PT-2026-54490 · Uvnc · Ultravnc

Published 2026-07-01 · Updated 2026-07-01 · CVE-2026-7840

Mitre

CVSS v3.1

9.8

Critical

Base vector

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

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.

Fix

Memory Corruption

Weakness Enumeration

CWE-787

Related Identifiers

CVE-2026-7840

Affected Products

Ultravnc

References · 3

• https://nvd.nist.gov/vuln/detail/CVE-2026-7840 · Security Note
• https://uvnc.com · Vendor Advisory
• https://github.com/ultravnc/UltraVNC · Note