Vulnerability Report

CVE-2026-7840

Title: UltraVNC Repeater Global Buffer Overflow (RCE)

Memory Corruption

Proof Of Concept

No public PoC currently indexed for CVE-2026-7840.

CWE Category CWE-787

Published Date Jul 01, 2026

Modified Date Jul 09, 2026

Exploit Status Not Found

Score 9.3 CVSS v4.0

Exploit Probability (EPSS)

2.63%

Vulnerability Summary

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.

Impacted Vendors

uvnc 1

Reference Links

https://github.com/ultravnc/UltraVNC https://uvnc.com/

CVSS v4.0

Source Entity 33c584b5-0579-4c06-b2a0-8d8329fcab9c

Severity CRITICAL

9.3

Attack Vector

NETWORK

Complexity

LOW

Privileges

N/A

Interaction

NONE

Confidentiality

N/A

Integrity

N/A

Availability

N/A

Scope

N/A

RAW VECTOR

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS v3.1