticated attacker who can reach the repeater HTTP port (default TCP 80) can overflow hdrbuf by at least 500 bytes with a single HTTP request containing a URI of 1500 bytes or longer, corrupting adjacent .bss-segment globals. The overflow occurs before any authentication check, making it reachable without credentials. A remote, unauthenticated attacker can achieve arbitrary code execution on the host running the repeater.

CVSS v3 9.8 CRITICAL

9.8^/10

CVSS v3 : CRITICAL

Vector :

Exploitability : 3.9 / Impact : 5.9

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact HIGH

Integrity Impact HIGH

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://github.com/ultravnc/UltraVNC	Product
https://uvnc.com/	Product
https://www.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-repeater-http-admin-ultravnc

Configurations

Configuration 1 (hide)

cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:*

History

09 Jul 2026, 06:16

Type	Values Removed	Values Added
References		() https://www.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-repeater-http-admin-ultravnc -

09 Jul 2026, 02:39

Type	Values Removed	Values Added
First Time		Uvnc Uvnc ultravnc
References	~~() https://github.com/ultravnc/UltraVNC -~~	() https://github.com/ultravnc/UltraVNC - Product
References	~~() https://uvnc.com/ -~~	() https://uvnc.com/ - Product
CPE		cpe:2.3:a:uvnc:ultravnc::::::::

01 Jul 2026, 05:16

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-07-01 05:16

Updated : 2026-07-09 06:16

NVD link : CVE-2026-7840

Mitre link : CVE-2026-7840

CVE.ORG link : CVE-2026-7840

JSON object : View

Products Affected

uvnc

ultravnc

CWE

CWE-787

Out-of-bounds Write

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