Vulnerability · Published Jul 1, 2026 · Assigned by securin
CVE-2026-7840UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
Critical 9.8
In plain English
A critical-severity out-of-bounds write in UltraVNC from uvnc. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 2.6% (higher than 86% of all known vulnerabilities).
Official description
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.
Affected products (as reported)
Weakness: CWE-787 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H