CVE-2026-7840
UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
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.
INFO
Published Date :
July 1, 2026, 3:33 a.m.
Last Modified :
July 1, 2026, 3:33 a.m.
Remotely Exploit :
Yes !
Source :
securin
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | CRITICAL | 33c584b5-0579-4c06-b2a0-8d8329fcab9c | ||||
| CVSS 4.0 | CRITICAL | 33c584b5-0579-4c06-b2a0-8d8329fcab9c | ||||
| CVSS 4.0 | CRITICAL | 33c584b5-0579-4c06-b2a0-8d8329fcab9c |
Solution
- Update UltraVNC repeater to a non-vulnerable version.
- Restrict access to the repeater HTTP port.
- Apply vendor-supplied patches or updates.
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