CVE-2026-7840: uvnc ultravnc Out-of-bounds Write

Vulnerability summary

CVE-2026-7840 is a critical (CVSS 9.8) Out-of-bounds Write vulnerability in uvnc ultravnc. EPSS estimates a 2.63% chance of exploitation in the next 30 days, higher than 85.2% of scored vulnerabilities.

Timeline

1. 01-07-2026
   PublishedVulnerability first disclosed.
2. 09-07-2026
   Last modifiedVulnerability information updated.

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.

• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.