UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
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.
Problem Types
| Type | CWE ID | Description |
|---|
| CWE | CWE-787 | Out-of-bounds Write |
Type: CWE
Description: Out-of-bounds Write
Metrics
| Version | Base score | Base severity | Vector |
|---|
| 3.1 | 9.8 | CRITICAL | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
| 4.0 | 9.3 | CRITICAL | 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 |
Version: 3.1
Base score: 9.8
Base severity: CRITICAL
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Version: 4.0
Base score: 9.3
Base severity: CRITICAL
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
Timeline
| Event | Date |
|---|
| Vulnerability discovered during security audit | 2026-06-02 00:00:00 |
| Reported to vendor (coordinated disclosure) | 2026-06-17 00:00:00 |
| Planned public disclosure (90-day window) | 2026-09-15 00:00:00 |
Event: Vulnerability discovered during security audit
Date: 2026-06-02 00:00:00
Event: Reported to vendor (coordinated disclosure)
Date: 2026-06-17 00:00:00
Event: Planned public disclosure (90-day window)
Date: 2026-09-15 00:00:00