{"id":"CVE-2026-7840","aliases":[],"url":"https://o3.security/vulnerability/CVE-2026-7840","summary":"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…","details":"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.","published":"2026-07-01T05:16:24.820","modified":"2026-07-09T06:16:22.340","cvss":{"score":9.8,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[],"fix":null,"references":[{"type":"WEB","url":"https://github.com/ultravnc/UltraVNC"},{"type":"WEB","url":"https://uvnc.com/"},{"type":"WEB","url":"https://www.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-repeater-http-admin-ultravnc"}],"provenance":{"sources":["OSV.dev","NVD","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T06:16:22.340"}}