CVE-2026-7840

Critical · CVSS 9.8

UltraVNC repeater — Global buffer overflow (pre-auth RCE) in embedded HTTP administration server via unchecked sprintf into fixed-size global buffer

CVSS: 9.8; nvd
EPSS: —
KEV: No
Class: other; CWE-787

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.

Search profile — drives PoC discovery

Symbols wi_senderrwi_replyhdrwebutils.chdrbufWI_RXBUFSIZEsprintfrepeater/webgui/webutils.c

Keywords CVE-2026-7840UltraVNC repeater buffer overflowUltraVNC repeater HTTP admin RCEUltraVNC repeater pre-auth exploithdrbuf overflow UltraVNCwi_senderr wi_replyhdr overflowUltraVNC repeater 1.8.2.2 exploitUltraVNC repeater webgui PoCCWE-787 UltraVNC repeaterUltraVNC HTTP port overflow

Versions: ≤ 1.8.2.2

References

Status: enriched · ingested 2026-07-09T12:00:39.000Z · profiled 2026-07-09T12:30:39.000Z